Potato Grower

32 POTATO GROWER | JUNE 2017 Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In ENTOMOLOGY | By Kay Ledbetter Texas A&M University Neonicotinoids' declining effi cacy vs. psyllids Losing Ground The potato industry may be losing a mainstay in the battle against psyllids, according to a recent Texas A&M AgriLife Research study. Ada Szczepaniec, AgriLife Research entomologist in Amarillo, Texas, said while there may be varying degrees of resistance to neonicotinoid insecticides in populations of psyllids across Texas, her recent study indicates they've lost their punch. "We are able to provide strong evidence that these insecticides no longer suppress populations of psyllids below desirable levels," Szczepaniec said. "However, there may be some biological control measures to help in this ongoing battle against the potato psyllid." The potato psyllid is a tiny insect with sucking, piercing mouthparts that transmits a bacterium, Candidatus Liberibacter solanacearum (Lso), which causes a disease called zebra chip and can cause tremendous losses to producers. Producers have used the neonicotinoid insecticides to protect their solanaceous crops, mostly potatoes, in the past. Szczepaniec said the study, funded by the Texas Department of Agriculture, indicates applications of neonicotinoid insecticides at planting, which are a considerable cost for producers, should be replaced with investments in post-emergence applications of insecticides other than neonicotinoids. Integrating predators in suppression of psyllids could also be considered, especially when incidence of Lso is low and the threat of zebra chip disease is decreased, she says. However, more research is needed before recommendations regarding biological control can be made. Szczepaniec's study included colonies of the potato psyllid from all major potato-producing areas of Texas. These colonies were established and maintained in the greenhouse for at least two generations before experiments examining their resistance to neonicotinoid insecticides began. "Over the course of a year and a half, we completed testing the majority of the populations of psyllids for resistance to imidacloprid and thiamethoxam," says Szczepaniec. Based on the Insecticide Resistance Action Committee definition of resistance—"heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve expected level of control"— potato psyllid populations across Texas are resistant to both neonicotinoid insecticides, says Szczepaniec. Szczepaniec conducted a second study to identify alternative biological controls. Surveys of all arthropods associated with three different potato types—a russet, a red and a chipping variety—were carried out at the AgriLife Research station near Bushland, Texas. "While we have only one year of data from which to draw our conclusions, the survey has revealed trends in predator abundance that have important implications for potato production," Szczepaniec says. "First, potatoes harbor a relatively diverse group of predators, and minute pirate bugs in particular are associated with potatoes throughout the season. This is important because they were identified as key predators of the potato psyllid, and their presence is important to suppression of the psyllid." She says other predators include long-legged flies, which are generalist predators, lady beetles, spiders, parasitoid wasps and syrphid flies. Russet potatoes had the highest prevalence of predators. Second, Szczepaniec says, it is important that minute pirate bugs are strongly correlated with thrips, which were also abundant in potatoes throughout the study. Minute pirate bugs are highly attracted to thrips, which are not key pests of potatoes. Texas A&M AgriLife Research entomologist Ada Szczepaniec discusses her greenhouse study on potato psyllid resistance to neonicotinoid insecticides. Photo by Kay Ledbetter

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