Selenium as a Potential Bait Active Ingredient Against the German Cockroach, Blattella germanica (L.) (Blattodea: Ectobiidae)

The German cockroach, Blattella germanica (L.), is a cosmopolitan indoor public health pest. Given its close association with humans, a German cockroach infestation can cause direct health risks through the production of allergens and the transmission of microbes and antibiotic resistance genes. Control of German cockroaches relies heavily on insecticide use, leading to the development of insecticide resistance. There is a serious need for a practical German cockroach control approach to manage insecticide-resistant populations while minimizing the risk of neurotoxic insecticide treatment. Selenium is a trace element that is found across different ecosystems. It is essential for growth at a narrow range, but beyond that range, selenium becomes toxic. Earlier studies on various insect species have found that selenium compounds exhibit insecticidal effects. It can repel, disrupt development, and kill insect pests. This study investigated selenium delivered as an oral toxicant.I evaluated two inorganic (sodium selenate and selenite) and two organic (selenomethionine and selenocystine) selenium compounds as liquid baits diluted in 3% sucrose against B. germanica in nonchoice bioassays. After the screening process with the UCR susceptible strain, sodium selenate was the most effective compound based on mean survival times. It was chosen as a suitable candidate for further evaluation. To determine lethal dose values, I developed a protocol for a droplet feeding method for an ingested selenium solution to determine lethal dose values. Subsequently, selenate was tested against three field strains (WM, RG386, and Ryan) that have been shown to have broad-spectrum resistance. I also evaluated five commercial bait products (Advion Evolution, Alpine Rotation Reservoir 1, Maxforce FC Magnum, Siege, and Vendetta) against these strains to make comparisons on their efficacy. The choice bioassays revealed that the 0.5% selenate liquid bait is comparable to or more effective than the commercial bait products. Although the field strains had some tolerance towards selenium, the selenate liquid bait achieved 395% mortality by 21 days. To investigate other characteristics of a bait toxicant, a horizontal transfer study was conducted where donor adult male cockroaches delivered a lethal dose of selenate to second and third instars through fresh regurgitate. In addition, video tracking software demonstrated that ‘distance traveled’ and ‘velocity’ after acute exposure significantly affected locomotor behavior over a two-day period. Lastly, I conducted histological procedures and found evidence that chronic sodium selenate exposure may cause alterations to the midgut. Based on the findings of my studies, I conclude that sodium selenate can be a potential bait toxicant against both susceptible and resistant strains of the German cockroach