Development of a Filter-Paper Bioassay for Sugarbeet Root Aphid, 1993

A bioassay was developed for first-instar SRA using micro-encapsulated diazinon (Diazinon 2FE; formulation 2340-1). Nymphs were collected by isolating individual, apterous adults in small cages fastened to hydroponic growth pouches (Campbell & Hutchison [1995], Can. Entomol. 127:[in press]). The colony had been in laboratory culture for approximately two years, with supplemental aphids added each year during summer months. Adults were allowed to larviposit for 24 h. The resulting cohort of first instars was assumed to have an average age of 12 h. A series of 6 dilutions was prepared from a stock solution of formulated product and distilled water. Filter paper discs (5.5 cm diam.) were soaked to saturation in each dilution (as well as a distilled water check) and placed in a 6.0- cm petri dish to dry. Approximately 60 nymphs were placed in each dish when the filter paper appeared dry, but was still moist enough to stick to the dish (1 h). Dishes were sealed with parafilm, held at 20°C for 24 h, and then checked for mortality. Dead nymphs and those exhibiting functional mortality (i.e., inability to walk, uncoordinated movement, etc.) were recorded as dead. The bioassay, using a new serial dilution for each, was repeated three times. Data were analyzed using the PROBIT procedure of SAS, which accounted for control mortality prior to fitting the model.