Cockroaches are flattened, fast-running, nocturnal insects that seek warm, moist, secluded areas. They are of ancient origin, and have not changed much during the 400 million years since the Devonian period of the Paleozoic era (Jeannel, 1960). They are gregarious and, because they develop from egg to adult by gradual metamorphosis, all stages and instars are found together in their preferred harborages. Cockroaches are now placed in a separate order: Dictyoptera.
These are among the most important of the household pests. In a survey of 8 large housing projects in North Carolina, 2 in each of 4 cities, cockroaches were found in 76.7% of the apartments in which control work was done by project maintenance men and in 12.7% of those that were regularly treated by licensed pest control operators (Wright, 1965a). Cockroaches are also known to cause severe injury to plants and their fruits, particularly in greenhouses (Roth and Willis, 1960).
This group of insects is well known to biology students because, like grasshoppers, they are commonly chosen for laboratory demonstrations of a somewhat generalized type of external and internal insect morphology. They are also commonly reared for research purposes, e.g., as test insects in the investigation of new insecticides. Nearly every entomological research group continuously maintains a culture of 1 or more species of cockroaches.
It is fortunate that during the last decade, insects of such great importance as the cockroaches, not only as pests, but also as experimental animals in teaching and research, have become the subjects of some excellent books containing a wealth of information of both academic and practical importance.In The Biology of the Cockroach (Guthrie and Tindall, 1968), one chapter deals with insecticides and control. Likewise, The Cockroach (Cornwell, 1968) has thus far appeared as Volume I which contains "An account of the biology oi the more common species, including details of their structure, physiology, behavior, and ecology." Volume II is expected to be published in 1975, and will be devoted to cockroach control methods. It should be available by the time our present book has been completed.
Cockroaches as the Ancestors of Termites
Certain morphological, biological, and behavioral characteristics of cockroaches indicate that they were the ancestors of termites, which appeared much later in the scale of geological time. These characteristics include the possession of intestinal protozoa (flagellates) by the primitive wood-eating cockroaches, Cryptocercus punctulatus and Panesthia javanica, the former being indigenous to the United States. These protozoa convert indigestible cellulose into soluble sugar, as they do in the more primitive families of termites (Trager, 1932; Cleveland et al., 1934; Rau, 1941). Of interest in this connection is the observation by Snyder (1935) that C. punctulatus and the termite Zootermopsis were found in California inhabiting the same log. Cockroaches and primitive termites also have intracellular symbionts (Brooks and Richards, 1955). This has led to the idea that the common ancestors of cockroaches and termites had both intracellular microorganisms and xylophagous flagellates. Cockroaches, except for the genus Cryptocercus, have retained the intracellular symbionts, and Cryptocercus has retained the intestinal flagellates. The primitive living termites (all the families except Termitidae), except for Mastotermes darwiniensis Froggatt, the most primitive of them all, have kept only the intestinal flagellates (M. darwiniensis has both). The Termitidae have lost the xylophagous intestinal flagellates, but have acquired a rich and complex bacterial fauna (Grassé and Noirot, 1959).
Mastotermes darwiniensis has an egg mass similar in appearance to the egg capsule (oötheca) of cockroaches. Its eggs are firmly cemented together by a light-brown, gelatinous secretion that fills the interstices (Hill, 1925; Snyder, 1935).
The ability many cockroaches have to mix bits of debris with mouth secretions in order to hide or disguise their oöthecae reminds one of a similar process used by termites to build nests. There are other interesting similarities (Rau, 1941; McKittrick, 1964, 1965; Cornwell, 1968).
Cryptocercus punctulatus leads a subsocial life in decayed logs, which serve as both food and shelter (Snyder, 1935). An Australian cockroach, Panesthia, lives in distinct family groups in burrows in the soil. Each group consists of an adult male and a viviparous female, and from 10 to 20 nymphs in various instars. Soon after reaching maturity, the adults bite off their own tegmina and hindwings; wings are inconvenient for insects that inhabit burrows. This indicates the probable origin of termites' social life in subterranean galleries and their discarding of wings (Tillyard, 1926). The subsequent evolution of termite social organization to such an astounding degree of complexity, superficially so strikingly similar to that of the social Hymenoptera, depended on a concomitant chemical evolution of pheromones and a highly specialized chemical communication among these insects. This is a good example of the important role of pheromones in insect evolution.
