Introduction
There are few structural pests as widely known as the cockroach, and while larger species are often the focus of popular media, no species of cockroach is more pervasive and problematic than the German cockroach (Blattella germanica). This species has a long history with humans and has evolved to exclusively infest and reside within built structures – impacting the indoor environment and placing those living alongside active infestations at risk. Pest management professionals have to make use of complex training, knowledge of insect behavior, and often multi-chemical management strategies to eliminate German cockroaches. In this publication, we will discuss our long history of connection to these pests, detailed information about their biology and behavior, as well as some of the characteristics that make these cockroaches particularly difficult to manage. Read on to learn more about the German cockroach and to equip yourself with the knowledge and know-how to identify and understand this tenacious pest.
History & Human Impact
As mentioned above, humans have a long and complex history of relationship with the German cockroach. The first known historical scientific record of the insect was roughly 250 years ago from central Europe – which is where their name came from. However, recent research that compared cockroach genetics over time (basically a massive family tree) found that the “German” cockroach actually arose out of Asia and spread along routes of human migration and settlement roughly 2,100 years ago! Their spread only quickened following the invention of mass- and intercontinental travel. Fast forward in time to now, where many societies live in climate-controlled environments, fast food and restaurants are abundant, and limited space leads to multi-unit housing – the German cockroach continues to thrive. This ideal manufactured ecosystem may be paradise to them, but it presents challenges for pest management professionals and leads to shocking health risks and negative outcomes among individuals living and working alongside active and heavy infestations (Figure 1).
At first glance most people may say that the worst part about having German cockroaches is that they are gross – and while this is true, there is a much more serious consequence of persistent infestation. German cockroaches produce an abundance of allergens, and when populations explode, the allergens quickly follow. A vast array of allergens are associated with infestation, with the largest culprits being two proteins known as BlaG 1 and BlaG 2. These two, and all other allergens can be found in German cockroach saliva, feces, eggs, and shed exoskeletons or cuticles (Figure 2). In fact, a single female German cockroach can produce up to 1,000 units of the major allergen BlaG1 each day, and while we may not know the exact amount that becomes airborne, this is an incredibly high amount of potential allergen being deposited into the environment. To frame it differently, imagine living in a house where pollen is constantly being introduced – into the air, your kitchen cabinets, the floors, countertops, and even in the pantry. This is the reality for homes with heavy German cockroach infestation. All hope is not lost, though! It has been shown that eliminating the infestation greatly reduces the amount of allergens present in the home – leading to healthier homes and reductions in asthma. Given the risk that large populations pose, let’s take a look at how they reproduce and how quickly an infestation can occur.
Figure 1. A hot water heater covered in German cockroach fecal spots - suggestive of a very high level of infestation.
Figure 2. Dead German cockroaches, feces, and exoskeletons spread across a countertop and stove in a heavily infested apartment.
Lifecycle & Reproduction
The German cockroach is not commonly found in the wild and occurs only near human dwellings. Though nocturnal, they occasionally appear by day if the population is too crowded. Populations rapidly increase due to their preference for organizing around aggregation sites in close proximity to food and water and due to their ability to eat the wide variety of foods that we consume.
From Egg to Adult
The German cockroach is a hemimetabolous insect (Figure 3). They go through incomplete metamorphosis, where babies look like small, wingless adults. This type of metamorphosis has three distinct phases: the egg, the nymph, and the adult. These groups go through gradual changes, and there is no pupal stage. The nymph often has a thin exoskeleton and lacks functional reproductive organs.
German cockroaches thrive in the same type of environments that we do. They thrive in slightly warmer temperatures of around 25-30 ℃ (77-86 °F). It can take anywhere from 70-100 days for a single German cockroach to reach adulthood. This is broken into a 20-30 days oothecal incubation period, 40-60 days of nymphal development, and a 6-10 days mating and egg production period – completing the cycle and increasing the infestation. Let’s take a closer look at each stage!
