Glucose aversion in Blattella germanica: the problem and a solution

Insecticide gels for cockroaches first came onto the market in the early 1990s. Until then, cockroach treatments meant fumigating whole rooms or floors overnight or at weekends. The arrival of these gels was a major revolution for the sector: because no safety interval was needed, they made the operator's job easier, reduced the risk of poisoning for applicator and client alike, and were more environmentally sound. In 1993 Jules Silverman and Donald N. Bieman published the first paper describing what, over the years, would become a serious problem for the control of one of the most widespread pests in the world, Blattella

germanica. Two populations had been found in Gainesville, Florida, that showed aversion to a glucose-rich diet (Silverman and Bieman, 1993). Silverman and Bieman had spent years working with sugar baits in that area, and they uncovered something that would gradually become widespread. In response to persistent anthropogenic selection by sugar-based gels,

German cockroach populations developed a strong aversion to glucose (Jensen et al., 2017). Many sugars are phagostimulants for cockroaches (Silverman and Bieman, 1993). In glucose-averse cockroaches, tasting a food containing a glucose concentration above 500 mM registers as bitter rather than sweet, so they move away from the product at once (Silverman and Bieman, 1993). Over the years this reaction was recorded more and more often, and cases in parts of the United States of treatments failing because cockroaches rejected the gel became increasingly common. As a result, most brands selling these products in the USA began marketing new glucose-free formulations, and treatments became effective again. As a manufacturer of pest control products, Mylva receives feedback and comments from customers constantly. In 2007 isolated reports began to arrive from the Canary Islands describing situations very similar to those previously recorded in the USA. For the first few years these were one-off problems, until late 2015 and early 2016 brought a marked increase in reports of Blattella germanica populations that might be averse to current gels. The behaviour observed in those populations closely resembles the one described by Silverman and Bieman, so the adaptation behind the aversion is suspected to have reached the Canary Islands too. The adaptation is semi-dominant and autosomal, controlled by a single allele and a major gene (Jensen et al., 2017) located in linkage group VIII of chromosome

9 (Ross and Silverman, 1995). It was at this point that Mylva set out to develop a possible solution. To try to prove or disprove that suspicion, individuals (AC) were collected from places in Santa Cruz de Tenerife where the behaviour had been observed and taken to the laboratory. They were kept for three months in sealed plastic tanks (25 x 20 x 20 cm) under controlled temperature (26 °C ± 1) and humidity (40% ± 10). Dog food was provided as a food source, along with water in a bird drinker with a piece of cotton to stop them drowning. At the end of the study all individuals were euthanised.

TO GLUCOSE

To check whether the cockroaches collected in the Canary Islands really were glucose-averse, two highly attractive diets were prepared: one containing 20% glucose (diet 13.38) and the other the same proportion of other sugars (diet 13.29). When both diets were offered to the cockroaches, a clear difference in behaviour between the two populations emerged. The cockroaches from our own laboratory fed on both diets without difficulty, whereas those from the Canary Islands barely touched the diet containing glucose (Figure 1) (Figure 2). The behaviour observed in the AC individuals was also highly significant, since it matched the one described by Silverman and Bieman in 1993. These results allowed us to confirm the presence of glucose-averse Blattella germanica populations in the Canary Islands.

Even so, by the time all these cases of unsuccessful treatments began to appear, Mylva's gels no longer contained glucose in their formulations. That prompted a new hypothesis: could the sugars present in commercial gels hydrolyse over time and give rise to glucose?

To test that hypothesis the earlier trials were repeated with an aged gel matrix, comparing it with the same glucose-free matrix used in the previous trial. The results were very similar to the earlier ones, both in consumption and in behaviour. The cockroaches approached the aged matrix and moved away within a few seconds; very few stayed to feed (Figures 3 and 4). With the glucose-free gel, by contrast, consumption was very high.

To confirm that this behaviour was due to the presence of glucose in the gel, the glucose content of the aged matrix used in the palatability trials was analysed. The result was 27.91% glucose in a product that contained 0% when first manufactured. Other gels on the market with which the same behaviour had been observed were analysed in the same way. Glucose was detected in all of them, at around 20–30%.

These results may account for the problems seen in the Canary Islands: gels that contain no glucose initially, but which over time end up producing the same effects in averse cockroaches as those that do.

Solution

To obtain a product effective against the averse Canary Island populations, Mylva developed a new food matrix using ingredients incapable of generating glucose over time, called Magnum Optimum. This delivers good palatability for longer and control of populations that had until then proved impossible.

Accelerated ageing trials were carried out with the new food matrix, and it was offered to the cockroaches in the same way as in the earlier trials. The results were highly satisfactory: consumption by the Canary Island cockroaches was good even with the aged gel.

Accelerated-aged Magnum Optimum was also sent for analysis to determine its glucose content. Those analyses returned 0% glucose, which shows that the sugars in this new matrix do not turn into glucose over time.

In light of these results, Magnum Optimum is offered as a solution for cases in which a Blattella germanica population cannot be controlled because of its aversion to current gels.

References

• Silverman, J. and Bieman, D. (1993). Glucose aversion in the German cockroach, Blattella germanica. Journal of Insect Physiology, 39(11), pp. 925-933.
• Jensen, K., Wada-Katsumata, A., Schal, C. and Silverman, J. (2017). Persistence of a sugar-rejecting cockroach genotype under various dietary regimes. Scientific Reports, 7, p. 46361.
• Jensen, K., Ko, A., Schal, C. and Silverman, J. (2016). Insecticide resistance and nutrition interactively shape life-history parameters in German cockroaches. Scientific Reports, 6(1).
• Crissman, J., Booth, W., Santangelo, R., Mukha, D., Vargo, E. and Schal, C. (2010). Population Genetic Structure of the German Cockroach (Blattodea: Blattellidae) in Apartment Buildings. Journal of Medical Entomology, 47(4), pp. 553-564.
• Jensen, K., Schal, C. and Silverman, J. (2015). Adaptive contraction of diet breadth affects sexual maturation and specific nutrient consumption in an extreme generalist omnivore. Journal of Evolutionary Biology, 28(4), pp. 906-916.
• Ross, M. and Silverman, J. (1995). Genetic studies of a behavioral mutant, glucose aversion, in the German cockroach (Dictyoptera: Blattellidae). Journal of Insect Behavior, 8(6), pp. 825-834.
• Devlin, T. (2015). Bioquímica. Barcelona: Reverté.