Mormon cricket control in Utah's west desert - Evaluation of impacts of the pesticide Diflubenzuron on nontarget arthropod communities

Abstract

Grasshopper and Mormon cricket (Orthoptera) populations periodically build to extremely high numbers and can cause significant economic damage in rangelands and agricultural fields of the Great Plains and Intermountain West. A variety of insecticides have been applied to control population outbreaks, with recent efforts directed at minimizing impacts to nontarget fauna in treated ecosystems. A relatively new insecticide for control of Orthoptera is diflubenzuron, which acts to inhibit chitin production, ultimately causing death during the molt following ingestion of the insecticide. All arthropods, including insects, mites, and crustaceans, use chitin to build their exoskeletons and will die if they are unable to produce it during the next molt. Diflubenzuron is not taxon specific—it affects all arthropods that ingest it, except adult insects, which do not molt. Consequently, application of this pesticide has the potential to significantly reduce not only target populations but all terrestrial and aquatic arthropods within treatment zones. Some research has been done in the Great Plains on the impact of diflubenzuron on nontarget arthropods in the context of grasshopper-control programs, but no work has been done in the Great Basin in Mormon cricket-control areas. This study was instigated in anticipation of the need for extensive control of Orthoptera outbreaks in Utah’s west desert during 2005, and it was designed to sample terrestrial and aquatic arthropod communities in both treated and untreated zones. Three areas were sampled: Grouse Creek, Ibapah, and Vernon. High mortality of Mormon cricket eggs in the wet, cool spring of 2005 restricted the need to control Mormon crickets to Grouse Creek. Diflubenzuron was applied (aerial reduced agent-area treatment) in May 2005. Terrestrial and aquatic arthropod communities were sampled before and after application of diflubenzuron in the Grouse Creek area of northwestern Utah in May and June of 2005. In July 2005, U.S. Geological Survey scientists sampled areas in Ibapah and Vernon that had been treated with diflubenzuron in 2004, along with adjacent untreated areas. Pitfall traps at four treated and four untreated sites were used to collect ground-dwelling terrestrial arthropods. Semiquantitative sweep surveys of aquatic habitats were made before treatment, 2 weeks after treatment, and 4 months after treatment (after leaf fall) at Grouse Creek. One-year post-treatment samples were collected by using the same methods for terrestrial and aquatic arthropods at Ibapah and Vernon in July 2005 (treatments applied in June 2004). More than 124,000 terrestrial arthropods were collected from the three study areas, and more than 200,000 aquatic invertebrates were collected in the aquatic samples. Direct effects of diflubenzuron on aquatic and terrestrial arthropod communities were not apparent in our data from Grouse Creek. The treatment was designed to avoid spraying pesticide on water bodies, and no measurable effects on aquatic communities from either springs or streams were observed, with the exception of the reduction of taxa richness at Vernon (a result confounded by elevational differences in the treatment and nontreatment zones). Some trends indicate diflubenzuron may affect some terrestrial taxa. Ant communities showed some differences, with possible lag effects at Ibapah and Vernon. Forelius was more abundant, while Tapinoma and, perhaps, Formica declined in treated zones in these two study areas. Solenopsis also was more numerous at treated Ibapah sites but varied without pattern at Vernon. Scorpions were abundant at Grouse Creek and Ibapah but rare at Vernon. Numbers did not change during several weeks at Grouse Creek, but at Ibapah, numbers at treated sites were much lower than at untreated sites. The Lygaeidae (in the order Hemiptera) were more abundant in the untreated zones at Ibapah and Vernon, although significantly so only at Ibapah. Lygaeidae were absent from the treated zone at Grouse Creek (before and after treatment) but were present after treatment in the untreated zone. Additional research is recommended to determine more explicitly whether these taxa are sensitive to diflubenzuron applications in the Great Basin.