The ancient cockroaches folded only a small area (anal lobe) of their hindwings, as can be seen in the fossil genus Pycnoblattina. The living Mastotermes darwiniensis also folds the anal lobe of its hindwings. The cockroaches gradually evolved more complex folding of the hindwings, while the termites, except for M. darwiniensis, completely lost this feature of their wing structure (Tillyard, 1936, 1937; McKittrick, 1964). McKittrick (1965) believed that cockroaches, walkingsticks, and termites belonged in the same insect order. She pointed out that Cryptocercus had xylophagous flagellates in common with primitive termites but not with other cockroaches. On the other hand, the primitive Mastotermes darwiniensis has intracellular bacteriocytes in common with cockroaches, but not with other termites. Also, in certain morphological features, Cryptocercus differs far less from Mastotermes than it does from many of the cockroaches.
The Place of Cockroaches in the Class Hexapoda
The place of cockroaches in the class Hexapoda has been the subject of considerable disagreement among experts during the last half-century. All the common cockroaches were formerly considered to be members of the family Blattidae of the order Orthoptera. This order included, besides the cockroaches, the grasshoppers, crickets, katydids, mantids, walkingsticks, and several other related families. In recent years, several new classifications have been proposed. The author has adopted the classification used by Cornwell (1968), based on the one suggested by Imms (1957) in the ninth edition of his A General Textbook of Entomology. According to this classification, the cockroaches should be placed in the order Dictyoptera, which is divided into the suborders Blattaria (cockroaches) and Mantodea (mantids). Blattaria in turn is divided into the superfamilies Blattoidea and Blaberoidea. Blattoidea contains 2 families, one of which (Blattidae) includes the well-known genera Blatta and Periplaneta. Blaberoidea contains 3 families, one of which (Blattellidae) contains the genera Blattella and Supella.
One way in which cockroaches differ from the closely related grasshoppers, locusts, and crickets is that they do not have a visible ovipositor. Also, they do not lay their eggs singly or in pods, but in a hardened, purseshaped egg case called the oötheca (plural, oöthecae). Most domestic cockroaches drop their oötheca as soon as it is formed, but the German cockroach (Blattella germanica), generally the species most frequently found in the household, carries hers until a day or two before the eggs are ready to hatch. She is often seen with an attached oötheca (figure 148).
Among domiciliary cockroaches, B. germanica is the only species with an oötheca subject to desiccation. The eggs are dependent on moisture supplied by the mother. The oöthecae of other species are remarkably resistant to desiccation. Pryor (1940) believed the oötheca of Blatta orientalis to be covered with an oily secretion, probably derived from the cuticle of the mother. It was so effective in retarding water loss that eggs hatched in about 46 days from oöthecae that had been kept in an atmosphere of 0% relative humidity from the age of less than 1 day (Roth and Willis, 1955).
Oöthecae also effectively protect the enclosed eggs from insecticides. Not until the eggs are hatched and the nymphs leave the protection of the oötheca and crawl over the insecticide residue are they subjected to the effects of an insecticide treatment, provided the residue retains its efficacy that long.
World-Wide Distribution of Cockroaches
Cockroaches are tropical or subtropical in origin, and can maintain themselves outdoors in such areas. Species that became adapted to living in human habitations - "domestic" or "domiciliary" species - all originated in north or tropical Africa. Species names such as americana, australasiae, germanica, or orientalis, and the corresponding common names, are therefore misleading.
The distribution of domiciliary cockroaches throughout the world was made possible by their ability to infest the various means of intra- and intercontinental transport, particularly ships. Originally, and probably to this day, their principal means of transportation between continents has been by ships. (Movie fans and readers of classics may be interested in the fact that in 1792, the much-maligned Captain Bligh combated cockroaches on HMS Bounty with boiling water.) However, transport of cockroaches in the baggage compartments and kitchens of aircraft has been abundantly documented (Roth and Willis, 1960).
For many years, the principal species infesting ships was the American cockroach, but in more recent times the German cockroach has become the dominant ship-infesting species (Cornwell, 1968).
To judge by the enormous numbers of cockroaches infesting ships before the advent of effective insecticides, these insects must have found abundant food and water. Even without food and water, however, cockroaches can live for considerable periods. Newly emerged adult females of the German cockroach, Blattella germanica, have been found to live at 27°C (81°F) and 36 to 40% relative humidity for an average of 20.1 days without food or water, 14.7 days with dry food (dog biscuits) and no water, 35.1 days with water but no food, and 82 days with both food and water. The corresponding figures for the American cockroach, Periplaneta americana, were 41.7, 40.1, 89.6, and 190 (Willis and Lewis, 1957). The apparent ability of the 2 species to survive slightly longer without food and water than with dry food alone is interesting, but in any case, cockroaches are well adapted to dispersal over great distances by man, even when food and water are lacking.
