Improved integrated pest management of two cucumber beetle species (Coleoptera: Chrysomelidae) in California melon agroecosystems

The biology of the western spotted cucumber beetle (WSpCB), Diabrotica undecimpunctata undecimpunctata Mannerheim, and western striped cucumber beetle (WStrCB), Acalymma trivittatum Mannerheim, in the California melon agroecosystem was investigated during 2008 and 2009.The first generation of WSpCB developed on alfalfa roots and began migrating into melon fields after the second seasonal alfalfa harvest in May. WSpCB populations in alfalfa have two peaks early in the summer that coincide with the first and second population peaks in melons. Beginning in August WSpCB populations in alfalfa declined while numbers in melons remained high. As the cucurbits senesce in the fall. WSpCB gradually migrate back into the alfalfa where they remain throughout the winter.Both species had predominately male populations during both years with female percentage ranging from 20 to 40%. In 2009, females were dissected and ovarian development was rated on a 1 to 5 scale. WStrCB and WSpCB collected from melons tended to have mostly developing ovaries. WSpCB collected from alfalfa had a much higher proportion of mature ovaries relative to females collected from melons. Cucumber beetles collected from the field were maintained in a colony and observed for the presence of parasitoids. No parasites were recovered from WSpCB, but WStrCB was parasitized very heavily by Celatoria setosa (Coquillett) especially later in the summer.Adults of both species were found to feed on weedy species located in and around melon fields, especially when melons were no longer flowering. Maize was the best host followed by tomatoes and alfalfa which were roughly equivalent. Melons appeared to be a slightly weaker host than tomatoes and alfalfa. Sunflower was a completely incompatible host.Sticky traps and visual plant counts were compared as sampling methods for monitoring cucumber beetle populations. Plant counts were made at the same grower fields where sticky traps were in place to monitor populations. In 2008, beetle numbers were quantified from three five-plant clusters in the area around each sticky trap. In 2009, six two-plant clusters were counted around each trap. The sticky traps and five plant counts had similar levels of precision around 30 to 40% relative variation. The two plant counts had the best precision with relative variations in the 20% range, and had relative net precision values more than twice that of the other sampling methods. Monitoring total cucumber beetle populations was the most precise, followed by WStrCB then WSpCB.Although the sticky trap numbers had significant linear correlations with both individual species and total beetle populations using the five-plant count method, the correlations with the two-plant counts were much more significant. WStrCB visual counts had higher correlation values with sticky traps than did WSpCB indicating that the traps were more sensitive to WStrCB.A comparative insecticide trial was conducted at the UC Davis farm in 2008 and 2009. The primary objective was to evaluate the efficacy of a cucurbitacin-rich gustatory stimulant called CidetrakRTM. The product was combined with 1/10 the registered rate of carbaryl and the full registered rate of spinosad with the goals of reducing the level of toxicant needed to control cucumber beetles with carbaryl and increasing the efficacy of spinosad in controlling cucumber beetles.In 2008, a high rate of carbaryl alone was compared to a much lower rate combined with Cidetrak. Although the Cidetrak treatment killed a significant number of cucumber beetles relative to the control in a field cage experiment, it did not consistently reduce beetle densities in the field and it did not reduce fruit damage relative to the control. In 2009, a low rate of carbaryl alone was compared to an equally low rate of carbaryl with Cidetrak. Both carbaryl treatments provided similar results to the Cidetrak treatment from 2008 indicating there was no sign of improved efficacy with the addition of Cidetrak.In both 2008 and 2009, spinosad was applied alone as a foliar spray and at the same rate combined with Cidetrak. In both years, both of the spinosad treatments failed to significantly reduce cucumber beetle numbers in either field cage experiments or field plots. There was no clear advantage to adding Cidetrak to this spinosad formulation which proved to be ineffective against cucumber beetles in the field. However, the same spinosad formulation was very effective against both cucumber beetle species in a laboratory bioassay indicating that the active ingredient has the potential to kill cucumber beetle adults but field conditions inhibited, in an undetermined manner, the efficacy. (Abstract shortened by UMI.)...