Effects of Ddt and Fenitrothion on Field-Collected Larvae of a Eucalypt-Defoliating Beetle, Paropsis Atomaria Ol. I. Mortality, Relative Toxicity, Development of Treated Larvae, and Effect of the Primary Parasitoids.

Abstract

Insecticide concentrations within the range 0.025-0.1% (DDT) or 0.002-0.02% (fenitrothion) were applied to populations of third- and fourth- instar larvae of Paropsis atomaria Ol. collected from the field. The susceptibility of larvae varied with insecticide and its concentration, larval age, generation, and with the level of parasitization in the batches of larvae tested. Fenitrothion was 10-14 times more toxic than DDT. Third-instar larvae were more susceptible than fourth instars to both insecticides. LC*50 for DDT at 48 h was about 0.02% w:v for third-instar, and 0.06% w:v for fourth-instar larvae; for fenitrothion it was about 0.004% w:v for both instars. For both insecticides, more than half of the batches of larvae showed significant heterogeneity of response. This was caused mainly by parasitization of some of the larvae, and in large part depended on which species of parasitoid was present. LC*50 for DDT applied to fourth instars was 0.066% for non-parasitized, 0.047% for parasitized larvae: 0.043% for those parasitized by the braconid Eadya paropsldis Huddleston & Short, and 0.06% for those parasitized by the tachinid flies. For fenitrothion, LC*50 for fourth-instar larvae was 0.0045% for non-parasitized, 0.0033% for total parasitized, 0.0031% for those containing Eadya, and 0.0038% for those containing tachinids. Non-parasitized prepupae and the adults from larvae which survived insecticide treatments showed a general decrease in body weight with increase in concentration: prepupal weight decreased further when parasitoids were present. Larval and pupal developmental periods were not significantly altered by treatments. The insecticides applied to the larvae stimulated egg production in the adults and speeded up embryogenesis, but did not affect the reproductive system and fecundity.