Domestic species of cockroaches never or rarely fly, but are readily carried about in and on such items as sacks, cartons, or packages (particularly corrugated cartons) of food, in laundry, or in kitchen appliances and furniture. Beverage cartons are particularly important means of distributing cockroaches. The cartons are often contaminated with spilled syrups or malts, which attract cockroaches. Empty and unrinsed softdrink or beer bottles in night clubs, restaurants, markets, and homes form a part of the infestation chain. More than 200 cockroach nymphs have been found in a single soft-drink bottle brought to a market for exchange (DeLong, 1962). Cockroaches may be found by the thousands in insulation in the walls of refrigerators and gas or electric ranges, surviving for months if necessary without access to their usual foods and feeding only on cast skins and dead insects. Cockroaches may become established in basements or crawl spaces, particularly, if these places are dark and damp, and they may then enter the building around utility pipes, air ducts or ventilators, or under doors.
Economic Importance of Cockroaches
Cockroaches rank with termites as the most important insects that must be dealt with by the owners or occupants of homes or other buildings or by pest control operators hired to combat insects. Flies and mosquitoes may be more important as household pests in some areas, but they breed elsewhere and then invade dwellings. Therefore, in the United States and many other countries, fly and mosquito control is generally considered to be primarily a community or regional responsibility, and the homeowner is generally left with the job of controlling only minor or seasonal infestation.
The Cockroach as a Domestic Nuisance
Cockroaches breed within the home, sharing with man his food, water, shelter, and warmth, and are thus active throughout the year. They consume any human or animal food or beverage, as well as dead animals and plant materials, leather, glue, hair, wallpaper, fabrics, and the starch in bookbindings. They contaminate food, imparting an unpleasant odor and taste, and may transmit disease organisms.
Although cockroaches are nocturnal, if they are abundant a few of them may be seen during daylight hours, particularly if articles in pantries, cabinets, and closets are moved about to disturb their hiding places. Other evidences of a cockroach infestation may be dead cockroaches, cast skins of the various nymphal instars, empty egg cases, and fecal droppings. The droppings are variable in size, and range from as small as "flyspecks" to as large as mouse droppings. Intersections and corners of shelves and the hinges of cabinet, pantry, and even closet doors show fecal stains when there are heavy infestations. The musty odor left on objects cockroaches contact may also be evident in heavy infestations.
Cockroaches as Disease Carriers
Many of the organisms causing diseases of man have been found on the tarsi and other body parts, or in the fecal pellets, of cockroaches, but these insects are most commonly implicated in the transmission of Salmonella, the causal agent of food poisoning (Roth and Willis, 1957, 1960; Rueger and Olson, 1969). When the feces of American cockroaches infected with Salmonella oranienburg were spread on human foods and on glass, the bacteria survived for the following periods: on corn flakes, 3.25 years; on crackers, over 4.25 years; and on glass slides, over 3.67 years. Mice placed in jars containing minute quantities of infected cockroach feces became themselves infected in as brief a period as 1 day (Rueger and Olson, 1969).
Parasitic toxoplasmosis is a disease that is believed to infect more than a third of all adult Americans at some time during their lives. It is usually a mild disease, but can be extremely serious in pregnant women, causing congenital defects in an unborn child while remaining mild or symptomless in the mother. The protozoan parasite, Toxoplasma, can be acquired by eating infected raw or undercooked meat. If the blood serum of an infected person is added to cultures of the parasites, the differing colors of fluorescence of the latter will indicate either a positive or negative reaction and thereby serve as a test for the disease (Anonymous, 1973a).
Cats that consume birds or rodents carrying the parasite can become carriers. Filth flies have been found capable of transmitting infectious Toxoplasma oöcysts to human food 1 or 2 days after consuming infected cat feces. In laboratory tests, Toxoplasma parasites were isolated from the digestive tracts and feces of cockroaches as long as 7 and 10 days, respectively, after the cockroaches had last been in contact with infected cat feces. Filth flies and cockroaches might consume infectious cat feces and contaminate human food. They might also serve as food for birds and small rodents that could in turn be eaten by cats (Anonymous, 1973a).
