German cockroaches (Blattella germanica) are one of the most common and resilient pests in urban environments. An ideal mechanical vector of pathogens, source of food contamination and cause of allergic reactions in humans, the German cockroach is an important public health pest. Although many pest management professionals consider German cockroaches easy to manage, their presence is still ubiquitous in human-built environments, both urban and rural. Their high reproductive potential, short life cycle, small size, cryptic habits and ability to develop insecticide resistance (both behaviorally and physiologically) are likely reasons for their persistence.
However, ineffective pest control practices are also cited as an issue in German cockroach management. Thus, the purpose of this article is to highlight some of the new research and technological advances from the past year (2020) that may assist in both current and future decision-making when it comes to German cockroach management. Additionally, we highlight the important role that pest management professionals play in public health and wellness through German cockroach abatement based on recent findings.
INSECT GROWTH REGULATORS
Insect growth regulators (IGRs) have been utilized in structural pest management for decades and impact insect development and reproduction. They are beneficial because they are typically target-specific and utilize a different mode of action than neurotoxic (poisonous to the nervous system) insecticides.
Novaluron is an IGR that is utilized for the management of other urban pests such as mosquitoes, termites and stored product beetles. Novaluron prevents insects from synthesizing chitin, one of the main components of their exoskeleton. Its relevance for cockroach management was recently investigated by Hamilton et al. 2020. Choice tests show that roaches, of all life stages, will readily feed on baits that contain novaluron. Feeding on concentrations as low as 0.1 percent was enough to disrupt molting in nymphs and prevent the development of viable oothecae (egg cases) in adult females (this active ingredient is not an adulticide, however). The authors noted acceptable efficacy through both topical applications and ingestion, but emphasized the relevance of novaluron as a gel bait ingredient. Ultimately, this active ingredient, in combination with adulticides, is a great candidate for cockroach population management and an alternative active ingredient to incorporate into insecticide resistance management (IRM) programs.
When we consider repellents for German cockroaches, DEET has received very little attention and some studies show that DEET is not very effective against this species (Huang et al. 2020). Nonetheless, researchers tested DEET for repellency against small, medium, and large nymphs as well as adult male and female German cockroaches (Reynoso et al. 2020). Small nymphs (likely first and second instars) were the most sensitive to DEET and avoided areas treated with the compound. Large nymphs and adults also avoided DEET, but to a lesser extent than small nymphs. Medium-sized nymphs exposed to DEET displayed the same behavior as nymphs in the control group (i.e., no repellent effect). Although the role of DEET in German cockroach management is marginal at this time, this study emphasizes the importance of understanding the behavioral effects of a repellent against all life stages of a pest, as they may differ dramatically.
Based on the number of emerging publications, it seems that interest in using essential oils (EOs) as cockroach repellents has increased in recent years and there is evidence that they may be effective. Huang et al. (2020) showed that EOs from Ilex chinensis (Oriental holly), Lavandula spp. (lavender) and Elsholtzia ciliata (Vietnamese balm) are effective repellents against German cockroaches. Additionally, they determined that combinations of essential oils may have synergistic effects, as combinations of these oils decreased the amount required to induce repellency.
Interestingly, using gamma-cyclodextrin (commonly used for pharmaceutical drug delivery), researchers were able to create a sustained release agent with essential oils, and some formulations had repellent activity up to 10 hours. Loading EOs onto cyclodextrins could be an important development in our ability to formulate sustained-release essential oil formulations since rapid degradation of EO constituents tends to limit their efficacy.
Liquid pesticides are still the primary method for controlling German cockroaches in spite of the fact that effective alternatives, such as gel baits, exist. Researchers also are exploring alternative control methods as insecticide resistance of German cockroaches to chemical products becomes more pervasive.