1. Egg stage: The eggs are enclosed and protected in an egg case (ootheca, 8mm long, 3mm high, and 2mm wide). The mean number of eggs per case is 40, where a single gravid female can find an infestation. It is held by the female at the tip of her abdomen until just before hatching – or, at times, even during hatching (Figure 4 & Figure 5). Because the female carries the ootheca throughout incubation, she is able to select and maintain ideal incubation conditions for her developing offspring by moving around the structure.
2. Nymphal stage: Upon emergence from their egg case, German cockroaches are referred to as “nymphs.” This species typically goes through five or six nymphal stages – also called “instars” (Figure 6). Although instars may have different behaviors, food preferences, and tolerances to environmental conditions, overall, all stages require protein for their development and show the same food preference. They steadily increase in size as they grow, where the first instar is about 3 mm, 2nd is 4.5 mm, 3rd is 5.6 mm, 4th is 7 mm, 5th is 9 mm, and 6th is 12.5 mm. It takes roughly 40 days for a newly emerged German cockroach to reach adulthood. This development is strongly affected by social interaction, as physical contact with others accelerates nymphal development.
3. Adult stage and reproduction: Adult German cockroaches are 13-16 mm long, dark brown to tan in color, and have wings. They are commonly identified by the two dark parallel stripes on the back of their head – this region is called the pronotum (Figure 7). Males generally have a lighter body color than females and are thinner with their abdomen narrowing towards the end. Females are slightly wider and thicker than males. Their abdomen is broadly curved at the end and does not narrow. Upon reaching adulthood, it takes a female 4-6 days to become reproductively active. During this window, they require carbohydrates (sugars) for development of unfertilized eggs (oocytes) before mating, which means they eat a lot more, and, as a result, can be more susceptible to baiting as a form of management during this period. Adult female German cockroaches can survive anywhere from 140-280 days, depending on their environment and resource availability.
Males take 6-10 days to become reproductively competent after reaching adulthood. Once active, mating behaviors are controlled and initiated through the release of odors (pheromones), which are the male’s attempt to encourage the female to mate with him. Females generally only mate once before their first ootheca, then produce 5-8 oothecae (egg cases), a total of 200-250 eggs in their life; so the stakes are high when trying to impress each individual female (Figure 8). Mating of males generally occurs multiple times with multiple females, as the males only live between 90-140 days. In the case of many cockroaches, males entice potential females through the use of a nuptial gift – much like planning a romantic dinner or proposing with a ring. In the case of the German cockroach, males produce a buffet on their backs, a special food source in the “tergal gland” and hidden beneath their wings until needed. The female smells and tastes this food during courting, and it often helps hold her attention while the male moves his abdomen into position for mating. This process can be influenced through over-exposure to sugar-containing baits and thereby can confound German cockroach management. Read more about baiting for German cockroaches and their control in our publication German Cockroach Surveillance and Management!
Population Growth and the Importance of Aggregation
As with individual growth and development, the growth of the total population strongly depends upon temperature. This is why they are typically only found indoors, as the outdoor environment is challenging and variable, the indoor environment is climate controlled and safe. Populations of German cockroaches are able to relocate within their environments in an effort to seek out ideal temperatures and other conditions. Within aggregation sites, this species has overlapping generations and generally 1:1 sex ratio of all stages – so roughly 50/50 male/female across the entire infestation. The system of aggregation is helpful, just as it is with humans, where close proximity promotes socialization (mating), safety (avoiding predators and dangers), and consistency (temperature regulation & water conservation). Physical contact with other members of the aggregation accelerates nymphal development and reproductive maturity. Proximity of individuals and their excrement may also aid in the practice of coprophagy (see below), which has been shown to benefit younger nymphs. German cockroach aggregation behavior is largely controlled and influenced by fecal compounds resulting in attraction and arrestment in all life stages. There are more than 40 chemical cues that serve as flashing lights to German cockroaches, saying “Vacancy, find us here!”
