Differences in Susceptibility of Two Species of Chrysopa (Neuroptera: Chrysopidae) to DDT

Abstract

Since 1919 it has been noticed in a number of peach orchards in the Niagara Peninsula that the larvae of Chrysopa plorabunda Fitch are highly resistant to DDT, whereas those of C. rufilabris Burm. are susceptible. Data obtained in a representative orchard since 1953 are summarized in Table I. Two 60- to 70-tree plots were sprayed with wettable sulphur or captan according to standard commercial schedules. Two adjacent plots were similarly treated with wettable sulphur and also with two applications of 50 per cent wettable DDT powder at 2 pounds per 100 gallons about 10 days apart in early July. Forty trees in each plot were sampled the same days at irregular intervals each season by brushing. This method, described elsewhere (Putman, 1955), is not a very efficient way of collecting chrysopid larvae because they are not easily dislodged from the foliage, but the numbers collected should be proportional to the actual populations. As sampling in this and in three other orchards showed that neither fungicide had any obvious effect on chrysopids, the two fungicide plots were considered as a single non-insecticide plot and the two DDT plots were similarly combined. Each value in the table represents the total number of larvae collected in each pair of plots in three to eight samplings during July and August after the first application of DDT.