Shelters or harborage sites: Their definition and implications for pest management

In structural pest control, we deal with various pests that exhibit different behaviors. Some pests that crawl, such as cockroaches and bed bugs, tend to hide in sheltered locations, either temporarily or permanently. While the terminology used to distinguish different hiding places can be helpful, it can also be confusing. For example, some people incorrectly use the word "nest" to describe a place where cockroaches rest while others use the word "shelter". Alternatively, pest management professionals (PMPs) in the U.S. use the word "harborage." Others may use the word "home" or "resting site," in the case of mosquitoes. Let's look at these terms in more detail.

Figure 1: German cockroaches inside cabinetary. Photo: Chris Pianto

Nest

The term "nest" is usually used for birds and other vertebrates. However, social insects are well-known for their nest-building behavior. While some non-biologists (e.g. Moshinsky and Mesa 1994) may use the word "nest" to refer to cockroaches and bed bugs hiding places, a nest usually refers to a space that is created or modified by an organism. For example, the Encyclopedia of Insects defines a nest as: "A nest may be defined as any modification of the environment by adult insects that provide shelter for the rearing of their offspring." (Jeanne 2009). Bed bugs and cockroach species found in urban areas are not generally known to build, or physically modify their hiding places. While their feeding (e.g., chewing cardboards), defecation, and egg laying behavior can alter the chemical and physical properties of their hiding place over long periods, such changes occur as side effects of their biological activity rather than an active form of environmental modification by these pests.

In contrast, termites, ants, wasps, and bees build extensive and intricate nests in soil or above ground. These nests are actively built by digging into substrate (e.g., soil, wood) and/or by mixing various materials such as saliva, mud, wood, fecal matter, and fibers to create a sticky cement to construct a specific structure (Jeanne 2009).

Harborage

The term "harborage" is usually used by American entomologists and pest management professionals (PMPs) to describe a place where insects hide or live. Although not an official definition, “harborage” is colloquially used to refer to more permanent shelters.

To elaborate, let's look at an example: If you chase a cockroach from the kitchen, it may run under a sofa in a living room; however, the space under that sofa may not be an ideal place for the long-term survival of that cockroach. There is limited food and, especially, limited water there. The cockroach simply ran under the sofa to escape from danger. If it had not been chased, that cockroach probably would never have gone under the sofa. Even if it did naturally wander under the sofa, it would most likely return to its original hiding place in the kitchen. As a result of the chase, the cockroach may spend a few hours under the sofa, but it will eventually return to its "permanent "harborage" in the kitchen, where it has easy access to food, water, and potential mates. Consequently, not every hiding place serves as permanent or long-term harborage. Nevertheless, let's assume that the cockroach population increases exponentially, and after some months, there isn't enough harborage space in the kitchen for cockroaches to coexist peacefully. This may force some of the cockroaches to leave the kitchen and find new hiding places. This is when you might see cockroaches entering unusual locations such as inside smoke alarms, wall corners on ceiling lines, inside cabinetry such as china cabinets, under living room furniture, behind wall pictures, or inside wall voids accessed through electrical outlets. Now, if these migrant cockroaches thrive in those new environments, they will spend more and more time there, and eventually we can call those spaces as permanent "harborages."

One important factor that distinguishes a temporary "hiding place" from a permanent "harborage" is their chemical properties. Cockroaches constantly drop feces in their harborage, and these feces contain various chemicals, including aggregation pheromone(s), which are known to attract other cockroaches. For example, research has shown that German cockroaches prefer entering new harborages with higher concentrations of fecal matter extracts as opposed to control harborages that lack such extracts. Moreover, higher concentrations of fecal matter further increased this preference by attracting additional cockroaches to the harborage (Miller et al. 1996). Similar aggregation chemicals have been identified for the common bed bug (Cimex lectularius) (Olson et al. 2017).

Shelter

The word "shelter" is generally used interchangeably with the word "harborage." However, it is more commonly used by European researchers and pest control experts. If you look at European-published literature on cockroaches, you will see a higher percentage of articles using the word "shelter" instead of "harborage." For example, the following articles on the German cockroach were authored or first-authored by Europeans. Pay attention to the word "shelter":

• Group Living Enhances Individual Resources Discrimination: The Use of Public Information by Cockroaches to Assess Shelter Quality. (Canonge et al. 2011)
  • All authors from: Brussels, Belgium
• Effect of Shelter on reproduction, growth and longevity of the German cockroach, Blattella germanica (Dictyoptera: Blattellidae). (Gemeno Marín et al. 2011)
  • First author from Spain

