Population dynamics, feeding strategies, and coccinellid predators of the larch woolly adelgid exules on the European larch

The larch woolly adelgid Adelges laricis Vallot (Hemiptera: Adelgidae) infests spruce Picea spp. and larch Larix spp. in the Northern Hemisphere. The present field and laboratory research embraced the hitherto unknown aspects of the biology and ecology of A. laricis on its secondary host, the European larch Larix decidua Mill. in Poland. The study showed that the A. laricis population on L. decidua was represented by exulis sistens females at dwarf stem bases, exulis progrediens mobile 1st instar nymphs (crawlers) on needles, exulis progrediens sessile “woolly” nymphs and “woolly” adults on needles, and winged sexupara females. The maximum abundance of the A. laricis population occurred from the end of April until mid-May, which coincided with the predomination of crawlers in the age structure of the population. The most important limiting weather component was high rainfall, affecting mainly the crawlers. Eight species of Coccinellidae were recorded on adelgid-infested larch trees and Exochomus quadripustulatus (L.) was the dominant species. The main differences in food consumption strategies among adelgid developmental stages were related to the duration of phloem sap ingestion. While the total duration of sap consumption at a given time was similar in all adelgid instars studied, the individual bouts of sap ingestion were much shorter in crawlers than in the sessile forms. The specific probing behavior of the mobile nymphs probably reflects the crawlers’ strategy to evaluate plant sap quality in different phloem vessels. It is likely that such strategy is a behavioral preadaptation for finding abundant food sources for permanent settling before crawlers develop into sessile nymphs and adult females. The feeding strategy of crawlers promotes the rapid dispersal of the population. Therefore, the control of A. laricis on L. decidua should focus on the early stages of the population development, which would most effectively reduce the final damage.