Grape Phylloxera Life Tables: Relevance of Whole Grapevine and Soil
A critical aspect of a pest’s biology is the way plant and environmental factors influence growth of populations. For grape phylloxera, our laboratory work has demonstrated the immense potential this pest has for population growth. However, surveys of actual populations on field vines has shown that such potential is not met and phylloxera population growth varies during the vine’s growing season. On susceptible vines, populations peak midsummer and then for unexplained reasons decline thereafter until a second peak occurs after-harvest. Populations are low in winter because low temperatures prevent insect activity. The purpose of the present research was to do life-table experiments to understand why populations decline midsummer. We set up phylloxera populations on roots while the roots were still attached to field vines and separated the insects from the soil environment by encasing them and then-feeding sites in Petri dishes. We then looked at these test populations at 3-5 day intervals and measured survival, developmental rate to the adult stage, and fecundity over a 45-day assay period. This experiment had a detached root control?that is, roots that were detached from the same test vine were infested and placed in Petri dishes in the ground adjacent to the attached-root Petri dishes. These insect life-table experiments were done three times, once at the vegetative, post-veraison, and post-harvest periods. The most striking result confirmed previous findings that attached roots have about a 2- to 8-fold inferior performance in comparison with detached roots from the same vine held under identical conditions. Since the soil was not in contact with the insects during this experiment, this finding was not due to a direct soil effect; the effect was either due to vine physiology or a soil factor mediated by vine physiology. The second striking finding was that the ability of the vine to support phylloxera changes markedly during the growing season and the major cause of the change is a factor causing phylloxera mortality prior to the insect’s 3rd instar. This is the time the insect is inducing the plant to form the gall. So, the cause of this mortality may be a toxin or a physiological factor inhibiting gall formation. With regard to nutritional factors, our data are not yet complete?a portion of the analyses being done by the DANR analytical laboratory are yet to be received. But so far the data suggest that involvement of nutritional factors is limited. In addition, greenhouse experiments in progress with rootstock cultivars will discover whether similar patterns are seen in plants with strong phylloxera resistance. These results will be available for the final report. Based on these results we are proposing new work to determine the relative role of gall formation, toxins and nutrition in the natural field mortality. In composite these findings are important for understanding vine susceptibility and resistance. Farmers will use them for managing infested vineyards prior to replanting and plant breeders will use them for producing better adapted rootstocks.