Goodwin (1973) recorded the successful passage of hepatitis B antigen (AB-Ag) through the alimentary tracts of American cockroaches, and identified the antigen in the feces of the insects. The hepatitis antigen was found in the feces of the cockroaches for up to 9 days after their initial exposure to the antigen-positive test meal.
Cockroach Bites
Roth and Willis (1957) discussed 20 reports of cockroaches biting man. However, such instances are rare and not serious; they are discussed here only because questions are often asked concerning the biting ability of cockroaches. There have been a number of reports in the literature of cockroaches being so numerous in ships that they gnawed the skin and nails of the men on board; in one case, the sailors wore gloves while sleeping to prevent hordes of the insects from gnawing off their fingernails (Roth and Willis, 1960). Cockroaches are most likely to gnaw fingernails, eyelashes (of sleeping children), and the callused portions of hands and feet, but can also cause small wounds on softer skin. Periplaneta americana and P. australasiae are the species most often implicated.
The Cockroach Allergen
Some people are allergic to cockroaches. In one investigation among 253 normal persons, 7.5% showed positive skin tests with extracts of cockroaches (Periplaneta americana and Blatta orientalis) compared with 28% of an unselected group of 114 allergic patients. Skin-sensitizing antibodies were present in the blood sera of positive reactors. The allergen is thermostabile. There is evidence that in some instances symptoms attributable to food allergy may be caused by cockroach allergen with which the food is contaminated (Bernton and Brown, 1964, 1970a, b).
In an investigation to test the age of onset of skin reactivity, 38 out of 102 allergic children, ranging from infants to 12 years old, gave positive cutaneous reactions to body extracts of the German cockroach, Blattella germanica, compared with only 5 out of 100 nonallergic children. A 4-year-old asthmatic child was the youngest to give a positive reaction (Bernton and Brown, 1970a). In a later investigation, it was found that an extract of B. germanica caused an attack of asthma in 10 asthmatic persons with skin hypersensitivity to the extract and other allergens, but not in asthmatics without such skin hypersensitivity (Bernton et al., 1972).
An allergen in the feces of B. germanica acts as an ingestant when it contaminates food and as an inhalant when dried fecal particles become incorporated with house dust (Bernton and Brown, 1970b).
Natural Enemies of Cockroaches
Natural enemies appear generally to be of little importance, although according to Roth and Willis (1960), E. C. Zimmerman stated that the hymenopterous egg parasite Comperia merceti (Compere) (Encyrtidae), when accidentally imported, practically wiped out the brownbanded cockroach in Hawaii. In some parts of Honolulu, almost 100% of the oöthecae of this species were parasitized. Muesebeck et al. (1951) recorded this parasite from the German as well as the brownbanded cockroach in Florida and Texas. Flock (1941) stated that another related parasite, Anastatus blattidarum Ferrière (Eupelmidae), seemed to be effective in controlling the brownbanded cockroach in certain areas in Arizona, and Hull and Davidson (1958) found some oöthecae of this species to be parasitized by A. blattidarum among the cockroaches they reared in Ohio. T. Maki considered the hymenopteron Tetrastichus hagenowii (Ratzeburg) (Eulophidae) to be an important egg parasite of cockroaches in Taiwan (Roth and Willis, 1960), and Muesebeck et al. (1951) recorded Blatta orientalis, Blattella germanica, Periplaneta americana, and P. australasiae as hosts in Florida and Louisiana.
Roth and Willis (1960) also stated that species from at least 6 families of Hymenoptera had been recorded as developing on cockroach eggs. They quoted Edmonds (1957) as stating that evaniid egg parasites were so abundant in a home in Ohio that the occupants considered them a nuisance, although the oriental cockroaches in the basement did not annoy them. Roth and Willis also listed many predators, including scorpions, spiders, dragonflies, mantids, bugs (e.g., reduviids), beetles (carabids, rhipiphorids [similar to mordellids], dermestids, and others), wasps, ants, toads, frogs, lizards, birds, poultry, and various mammals. Cannibalism among cockroaches, including the devouring of oöthecae, has been noted by many investigators even when food was adequate.
When humidity is too high in cockroach culture colonies, mites may become numerous, and have been known to cause German cockroaches to drop their oöthecae prematurely, resulting in a low percentage of eggs hatching. Control of mites with acaricides has been known to increase the vigor of cockroach colonies (Roth and Willis, 1960). A pterygosomid mite Pimeliaphilus podapolipophagus Trågårdh, must actually feed on live cockroaches to survive; it cannot subsist on feces, cast skins, or dead insects (Cunliffe, 1952). It has been accused of biting people, and its presence in homes is an indication of cockroach infestation (Baker et al., 1956).