And the following articles are authored by American entomologists. Pay attention to the word "harborage":

• Harborage width preferences of German cockroach (Dictyoptera: Blattellidae) adults and nymphs. Journal of economic entomology (Koehler et al. 1994)
  • All authors from the U.S.
• Influence of German Cockroach (Dictyoptera: Blattellidae) Fecal Extract on Food Consumption and Harborage Choice. (Miller et al. 1996)
  • All authors from the U.S.
• Harborage limitation and habitat isolation for German cockroach control (Farmer 1982)
  • Author from the U.S.
• Adult German cockroach (Orthoptera: Blattellidae) feeding and drinking behavior as a function of density and Harborage-to-resource distance. (Silverman 1986)
  • All authors from the U.S.

Home

Another layperson term used for pests is the word "home." Although it is not an official technical term in professional pest management, it appears in some publications to help readers understand the concept of pests returning to a central place such as a nest, harborage, or shelter. Using such a term makes it easier for the average reader, especially the general public, to better understand pest behavior and recognize signs of infestations.

Resting site

The term equivalent to "harborage" for mosquitoes is "resting site." These are the areas where mosquitoes hide when not actively feeding, mating, or reproducing. Examples include shady areas in the landscape, areas under eaves, and empty containers. Treating resting sites can kill and repel mosquitoes from an area. However, unlike cockroaches and bed bugs, mosquito resting sites are more difficult to find and study. Also, mosquitoes are not known to show similar level of fidelity to a particular resting site.

Implications for pest control

By treating nests and harborage sites, we can maximize the efficacy of pest control efforts. Because insects tend to aggregate inside harborages, an insecticide directly applied to such areas can quickly reach and kill a large proportion of the population. In addition, insecticide baits and some contact insecticides can be carried by insects and transferred to other individuals in the population. This process is called "horizontal transfer." For horizontal transfer to occur, insects must somehow exchange the insecticide with one another. Such chemical exchange can happen when insects share food (trophallaxis) or when they feed on fecal matter contaminated with insecticides. For contact insecticides, tarsal and antennal contact can facilitate chemical exchange. Horizontal transfer can make an insecticide many times more effective because exposed insects act as carriers and distributors of the insecticide, leading to a more complete distribution of insecticide in the population or colony.

References

• Canonge S, Deneubourg J-L, Sempo G. 2011. Group Living Enhances Individual Resources Discrimination: The Use of Public Information by Cockroaches to Assess Shelter Quality. PLOS ONE. 6(6):e19748. https://doi.org/10.1371/journal.pone.0019748

• Farmer BR. 1982. Harborage limitation and habitat isolation for German cockroach control [PhD Thesis]. Virginia Polytechnic Institute and State University. [accessed 2025 Sept 24]. https://vtechworks.lib.vt.edu/items/17368798-44c6-4309-93eb-779ff09ec311.

• Gemeno Marín C, Williams GM, Schal C. 2011. Effect of shelter on reproduction, growth and longevity of the German cockroach, Blattella germanica (Dictyoptera: Blattellidae). [accessed 2025 Sept 24]. https://repositori.udl.cat/bitstream/10459.1/59218/1/017406.pdf.

• Jeanne RL. 2009. Nest building. In: Encyclopedia of Insects. Elsevier. p. 688–691. [accessed 2025 Sept 24]. https://www.sciencedirect.com/science/article/pii/B9780123741448001880.

• Koehler PG, Strong CA, Patterson RS. 1994. Harborage width preferences of German cockroach (Dictyoptera: Blattellidae) adults and nymphs. Journal of economic entomology. 87(3):699–704.

• Miller DM, Koehler PG, Patterson RS. 1996. Influence of German Cockroach (Dictyoptera: Blattellidae) Fecal Extract on Food Consumption and Harborage Choice. J Econ Entomol. 89(3):668–672. https://doi.org/10.1093/jee/89.3.668

• Moshinsky M, Mesa A del S. 1994. A relativistic cockroach nest. Can. J. Phys. 72(7–8):453–465. https://doi.org/10.1139/p94-061

• Olson JF, Vers LMV, Moon RD, et al. 2017. Two compounds in bed bug feces are sufficient to elicit off-host aggregation by bed bugs, Cimex lectularius. Pest Management Science. 73(1):198–205. https://doi.org/10.1002/ps.4286

• Silverman J. 1986. Adult German cockroach (Orthoptera: Blattellidae) feeding and drinking behavior as a function of density and harborage-to-resource distance. Environmental entomology. 15(1):198–204.