SUSCEPTIBILITY OF APHIS-GOSSYPII (GLOVER) TO INSECTICIDES AS AFFECTEDBY HOST-PLANT USING A RAPID BIOASSAY

The susceptibility of Aphis gossypii (Glover) reared on watermelon or cotton to seven insecticides was determined using a Petri dish bioassa y. Baseline susceptibility values to each insecticide for susceptible laboratory A gossypii colonies varied between host plants, but aphids reared on cotton were generally more tolerant to insecticides than aph ids from watermelon. The ratio of relative susceptibility of cotton ap hids to melon aphids was as much as 1000 with dimethoate or 415 with b ifenthrin, however, no significant differences in susceptibility was o bserved with chlorpyrifos between aphid populations from the two host plants. Orders of toxicity for the seven insecticides varied between h ost plant, but on watermelon, the order of toxicity was bifenthrin > o xydemeton-methyl > methomyl > dicrotophos > dimethoate > chlorpyrifos > endosulfan. Because of the wide range of response to insecticide dos es observed with bifenthrin on melon aphid and with dimethoate and end osulfan against cotton aphid, use of the Petri dish bioassay method as a discriminating-dose field bioassay for these insecticides may not p rovide consistent estimations of the resistant nature of field populat ions. Bioassay data taken at 3 h were generally more consistent and pr ovided a more predictive mortality model than those taken at 2 or 4 h for most insecticides. LC50 values estimated for dimethoate with melon aphids using leaf-spray or leaf residue bioassays differed little fro m LC50 values estimated with the Petri dish bioassay. Because Petri di sh bioassays cost less than half as much as plant-based bioassays, pro vide comparable results, and require less assay time, this method is m ore suitable for use in monitoring for insecticide resistance in melon aphid.