Roth and Willis (1960) reported 2 species of spiders in the family Theridiidae, the South African Latrodectus indistinctus and the North American L. mactans, the latter the venomous "black widow," as predators of cockroaches. The author observed another theridiid spider, Steatoda grossa (C. L. Koch) (figure 212, chapter 9), which resembles the black widow but is one of its natural enemies, in great abundance in an infestation of German cockroaches confined to experimental mockup closets at the University of California (Los Angeles). Remains of cockroach nymphs were found in their webs. It is also of interest to note in this connection that the related S. fulva (Keyserling) blocks the nest entrances of the harvester ant Pogonomyrmex badius (Latreille) during the early afternoon, when temperatures are high and the ants are inactive, and feeds on the ensnared ants (Hölldobler, 1970). Steatoda albomaculata (De Geer) also feeds on ants (Levi, 1957).
From their extensive review of the literature, Roth and Willis (1960) concluded that with the exception of a few instances of egg parasitism such as just cited with respect to Comperia merceti and Tetrastichus hagenowii, there had been too little information to enable them to evaluate the effectiveness of biological control in reducing the numbers of pest cockroaches, and that further investigation would be justified.
A large percentage of any cockroach population generally contains internal parasites. Two species of nematodes and 7 of protozoan parasites were found in 105 Blattella germanica in New York city.
One of these nematode species, Blatticola blattae (Graeffe), was present in 96.2% of the cockroaches collected. Among the protozoa, Nephridiophaga blattellae was found in 82.8% of the cockroaches, and 3 other species were abundant (Tsai and Cahill, 1970).
Rearing Cockroaches Experimentally
A continuous supply of several species of cockroaches has been reared for experimental purposes in our laboratory at UCLA. These species are, in the order of what we consider to be their importance in California, the German, oriental, brownbanded, and American cockroach. Later the Australian and the brown cockroach were added. The insects were originally reared in 15 gal (57-L) garbage cans with an electric barrier (Wagner et al., 1964), later replaced by 20-gal (76-L) plastic trash cans (Ebeling et al., 1966). A double layer of strips of aluminum foil with a strip of insulation tape between them, and with the terminals from a 22.5-volt battery connected to the aluminum strips, serves as an electrified barrier to the escape of the cockroaches. In each trash can there is a poultry-watering device, Purrina Dog Chow, and 3 mazes of concentrically rolled and closely approximate layers of corrugated paper to serve as the harborage areas that cockroaches require for optimum development. The trash cans are covered with their usual plastic lids, provided with screens for ventilation.
It is important to prevent contamination of a colony by stray individuals of other species. It is especially important to keep German cockroaches out of colonies of other species because of the ability of this insect to thrive and rapidly increase its numbers at the expense of the others.
In the cockroach rearing room, boric acid powder applied under benches, radiators, appliances, cabinets, etc., will serve to permanently prevent a buildup of the cockroach population, in the rearing room and adjacent rooms, from those insects which escape when they are being removed for experimental work.
Twenty cans of German cockroaches and a can or more of each of the other 5 species are continuously maintained in our laboratory. The temperature is kept near 80° F (27° C) in the rearing room. Provided that a water supply is constantly maintained, the author and his colleagues have had no indication that the maintenance of relative humidity above ambient is of any advantage in the rearing of cockroaches, at least in the rearing cans just described. With ambient humidity and monthly removal of the debris at the bottoms of the cans (with a vacuum-cleaner device), infestation of German cockroaches by parasitic mites has been avoided. American cockroaches drop their egg cases to the bottoms of their rearing cans; therefore, the cans are vacuumed only about twice a year, but their colonies do not produce so much debris and do not seem to be so susceptible to mite infestation as those of the German cockroaches.
German Cockroach, Blattella germanica (L.) (Blattellidae)
The German cockroach is generally the most important cockroach species in the United States and many other areas of the world in homes, apartment buildings, restaurants, and in the food preparation and food-storage rooms of hotels, hospitals, and other private and public buildings in which food is prepared, stored, or served.
Description. Adult German cockroaches are 1.3 to 1.6 cm long, pale brown or tan, and have 2 parallel dark streaks on the pronotum (plate III, 1). They have chewing mouthparts. (See figures 44and 45, chapter 4, showing the similar American cockroach.) Their movements are very rapid when they are disturbed. They are nocturnal. If a few are seen crawling about in open spaces during daylight hours, this indicates that the infestation is already severe. The female is darker and has a broader abdomen than the male, and is rounded posteriorly. If German cockroaches are placed on their backs, the males are easily distinguished by the yellowish, slender abdomen, tapering gradually to the posterior end. The abdominal cerci of the males have 11 segments and those of the females have 12.
