Factors Influencing the Susceptibility of the Plum Curculio to Lead Arsenate1

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Abstract
Susceptibility to lead arsenate was highest among plum curculio adults of the overwintering brood. Some died without feeding, apparently due to dehydration resulting from the repellent action of the residue. Susceptibility was highest early in the season, decreasing thereafter for several weeks and rising again late in the season. The early susceptibility is believed to be associated with low vigor following emergence from hibernation and the increased susceptibility later in the season with senility. The response of single-(northern strain) and multi-brooded-(southern strain) summer adults was compared. The single-brooded females had the highest LD50 values and caused more feeding injury. Relationship between rate and extent of feeding and mortality was considered. Effectiveness of lead arsenate was highest where test insects had access to surface moisture. Number of insects feeding and extent of feeding were greater on dry residue. Oviposition was likewise less on wet residue, being roughly proportionate to feeding. These differences were apparently due to the ingestion of toxicant taken with surface moisture. The effect of spray residue on behavior of the insects was considered. It is believed that the factors considered here account for some seasonal variations in the effectiveness of lead arsenate under field conditions.

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