Biology Of The German Cockroach

What are German cockroaches? 

The German cockroach (Blattella Germanica) is a cosmopolitan pest that exclusively lives with humans. It produces allergens that may trigger asthma and can create an unsanitary environment. To emphasize the importance of the problem, it is worth mentioning that up to 85% of U.S. inner-city homes test positive for cockroach allergens, and 60–90% of inner-city children who have asthma are sensitized to these cockroaches (Rosenstreich 1997). Insecticides are necessary to control German cockroaches in many situations. However, since the 1950s, this species has become resistant to many insecticides, making their management more difficult.

In search of a cure to the resistance issue, researchers have conducted pivotal field studies to compare various intervention strategies and evaluate resistance across multiple generations of the German cockroach. One study showed that cockroach populations could be eliminated using only a single active ingredient when the resistance levels are low. Also, they found that rotational treatments using low-resistant insecticide may reduce the insecticide selection pressure and make the treatment more effective. A combination of thiamethoxam and cyhalothrin active ingredients (AIs) was found to be ineffective and highly repellent (Fardisi et al. 2019).

The German cockroach is also a vector of human enteric pathogens such as Escherichia coli, Salmonella, and Enterococcus. They are capable of hosting many other bacteria in their digestive system, including antibiotic-resistant strains. Many other pathogenic microbes, including protozoa and fungi, have also been linked to the German cockroach.

Where do they come from?

The German cockroach is found worldwide exclusively near human settlings and may not survive without human populations. Despite the word “German”, the German cockroach is not originated from Germany or even Europe. Its true origin is not clear, but it may have originated from Asia. A significant factor preventing its survival outdoors seems to be cold temperatures. Studies have reported that German cockroach cannot colonize vacant structures during cool seasons and does not survive structures without central heating in northern climates.

German cockroaches usually hide in dark, sheltered spaces during the day and come out at night to feed. However, they may be commonly seen during the day, especially in heavily infested environments. Inside homes, they could be found around the drainboard, under the kitchen sink, in cracks, around or inside cabinets, in drawers, behind loose baseboards, around window or door frames, and in any closet around the house. However, you may mostly find them in the kitchens and bathrooms.

Life stages

Like many other insects with incomplete metamorphosis, the German cockroach has three different life stages: egg, nymph, and adult. Below, we will describe each stage:

• Egg. Females carry the eggs in an egg case (ootheca) until just before they hatch. The egg case is a small, brown capsule that is about ~5/16 inch (8 mm) long, ~1/8 inch (3 mm) high, and ~1/16 inch (2 mm) wide and typically contains 30 to 40 eggs. The ootheca protrudes from the posterior end of females from what is called the genital chamber. 
• Nymph. This stage begins with eggs hatching and ends when the adult emerges. Nymphs are dark brown to black, and they have distinct parallel bands running the length of their pronotum. Nymphs do not have wings. There are usually six molts required to reach the adult stage. However, this number varies. At room temperature, it takes around 60 days for nymphs to complete their development and for this to happen, they need food and water.
• Adult. The adult is ~3/8 inch (10mm) to ~5/8 inch (15 mm) long, brown to dark brown, with the two distinct parallel bands running their pronotum.

• Males have a thin body, tapered posterior abdomen, and an exposed visible terminal abdominal segment.
• Females have a stout body, with a rounded abdominal tip that is entirely covered by tegmina.

Life span

It takes around three months (~100 days) for a German cockroach to reach the adult stage from an egg. However, this period can significantly change depending on the ambient temperature and the availability of food. Since they live indoors, German cockroaches can breed continuously all year round and can have many overlapping generations.

Reproduction

German cockroaches reproductive continuously and very rapidly in structures. The mating behavior in German cockroaches is mainly triggered by pheromones secreted by females, which are detected by the antennae in their male counterparts. 

They breed throughout the year but breed slower in cooler months of the year. They have internal fertilization in which females store the sperm for gradual release. Throughout their lifetime, females produce 4 to 6 capsules, each containing 30 to 40 eggs.

Diet

German cockroaches are omnivores and therefore feed on a wide variety of foods. In a study, German cockroaches were fed with three different diets, including poultry, sugar, wheat, and rusk. It was shown that those feeding on poultry feed had the highest hatching rate and longevity in both males and females. They specifically like starches, sweets, beer, and meat products. Last but not least, they feed on leather, bakery products, pieces of dried skin, dead animals, and plants. In another study, adult female German cockroaches were studied for food and water consumption in relation to their reproductive cycle. The researchers recorded their daily consumption and then related it to the events in their reproduction cycle. The results showed that the peaks of feeding and drinking are usually in the egg maturation phase but feeding stopped quickly when the egg case (ootheca) appears. In other words, female German cockroaches fed significantly less when carrying an ootheca (Cochran, 1983) which may decrease their susceptibility to insecticidal baits.

Temperature and moisture requirements

Since insects are cold-blooded organisms, they hardly survive in extreme temperatures-cold or hot. Every species has its optimal temperature and humidity conditions in which they thrive the most. Despite the differences between species, the domestic cockroaches are adapted to temperatures we maintain in our homes. They tend to develop and reproduce in temperatures between 45° F (7.2 °C) and 115° F (46 °C). New studies have shown that temperatures around 87 °F (31℃) were optimum for German cockroach development and survival. In the same study, it was found that they could not mate or survive at 40℃. This is probably due to the lack of expression of some genes or malfunction of the expressed proteins at such a high temperature. (Yitian, 2017).

References:

• Cochran, D. G. (1983). Food and water consumption during the reproductive cycle of female German cockroaches. Entomologia experimentalis et applicata, 34(1), 51-57.
• Fardisi, M., Gondhalekar, A. D., Ashbrook, A. R., & Scharf, M. E. (2019). Rapid evolutionary responses to insecticide resistance management interventions by the German cockroach (Blattella germanica L.). Scientific reports, 9(1), 1-10.
• Rosenstreich, D. L., Eggleston, P., Kattan, M., Baker, D., Slavin, R. G., Gergen, P., ... & Malveaux, F. (1997). The role of cockroach allergy and exposure to cockroach allergen in causing morbidity among inner-city children with asthma. New England Journal of Medicine, 336(19), 1356-1363.
• Xu, Y., Chen, S., Yang, Y., & Zhang, W. (2017). Development-temperature relationship and temperature dependent parameters of German cockroach, Blattella germanica L. Arthropods, 6(3), 78.