In their investigation of the German cockroach, Willis et al. (1958) found the ratio of females to males to be 1.12:1. In similar work, Ross (1929) observed that the sexes occurred in approximately equal numbers.
Mating Behavior. Roth and Willis (1952) determined that the male of the German cockroach could not detect the female from a distance, even when in close proximity, but had to make physical contact, ordinarily via the antennae. The antennae of the female, as well as other body regions, contain a chloroform-soluble, nonvolatile substance that will stimulate the male sexually. Thus, sex discrimination by males is mainly owing to "contact chemoreception." It follows that the male German cockroach probably cannot be attracted from a distance by means of synthetic sex attractants (provided they are available) as some other insect species can be.
When male and female meet, their antennae touch and vibrate against each other. The male then turns around, raises his wings to expose the orifices of a pair of dorsal glands located on the seventh and eighth tergites, and extends his abdominal segments to expose the openings of the 2 pairs of glands. The glands are not normally visible, being covered by the wings and by the margins of the preceding abdominal sclerites. Both wing-raising and extension of the abdomen are required to uncover the glands. The female eats a secretion from these glands. After she has fed for a few seconds, the male-pushes his abdomen farther back, and connection of the genitalia is made. The male then moves out from under the female, and the pair remain attached in a linear position for an average of about 86 minutes (Roth and Willis, 1952). These authors also pointed out that the fact that German cockroaches do not need to be attracted from great distances, as do many other insects for the meeting of sexes, is understandable in view of the habits of cockroaches in general. The chance meeting of opposite sexes is enhanced by the fact that they are negatively phototactic and positively thigmotactic (principally guided by contact), and that they are gregarious, with large numbers seeking the same environment. They seek secluded and particularly very narrow hiding places, such as cracks, crevices, and voids, especially those having optimum temperature and moisture conditions. Attraction to the odor of the species is another factor that favors aggregation. Once the sexes have been brought together by these different stimuli, more refined stimuli lead to sexual discrimination.
Life Cycle. The German cockroach lays more eggs and has more generations per year than the other common domiciliary species, and therefore tends to increase in numbers much more rapidly. There may be 3 or 4 generations per year. In one investigation at a temperature of 76° F (24.4° C), the developmental period varied from 54 to 215 days, with an average of 103 (Gould and Deay, 1940). It has been found to be as little as 60 days for males and 65 days for females at 95° F (35° C) and 90 to 95% relative humidity (Ross, 1929). An individual German cockroach, however, lives an average of 200 days at room temperature (Gould and Deay, 1940), and may live as long as 10 months (Truman, 1961a). A comparison of the developmental periods of the 4 principal domiciliary cockroaches at 82° F (28° C) and at ordinary room temperature is shown in figure 149.
The Oötheca (Egg Capsule). The German cockroach female produces her first oötheca 11 or 12 days after becoming an adult (Ross, 1929). 0öthecae can be produced parthenogenetically (without fertilization), but although embryos develop, the eggs do not hatch (Roth and Willis, 1956). Unlike other domiciliary species, the German cockroach female carries her oötheca for as long as a month, until a day or two before the eggs are ready to hatch, then drops it anywhere. The eggs occasionally hatch while the oötheca is still attached. The female generally deposits 4 or 5 oöthecae, but there can be as many as 8. The females of other cockroach species drop the oötheca as soon as it is fully formed. Thus, the abundance of females with their attached, brownish, purseshaped oöthecae (figure 148) is a characteristic of a German cockroach infestation.
The oötheca is about 8 x 3 mm-rather large in relation to the size of the female. As with all common cockroach species, the eggs are located in 2 parallel rows. The locations of the eggs are indicated by corresponding divisions on the outer wall of the brownish oötheca. When the embryos have developed to the point where they exert enough pressure, the oötheca splits open at the top and the nymphs wriggle out. The female will eat the young nymphs if no water is available to her (Ross, 1929).
There are usually 30 to 40 eggs per oötheca, but there can be as many as 48. In one investigation, an average of 29.9 nymphs were hatched per oötheca in an average of about 28 days at ordinary room temperature (Gould and Deay, 1940), while in another investigation at 95° F (35° C), eggs hatched in 14 days (Ross, 1929). After the fourth oötheca is produced, the number of eggs per oötheca gradually decreases to about 75%of the original number in the seventh and eighth oöthecae (Willis et al., 1958).