Figure 3. Lifecycle of the German cockroach - they are hemimetabolous, or undergo incomplete metamorphosis. From the time they emerge from their egg case (ootheca) they look like tiny wingless adults, getting larger with each molt. They have three distinct stages: the egg (pink), nymph (green), and the adult stage (blue). Copyright Wada-Katsumata 2024
Figure 4. A gravid female German cockroach carrying her ootheca.
Figure 5. A female German cockroach carrying her ootheca as first instar nymphs emerge.
Figure 6. German cockroach nymphs.
Figure 7. An adult male German cockroach with the clear parallel dark bands easily visible on the back of it's pronotum.
Figure 8. A mating pair of German cockroaches - female (left) and male (right), takes 60-90 min.
Dynamics of Infestation
Given the history of cohabitation with humans, the health risks of infestation, and the complex nature of their mating behaviors and reproduction, it is important to discuss certain German cockroach behaviors which can complicate management. These behaviors can contribute to the introduction of German cockroaches into a structure and their persistence despite treatment, but at times have also been exploited to achieve more complete control.
Hitchhiking & Multi-Unit Housing
Often, the first question on someone’s mind when German cockroaches first appear is, “How did these end up here?” This is often followed by statements like: “We aren’t 'dirty,' we clean up all the time,” or “We have a pest management service; why aren’t they doing their jobs?” The reality is that even with the most stringent cleaning regimen and despite regular pest management, German cockroaches can still find their way into your structure. Since these cockroaches are not free-living in the environment, they arrive in one of two ways: through hitchhiking or transfer between units in multiunit housing. Let’s discuss these two routes of introduction in more detail.
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Hitchhiking: This term refers to the introduction of any number of pests into a previously uninfested structure through contact with or through acquiring previously infested material or products. In the case of German cockroaches, this can occur via pieces of furniture, grocery products (both commercial and from places like farmers markets), or even from shipments of goods. These introductions often occur unknowingly, and even with the most stringent inspection small nymphs can easily be overlooked (Figure 3). In these instances, it is best practice to avoid placing blame on a likely unaware party and instead work quickly to address the issue and prevent infestation. If items are easily replaceable, it is often easier to discard them than attempt to remove the cockroaches. In some cases, it is possible to disassemble items, treat respective components, and reassemble the item after thoroughly cleaning and removing insecticide residues. When addressing a sudden introduction of German cockroaches, you should always consult a pest management professional, as inadequate management can further complicate the situation.
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Transfer Between Units: This refers to situations where shared walls between residential or commercial units become doorways for German cockroaches into new and unexplored territories. This can occur as a result of decreases in available resources, overpopulation, and crowding, or even exploratory behaviors. In these cases it is often difficult to “eliminate” the infestation without surveying and treating multiple units, as the insects can simply reinfest areas where they have been previously controlled. In these situations, it is essential to avoid blame, as identifying the specific unit of origin is likely impossible. Instead, contact a pest management professional to discuss options for control and elimination.
Foraging
The term ”foraging” deals with the food and water preference of individuals and populations, and how these preferences shape stage-specific foraging behavior. The German cockroach is a generalist omnivore and has evolved a suite of internal safety measures to ward of foreign chemicals and pathogens. This even includes a remarkable ability to resist starvation. Flexibility in their foraging and food choice is supported by their excellent olfactory (scent) and gustatory (taste) systems, as well as their ability to learn and remember (Figure 9). They obtain a nutritionally balanced diet from various foods in the household, are resilient to nutritional imbalance, and have a capacity to rebalance overall nutrient intake and minimize the impacts of previously poor nutrition when new and rich foods appear. All nymphal stages, especially early nymphal stages, show high food consumption. Adult males show constant daily food consumption, while adult females consumer massive amounts of food while developing eggs (5x that of adult males), but rapidly reduce food intake after mating. Females again increase food consumption a day or two before egg hatch, in preparation for the next reproductive cycle. Foraging activity is further influenced by various environmental factors, such as population density, as crowded conditions increase competition for limited resources.