Specifically, a recent review (Pan and Zhang 2020) focused on the potential of insect-killing bacteria, fungi and viruses and their role in German cockroach management. For instance, authors noted that some Bacillus species (bacteria) are capable of causing cockroach mortality. One species, Bacillus sphaericus, is successfully used in mosquito control, prompting the authors to state that there may be relevance of this pathogen for cockroach management. Multiple fungal pathogens show efficacy against German cockroaches in laboratory experiments (Hernandez-Ramirez et al. 2020), but fungal insecticides often show limited field effectiveness due to their long action time, unstable virulence and sensitivity to the environment, as well as varying effects on the target organism depending on its sex and life stage.
For instance, in the Hernandez-Ramirez et al. (2020) study, Beauvaria bassiana was most effective against adults and first instars. Combining fungal pathogens with other compounds may increase efficacy, though. Metarhizium anisopliae can work synergistically with hydramethylnon (Chao et al 2020). This combination was tested both orally and topically and topical applications are seemingly more effective. The authors speculate that hydramethylnon promotes the survival of Metarhizium anisopliae by altering the German cockroach gut microbiome. Fungal pathogens paired with conventional active ingredients (AIs) could be used to control German cockroach populations while mitigating insecticide resistance. Finally, densoviruses (DNVs) are highly effective against German cockroaches and have been integrated into some cockroach baits. However, better understanding of their field efficacy as well as the potential of cross-infection of non-target organisms likely will need further investigation before widespread field deployment can occur.
Although much research has been aimed at the biological control of cockroaches, ultimately many researchers state that more in-depth research is needed for any of these technologies to become feasible or mainstream in German cockroach control. Larger-scale field efficacy studies are needed as well as products that can compete with chemical insecticides in both efficacy and cost-effectiveness. As of now, biopesticides tend to fall short based on these criteria. However, further development and research in this emerging field will hopefully lead to tenable conventional insecticide alternatives in the future.
Because German cockroaches are resistant to a number of insecticidal compounds, the use of baits has become exceptionally important for effective population management. This has led to investigation and scrutiny of bait efficacy. Researchers recently evaluated abamectin, dinotefuran, imidacloprid, and pyriproxyfen + abamectin in a head-to-head efficacy study. While roaches were attracted to all baits assessed in the experiment, baits containing abamectin B1 and pyriproxyfen + abamectin B1 were eaten more readily by roaches. However, greater palatability does not necessarily lead to improved efficacy — baits containing dinotefuran and imidacloprid resulted in faster mortality (~2 days to 100 percent mortality) than the other baits (~7 days to 100 percent mortality) (Salehzadeh et al 2020).
Researchers also are evaluating the efficacy of gel baits in field settings. Unfortunately, we know that many German cockroach infestations in the United States are concentrated in affordable multi-unit housing, which is where Miller and Smith (2020) focused efforts on a long-term study conducted in North Carolina and Virginia. Utilizing an assessment-based pest management (APM) strategy (using the number of cockroaches present to determine the amount of bait to apply), Miller and Smith (2020) found that PMPs can reduce populations of cockroaches by more than 90 percent using gel baits alone. Reinforcing findings in previous studies (not discussed here), the treatment protocol used in this study did not require residents to clean or change their surroundings prior to treatment. Importantly, the authors mentioned the need for rotation of baits at least every three months to avoid development of pesticide resistance and bait aversion.
Avoiding the evolution of insecticide resistance is crucial for maintaining the efficacy of gel baits. Hu et al. (2020) showed that high levels of deltamethrin resistance in some strains of cockroaches could negatively affect the performance of indoxacarb, fipronil and imidacloprid gel baits. Losing the efficacy of both pyrethroid liquid sprays and gel baits would be devastating for cockroach management, emphasizing the importance of product rotation for IRM in this pest.