The eggs become desiccated if the oötheca is removed from the female any time before its normal time to drop, for they must obtain moisture from the body of the mother. The end of the oötheca that is attached to the female is relatively soft and permeable to water, and is not so heavily sclerotized as the posterior end. Roth and Willis (1955) found that German cockroach oöthecae placed with their anterior ends on wet filter paper gained weight, whereas those with their posterior ends on the same wet paper lost weight, even though the humidity was high in the covered petri dish in which the oöthecae were kept. Probably, the wall of the oötheca in contact with the female's genital pouch is permeable to water (Ross, 1929; Parker and Campbell, 1940).
Even before the oötheca begins to emerge, the abdomen of the female is greatly distended. Once the translucent tip of the oötheca becomes visible, the entire oötheca will be fully developed and entirely visible by the following day, changing from white to pink within a few hours. Within a day or two, it becomes light brown and finally chestnut. It is turned with the keel to the left or right (Haber, 1919; Gould and Deay, 1937, 1940).
Nymphal Instars. Wright (1968) found that among 749 German cockroach nymphs, 5.7% emerged while the oöthecae were still attached to the body of the female, 92.9% emerged within 24 hours after the oöthecae were dropped, and only 1.4% emerged thereafter. Among field-collected German cockroaches, 27.9 nymphs per oötheca emerged from those dropped by females collected in summer, compared with only 9.7 from those collected in winter.
German cockroach nymphs have 6 or 7 instars. As in the case of all insects with gradual or incomplete metamorphosis, there is no abrupt change in appearance between immature and adult forms, except that the adults have wings. Nevertheless, the layman will not necessarily associate the tiny, newly hatched German cockroaches or even the more mature nymphs with the adult insects unless he sees all instars and stages together in considerable numbers. The smaller nymphal instars are sometimes isolated from the remainder of the colony, particularly when they have gained access to a crack or crevice too narrow for the older nymphs and adults.
The first-instar nymph is only 3 mm long. The body is dark gray to almost black, except for the second and third thoracic segments, which are pale brown. The pale-brown band conspicuously characterizes the first-instar nymph. In succeeding nymphal instars, the light band becomes narrower and extends in both directions to become a median longitudinal stripe (plate III, 7). In its anterior extension, it eventually becomes the median pale-brown stripe dividing the "two parallel dark streaks" (already mentioned) that characterize the pronotum of the adult. The remainder of this stripe is covered by the wings.
Completely white nymphs or adults may be seen in a cockroach colony. These are newly molted individuals that have not yet had time to harden their cuticles and to acquire the normal color for the species.
A high degree of mortality occurs during the molting periods. Ross (1929) found mortality to be about 50% during each molt except for the last, when it was 40%. He found that about half the insects died of natural causes before reaching maturity. Molted skins were quickly eaten by the nymphs that emerged from them or by other cockroaches that happened to be near-by. On the other hand, Willis et al. (1958) found in their cultures that 85% of the hatched insects reached maturity.
Aggregation Pheromone
As already stated, because cockroaches have a gradual metamorphosis, all immature instars may be seen together with the adults. They tend to congregate in a single area, leaving it to search for food and water, then returning. An aggregation pheromone, present in the feces and on the bodies of the insects (it spreads easily on the greasy epicuticular lipid), is responsible for this tendency to aggregate. The gregarious behavior of nymphs favors their growth and development; they do not develop so well when isolated (Ishii and Kuwahara, 1968). Isolated nymphs that could only see or smell the others, or received contact only with the washings of the body surfaces of other nymphs, or with filter paper contaminated with feces, did not grow at the group rate. This indicated that a "psychological factor" affecting the endocrine system by means of tactile stimulation, chiefly via the antennae, was the principal factor involved in growth acceleration (Izutsu et al., 1970).
Effect of Excessive Crowding. The author investigated the effect of crowding in a "choice box," 30 cm square and 10 cm high, with a 12-mm hole at the top of the partition wall between the 2 halves (Ebeling et al., 1966). When there are only 20 to 40 cockroaches in the choice box, most of them will spend nearly all their time in the dark half, for they are negatively phototropic. The insects tend to congregate at the intersection of 2 plane surfaces or in corners. When such areas are filled, the insects will gather in small, scattered groups on plane surfaces. However, despite the aggregation pheromone, German cockroaches have some aversion to being crowded too closely together. They prefer to leave some space between themselves and their neighbors. They sometimes initially display aggressive behavior toward their neighbors when actual physical contact is made. However, if large numbers are forced to occupy limited space, they soon appear to get used to this situation.
