Susceptibility of Adult Microplitis croceipes (Hymenoptera: Braconidae) to Insecticides Used for Heliothis spp. (Lepidoptera: Noctuidae) Control

Abstract

Microplitis croceipes (Cresson) is a larval parasite of both Heliothis zea (Boddie) and H. virescens (F.) (the bollworm complex) (Lewis & Brazzel 1968). In Arkansas, M. croceipes is the predominant parasite of Heliothis spp. particularly in late spring and early summer cotton fields (Burleigh 1975). In Integrated Cotton Pest Management systems the process of insecticide selection often does not consider the impact to beneficial insects. This is partly due to the lack of information about the susceptibility of specific predators and parasites to various insecticides. This study was undertaken to provide information on the susceptibility of M. croceipes to insecticide formulations used in cotton pest management. The same testing procedure was used throughout the study. A 5.5 cm diam filter paper disk was dipped into varying concentrations of insecticide, air dried for ca. 2 h and taped to the side of a 0.27 1 waxed paper cup. A moist cotton ball and a raisin were placed in the cup for food and water. Adult M. croceipes were aspirated into a cup which was covered with a lid. Mortality was recorded at 1, 16, 24 and 48 h. The insecticide concentration was based on recommended rates for Heliothis control in cotton using an application volume of 18.9 1 of water per 0.4 ha (Table 1). Concentrations used were 5, 3, and 2 times field rate, field rate, and dilutions of 1 part field rate to 5, 10, 50, 500, and 5000 parts water. The adult M. croceipes used in testing had not been exposed to Heliothis larvae and were less than 3 days old. No attempt was made to equalize the number of males and females exposed to any particular concentration of insecticide. When mortality occurred, the tests were repeated at insecticide concentrations above and below the concentration that caused mortality. Insecticide susceptibility testing was conducted on adult M. croceipes reared from Heliothis spp. larvae collected from cotton fields during the fall of 1983 and using adults from a laboratory colony during 1984-5. This colony was obtained from the Department of Entomology, University of Arkansas, Fayetteville. A total of 10 insecticides were tested using 391 adults. Insecticide susceptibility was evaluated by determining percent mortality at concentrations more than field rate, field rate, and less than field rate. Data were analyzed using a two way ANOVA, the combined concentration levels and the different insecticides were used as treatments. Only insecticides which caused mortality were included in the analysis. The results of the insecticide susceptibility tests are shown in Fig. 1. No mortality occurred in the controls during testing. The biological control agents, B. thutringiensis