ESSENTIAL OILS/BORIC ACID
As previously stated, the overuse of synthetic pyrethroids and subsequent development of resistance in German cockroaches has led to interest in alternative lethal active ingredients, such as plant-derived essential oils (EOs). Multiple EOs and their constituents have both lethal and sublethal effects against cockroaches. However, the low water solubility and high volatility of EOs affects their efficacy against target pests and their persistence in the environment, limiting their field relevance. A recent study set out to test the effectiveness of some EOs (peppermint, palmarosa, geranium, lavender and rosemary) against cockroaches as well as the ability of a polymeric nanoparticle to extend the longevity of EO compounds (Yeguerman et al. 2020). The study found that for controlling German roach populations in the home, peppermint and palmarosa oils were the most effective. Additionally, polymeric nanoparticles can increase lethal activity of EOs and enhance repellency. This mirrors findings of the aforementioned Huang et al. 2020 study and notes the potential importance of developing acceptable carriers for EO compounds to increase their lethal activity against cockroaches, as well as extend their longevity in the environment.
Boric acid is often a preferred alternative active ingredient for powders, pellets or blocks used against German cockroaches as it is low cost and weakly toxic to mammals at low concentrations. Additionally, boric acid can be used to manage insecticide-resistant roaches due to a unique mode of action that has yet to be fully elucidated.
Although multiple boric acid product forms exist, few gel baits containing boric acid have been developed to date. Neupane and Kafle (2020) developed and tested a 40 percent boric acid gel bait, which did require more time to kill German cockroaches compared to commercial baits with ingredients such as fipronil.
However, boric acid gel baits performed equally to commercial boric acid baits in terms of detection time, amount of bait consumed and overall mortality. Even roaches that did not consume laboratory boric acid gel bait were killed through cannibalism of roaches that did feed on boric acid baits. In fact, boric acid gel baits provided longer-term secondary mortality than commercial fipronil baits. Although field efficacy trials of new formulas will be necessary to confirm relevance to German cockroach management programs, this low-toxicity bait may be another effective tool in the toolbox in the years to come. The persistence of an existing boric acid gel bait in the market (Nisus Magnetic Roach Bait), developed in 1989, indicates that new formulas may also be field relevant and effective.
Decades of research has revealed that cockroaches, especially German cockroaches, are one of the most common sources of indoor allergens worldwide (Pomés and Schal 2020). Proteins isolated from cockroach excrement, saliva and cast skins have been identified as important allergens that can cause allergic diseases, such as asthma. Due to the interconnectedness of cockroach allergens, cockroach infestation severity and resulting negative health outcomes, researchers have repeatedly stated that a reduction in allergen levels and cockroach activity will inevitably improve human health. Previous research supports this claim, with some studies showing that even single interventions (bait application alone) significantly reduce the number of cockroaches in the home post-treatment and, as a result, affected children show rapid improvement in asthma symptoms (Rabito et al. 2017). Other studies definitively showed that cockroach abatement reduced the concentration of cockroach allergens in dwellings (Sever et al. 2007).
Previous studies attempting to show reductions in cockroach allergens after cockroach abatement typically measured the concentration of cockroach allergens in open spaces around the home. However, a recent study by Wang et al. (2020) was conducted to determine whether cockroach abatement strategies could reduce cockroach allergen concentration in enclosed areas as well. After treatment by entomologists, cockroach presence was reduced by approximately 99.9 percent in infested dwellings after 12 months. Cockroach allergen levels also decreased significantly after treatment in enclosed areas, but the authors concluded that even after cockroach elimination, it takes a few months for allergen levels to decrease below the asthma morbidity threshold. This study reinforces previous findings that effective cockroach control likely leads to significant health improvement of dwelling occupants as cockroach allergen levels decrease significantly, even in cryptic areas, once populations of cockroaches have been eliminated. PMPs play an important public health role through cockroach management that we should celebrate and revere.
It is clear that researchers are working on alternative products and active ingredients for German cockroach management based on research published in 2020, with a clear focus on alternative compounds and product formulations, such as gel baits and biologicals. This is likely due to the evolution of insecticide resistance, both behaviorally (bait aversion) and physiologically (metabolic and genetic resistance to pyrethroids and other compounds). Gel baits clearly remain an effective way to manage German cockroaches, even at high densities. Bait and product rotation in general is important in managing the evolution of resistance in this pest. Finally, look for and be open to interesting alternative technologies, as preliminary research shows promise for effective novel products in the future.