When 200 adult male German cockroaches were placed in a choice box, only 11% were in the light half 3 days later, compared with an average of 16.2% in the light halves of 5 choice boxes, each containing 20 adult male cockroaches. On the other hand,, when 959 adult German cockroaches of mixed sexes and 530 nymphs were placed in the dark half of a choice box, they were so crowded that they made physical contact with one another, and in some cases were one on top of the other. They were left that way for 2 hours, and then a cork occluding the 12-mm hole in the partition wall was removed. Large numbers of cockroaches left the dark compartment. On the third day of the experiment, the insects were anesthetized and,counted. Of the live insects, 36.1% of the adults and 41.7% of the nymphs were in the light compartment of the choice box, preferring the light to an excessively crowded condition in the dark. If German cockroaches are found in parts of an untreated building far removed from food and water or from preferred habitats, it is an indication of excessive crowding in their customary harborages.
Crowding in Narrow Spaces. Adult German cockroaches can move about in space only 1.6 mm in width or depth (Wille, 1920). When given a choice among 8 spaces between masonite plaques placed one above the other and ranging in distances from 1.6 to 12.7 mm apart, in 1.6-mm increments, 67% of adult German cockroaches gathered in the 4.8-mm space. When the plaques were vertically arranged, this percentage increased to 85. Only a few adults gathered in the 1.6-mm space (Berthold and Wilson, 1967). In our laboratory we found that only the first 3 instars of a population of all ages of German cockroaches crawled from a light to a dark compartment of a 5-cm-wide wooden box for a distance of 2 cm under a strip of masonite that was only 1 mm above the floor of the box. Only 80% of the adult population (6 males and 2 females without oöthecae) crawled through a 2-mm aperture in a 24-hour period, even though the insects were driven by hunger and negative phototropism to seek food and darkness. No adults were able to crawl from the light to the dark compartment when the 2-mm aperture was between the wooden partition wall (1 cm thick) and a masonite ceiling.
The author once observed a severe infestation of German cockroaches in the food-storage room of a large "rest home." Leaking or spilled packages of flour, cereal, or similar foods on the shelves provided plentiful food for the insects. Cockroaches were crowded into a space of 2 or 3 mm between a shelf and the wall. They were completely hidden from view, except for a continuous band of waving antennae that extended from the aperture, and could be seen by looking beneath the shelf.
This.sort of thing is a common occurrence, well known to pest control operators. However, it appears that the first mention of this phenomenon in the literature was in P. B. Cornwell's The Cockroach, and figure 150 is taken from his book.
Habitat
In the living space of a house, German cockroaches are most abundant in the kitchen, and this is also true of other domiciliary species. During the day they may be found under stoves or ranges and refrigerators, and in the insulation in the walls of appliances; under the sink and especially in the "dead space" between the sink and the wall; under and behind the water heater, particularly if it is enclosed in a cabinet; in, under, or behind cabinets, pantries, and closets; behind baseboards and moldings; and in other cracks, crevices, and dark, protected areas. They venture forth at night in search of food and water.
German cockroaches are also likely to be found in the bathroom, particularly if the kitchen and bathroom share a common wall, for then they can infest a common wall void and pass from one room to the other via areas surrounding utility pipes or the louvers and razor-blade receptacle of the built-in medicine cabinet. In moderate to severe infestations, they can be found throughout the house in protected areas, such as closets, dressers (usually above the top dresser drawer), sofas, electrical appliances such as radios and television sets, under stairwells, behind moldings, picture frames, etc. All such places must be thoroughly treated with insecticide for effective control.
Television sets in particular are becoming increasingly important harborages for both German and brownbanded cockroaches. Not only the warmth, darkness, and the edible glue used in construction, but often the bits of human food commonly found in the vicinity of television sets are attractive to cockroaches and combine to provide an ideal harborage.
In severe infestations, enormous numbers of German cockroaches may sometimes be found in the attic and in wall voids. They use the wall voids to move either laterally or vertically from one room to another or from one apartment to another. They may also be found in the crawl space under the house, at the bases of foundation walls, in cracks in the sidewalk, in the lawn and under shrubbery, and in outdoor incinerators or garbage-disposal bins.