Reproductive Capacity of Grape Root Borer, Vitacea polistiformis (Harris), and Implications for Pheromone Based Management

Abstract

The increasing incidence of insect resistance to insecticides, and proposed governmental restrictions of pesticides on food crops have led many fruit growers to consider the use of synthetic pheromones as a method of managing lepidopteran pests. Synthetic pheromones used in mating disruption disrupt olfactory sexual communications and are believed to result in either delayed or greatly reduced numbers of matings for a proportion of females in the population. In an effort to improve the efficiency of pest management programs utilizing synthetic pheromone for V. polistiformis on grapes, experiments were conducted to examine four components of this insect's biology and behavior that may be influenced as a result of the use of synthetic pheromone. The components studied were the incidence of polygyny, the effect of polygyny and delayed female mating on reproductive capacity, the evaluation of pheromone trap placement on the capture of males in a vineyard, and oviposition behaviors of gravid females in an in-field wind tunnel.

Polygyny was observed in V. polistiformis. Males were able to mate at least one time daily for six consecutive days. Male mating history significantly affected the duration of mating. The number of matings per male had a significant effect on female fertility but not on fecundity or reproductive capacity.

Virgin female V. polistiformis were mated on day of eclosion or delayed to 3, 5, or 7 days after eclosion. Fecundity, fertility and reproductive capacity of females mated on day of eclosion or 3 days after eclosion were significantly greater than females mated 5 or 7 days after eclosion. Average fertility and reproductive capacity of females mating 7 days after eclosion were significantly lower than all other treatments. Delaying mating by 7 d has the potential to reduce population densities over five generations.

Male capture in pheromone traps is known to be influenced by numerous factors. The placement of pheromone traps in vineyards and wind direction were examined to determine their influence on capture frequency. The frequencies of daily per trap capture were compared among traps located in an inner and outer array, and upwind, downwind, and crosswind sectors of the vector-average wind direction in a vineyard. Trap captures were higher with traps located in an inner array compared with the array that was located near the outer edge of the vineyard. Male capture was greater in downwind traps compared with upwind or crosswind traps.

Limited information is available on cues that influence the ovipositional behavior of V. polistiformis. Simultaneous choice trials were conducted in a wind tunnel to determine if gravid females discriminate between a grape and non-host plant of similar appearance, and oviposit on grape with greater frequency. Most females oviposited indiscriminately near the release point, or remained motionless. Indiscriminate maternal choice of oviposition sites may be related to the random distribution of wild grape vines, the insect's primary host, in wooded areas. Random oviposition may reduce high larval populations at any single site, and hence the probability of resource depletion and cannibalism.