Understanding foraging patterns and behaviors is critical for effective development of traps and baits, but also plays a major role in the transmission of bacteria by foraging German cockroaches. Quality of diets and foraging patterns play an important and direct role in the acquisition of the gut community and its structure. Further, we know that German cockroaches are able to mechanically transmit diseases picked up in their environments, including Salmonella. This is in large part due to their omnivorous nature, leading to foraging for and sampling of a wide variety of organic food. These range from breadcrumbs, plant material, and dead insects, to decomposing material from trash cans. The world is their buffet! All these foods carry a suite of microorganisms that are ingested and have the potential to proliferate in the gut, forming an important route for acquisition of microbes. In addition to food consumption, water intake is a possible means of entry of microbes into the cockroach gut, as it could be contaminated in a similar fashion.
Coprophagy
Coprophagy is the consumption of fecal material, and is well-documented across detritivores and social insects. While all the developmental stages of the German cockroach may feed on feces, it is apparent that aggregation behavior and coprophagy are most prevalent in the early instars. Small nymphs have lower mobility than larger nymphs and adults, and they tend to stay at aggregation sites. In that situation, fecal pellets from other cockroaches in the aggregation (conspecifics) are concentrated nutrient sources containing all they need to develop and succeed (Figure 10). Additionally, feces contain components of the gut microflora, such as protozoan cysts, bacterial cells, and spores. Thus, aggregation with groups of conspecifics accelerates nymphal development and reproductive maturation while serving as an important mechanism of acquiring their initial gut microbiome. Therefore, fecal aggregation pheromones not only indicate a suitable harborage and potential food source to early instars, but likely also support their health and the future expansion of the infestation.
Necrophagy
Alongside their sometimes challenging aggregation behaviors, and strange enjoyment of eating poop, they also are expert recyclers and regularly consume other German cockroaches who have died. This most often occurs within aggregations, and is an especially relied upon food source for those life stages that tend to spend less time foraging and more time hidden away within the aggregation. Insects, in general, live life on the margin, and must make use of the resources available to them – even if it was your sibling or cousin. As mentioned previously, this is one behavior we are able to exploit for infestation management through the use of secondary-, tertiary-, and even, at times, quaternary kill. In other words, the first cockroach eats an insecticidal bait and returns to the aggregation, where it dies; this is our primary kill. Within the aggregation, other cockroaches feed on that dead and insecticide-filled cockroach. These now die, although at a slightly lower rate, which gives us our secondary kill. This domino effect continues until the concentration of insecticide is no longer lethal, or until the cockroaches are eliminated – pretty cool, huh? This is the definition of using your enemy’s strategies against them and is one of the strongest selling points of the reliance on baiting for German cockroach management.
In summary, the German cockroach is a persistent pest with an incredibly long and intimate history with human migration and expansion, and left unchecked, their infestations pose immense health risks. They have unique aspects of their biology (glucose aversion, ootheca carrying) and behavior (aggregation, necrophagy) that can complicate management programs, but have also been leveraged to achieve higher levels of control. Regardless of the method of introduction, and given the aspects discussed here, it is essential to involve and rely upon professional pest management services for German cockroach control as soon as this pest is identified. Check out our publication German Cockroach Surveillance and Management for more information on management tactics, conducive conditions, and the challenge of insecticide resistance.
Figure 9. Male German cockroaches eating peanut butter in the kitchen. Carbohydrates, proteins, and Lipids are important nutrients for development and reproduction.
Figure 10. “Coprophagy” Nymphs (especially younger nymphs) tend to stay at aggregation sites and eat conspecific feces to obtain fecal nutrients and gut bacteria metabolites.
Publication date: Jan. 23, 2025
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