Monomorium pharaonis insect control using porous adsorbent materials

While insecticides in the past evolved from natural insecticidal powders to synthetic chemical pesticides, the devastating effects of many of the early synthetic chemicals such as the organochloride DDT (Dichlorodiphenyltrichloroethane) were soon revealed and they were consequently banned from use. When we add to this the increasing demand of the current market for increasingly more environmentally friendly products with fewer health risks, we obtain the perfect context to return to some of the older, more natural pesticide products. In order to facilitate this evolution, the efficiency of these products has to be increased to be competitive in the current day pesticide market.

Research into insecticidal dusts dates back to the 1960’s, when researchers theorized, based on the observation that insects died from desiccation when exposed to the powders, that the mode of action was either adsorption or abrasion of the insects’ epicuticular wax layer. The hardness of the material ultimately determined whether it was adsorption or abrasion. Since these are non-targeted modes of action, most research was conducted on stored product pests, which are managed inside enclosed spaces. This research was put on the back burner when the movement towards synthetic chemicals started, leaving many of the more recently developed porous materials untested.

We took the challenge to not only test the insecticidal efficacy of these materials, but also to determine which material properties most strongly influenced the insecticidal effectiveness of the materials. For this purpose we used the pharaoh ant as a test species, since it is a notorious insect, residing inside buildings and hospitals, posing a threat by spreading pathogens and damaging electrical equipment. We devised a lab scale setup where we determined the insecticidal effectiveness of 32 different materials from several material classes. Diatomaceous earth was used as a benchmark. We also determined the material properties to investigate their influence on insecticidal effectiveness. The results showed that activated carbon was the most insecticidal material, with a mean forager survival time of only 25 minutes. Zeolites also performed well, but their shortest mean survival time of 40 minutes only landed them in second place. We also showed that the material properties determining insecticidal effectiveness depended on the type of material used. For zeolites, the most important material property was the BET specific surface area, for the ordered zeolites this proved to be the large mesopore surface area and for the activated carbon materials, the particle size had the greatest effect on insecticidal effectiveness.

We then continued with the assessment of the best delivery method for the activated carbon. For this purpose, the queen mortality was monitored, since this ultimately defines colony survival. A bait formula, mimicked by mixing food with either activated carbon powder or pellets, proved to be the least effective delivery method. While the food was entrained and the powder treatment even resulted in high forager mortality, no queens died as a result of the treatment. Application of activated carbon as a barrier proved to be more effective, with a median survival time of 47 hours, but the practical application seemed limited. A surface treatment, directly targeting the nest, proved to be the best delivery method, resulting in a median survival time of 1.5 hours. However, queens escaping this treatment did not return to the nest, which could result in a relocation of the problem. For this reason we suggest incorporating it in an IPM system.

This left us with one last goal, defining the mode of action of the insecticidal powders. A first step was analyzing the epicuticular wax layer by immersing pharaoh ants in n-hexane. This provided us with the composition of the wax layer as well as the major alkaloids excreted by the abdominal glands. It also resulted in the identification of 4 alkaloids which were not previously found in Monomorium species. The zeolites selectively adsorbed specific compounds from the epicuticular wax layer, leading to an identification of three compounds which were not found in the hexane wash. We discovered both pronounced shape and size selectivity in the zeolites with the smallest pore sizes and inverse shape selectivity in H-BEA-300. These are phenomena that are also of great interest in the fields of refinery processes and catalysis and lead us to the conclusion that pore mouth and key-lock adsorption are also at play in the adsorption mechanism of insecticidal powders. Since one of the most adsorbent zeolites showed very little insecticidal effectivity, we conclude that not the adsorption capacity, but rather adsorption selectivity lies at the basis of insecticidal effectivity.