- Fruits: apples, peaches, pears, general tree fruits
- Pest Management: cultural control, field monitoring/scouting
Leafrollers (LR) are bivoltine tortricid (Lepidoptera) pests of pome and stone fruits in Washington. While most apple growers and many cherry growers spray twice per season to control LR populations, surveys of LR parasitism indicate that there is a large parasitoid complex that target LR in unsprayed orchards. There is substantial economic and ecological benefit to be gained by greater reliance on natural enemies for LR management, but limited knowledge of these parasitoids is a major deterrent. In this study we are focusing on the two most abundant parasitoid species affecting LR populations. These are flies from the family Tachinidae. One key issue that has been identified as a major barrier to improved biological control of LR in orchards is the unsuitability of LR as overwintering hosts for these parasitoids. The flies must rely on alternate hosts to overwinter, and this puts constraints on the population growth and limits parasitoid impact on the overwintering LR generation. The goals of this project are 1) to identify the habitats and alternate host species that are important to maintaining the populations of these important parasitoids, and 2) to identify spatio-temporal migration patterns of these parasitoids around orchards. These data will improve understanding of how these parasitoids interact across the landscape and should help identify particular habitats or host species that may be targeted for conservation or enhancement for the purpose of improving LR biological control.
Project objectives from proposal:
Objective 1. Determine the types of habitats and host species that are utilized by the parasitoids in the regional landscape and identify potential overwintering hosts.
Because orchards cannot support overwintering populations of the two tachinid parasitoids of interest, and no other orchard species host the flies, landscape composition is an important determinant of parasitism rates in orchards, and has implications for the stability and resilience of this system. One potential source of parasitoids are woodlands, where these tachinids are known to attack certain forest pest Lepidoptera. Field crops, windbreaks, and riparian zones may also provide suitable hosts for summer parasitism and overwintering. Another study has indicated that the principal natural habitat type in the fruit producing regions in central Washington, shrub steppe, does not support parasitoids important in orchard biocontrol.
To accomplish Objective 1, potential hosts will be collected and reared from extra-orchard habitats, and their potential suitability as overwintering hosts determined. Collection efforts will be concentrated in riparian habitats where the highest plant diversity and the greatest number of host larvae are expected. Species-specific life history data from literature surveys and inference from relatives will be used to determine life history of collected larvae, and their potential to host overwintering tachinid parasitoids. These parasitoids most likely diapause as larvae in hosts that overwinter as late instar larvae or pupae. Surveys will occur in a minimum of five sites bi-monthly and will use hand collection, sweeping, and adult tachinid trapping using volatile attractants. Several sites have already been determined and sampled for larvae and several tachinid host species have been found. Collected larvae will be reared for identification and to determine parasitism.
Objective 2. Determine the spatial and temporal migration patterns of the tachinid parasitoids around orchards.
To examine tachinid migration patterns, sites under soft pest management regimes that have leafroller and tachinid populations and are bordered by different crops or uncultivated habitats will be selected. Three evenly spaced distances from the center of the orchard in each direction will be sampled such that the orchard interior, the orchard edge, and the extra-orchard habitat beyond the edge are represented. Three sampling methods will be used in each of three regions: volatile attractants, passive pane traps, and leafroller sentinels on foliage bouquets. Arrangement of the sampling stations will be randomized within each region. Statistical analyses will be used to test whether there are differences in detection/trapping rates between the regions (distance effect), and whether there is interaction between the distance and the sampling method. Directional comparisons should be informative as to where parasitoid activity is highest, possibly indicating sources of extra-orchard habitats where orchard immigration is important. Season-long observation may identify temporal migration patterns.