Obliquebanded and Pandemis leafrollers (OBLR and PLR) are destructive pests in Washington apples and sweet cherries. Parasitism surveys indicate that two tachinid parasitoids, Nemorilla pyste (Walker) and Nilea erecta (Coquillett) account for the majority of leafroller parasitism in orchards. These parasitoids are generalists and probably cannot overwinter on OBLR or PLR in the orchard. Thus, availability of alternative hosts may be important to improve biological control and thus sustainable leafroller management. The goals of this study were to determine whether alternate, non-orchard habitats are utilized by these flies so that these habitats can be targeted for conservation or enhancement.
Obliquebanded (Choristoneura rosaceana) and Pandemis (P. pyrusana) leafrollers (OBLR and PLR) are persistent bivoltine tortricid (Lepidoptera) pests of pome and stone fruits in Washington (Jones et al. 2005). These pests originally occurred on native host plants but have become serious pests of deciduous tree fruits in the last 20 years. Historically, leafrollers were controlled through broad-spectrum insecticide use targeting codling moth, which is the most severe pest in tree fruit. Leafrollers have gained importance as management of codling moth has become increasingly reliant on mating disruption, biological insecticides such as granulosis virus (CM-Gv) and “reduced risk” insecticides in place of broad-spectrum materials (Brunner et al. 2003). One positive aspect of this shift is that biological control for leafrollers has become more feasible.
Relative to codling moth, the economic threshold for OBLR and PLR is high, and the natural enemy complex is diverse; thus, leafrollers represent a viable, low-risk target for biological control in orchards. Nonetheless, insecticides targeting leafrollers are typically applied twice per season (Brunner et al. 2003), and these treatments are often prophylactic (Jones et al. 2006). Prophylactic insecticide use violates the mandates of integrated pest management and sustainable agriculture.
Surveys of parasitism in central Washington (V.P. Jones, unpublished), British Columbia (Vakenti et al. 2001, Cossentine et al. 2004) and Michigan (Wilkinson et al. 2004) demonstrate that the leafroller parasitoid complex is diverse. In central Washington, the majority of OBLR and PLR parasitism on young apple trees is attributed to the tachinid flies Nemorilla pyste (Walker) and Nilea erecta (Coquillett; V.P. Jones, unpublished), but these species are less important in British Columbia (Cossentine et al. 2004) and Michigan (Wilkinson et al. 2004). Unfortunately, OBLR or PLR are not viable overwintering hosts for N. pyste or N. erecta, and this is an important factor driving the population dynamics of these parasitoids in orchards. Nemorilla pyste and N. erecta attack only large leafroller larvae (4th– 6th instars; O’Hara 2005, V.P. Jones, unpublished), and OBLR and PLR overwinter as minute 2nd or 3rd instars. Thus, an alternate host is probably required for the flies to complete seasonal development.
Because N. pyste and N. erecta attack larvae from many families of Lepidoptera (O’Hara 2005), yet cannot overwinter on OBLR or PLR, availability of alternative hosts in habitats outside of orchards may be an important factor affecting leafroller parasitism rates within orchards, particularly for the overwintering leafroller generation that attacks fruit trees in the spring. Parasitism rates are generally much lower in the overwintering generation relative the summer generation (Brunner 1996, Pfannenstiel and Unruh 2003, Cossentine et al. 2007), reflecting an overwintering bottleneck that constrains early population growth of parasitoids such as Tachinidae. The bottleneck may reflect on rarity of alternate hosts, or perhaps large distances between orchards and overwintering habitats. Identification of the habitat types and alternate host species that these flies utilize is key to enhancement of biological control through conservation of existing extra-orchard habitats and possible manipulation of orchard habitats to accommodate the overwintering needs of tachinid parasitoids (see Pfannenstiel et al. 2010). The goal of this project was to assess the potential role of extra-orchard habitats in the population biology of N. pyste and N. erecta, so that habitats and/or host species could be identified for conservation or enhancement.
Brunner, J. F., W. Jones, E. Beers, G. V. Tangren, J. Dunley, C. Xiao, and G. G. Grove. 2003. A decade of pesticide use and ipm practices in Washington’s apple orchards. Agrichemical and Environmental News 205:1-16.
Cossentine, J., A. Bennett, H. Goulet, and J. O’Hara. 2007. Parasitism of the spring leafroller (Lepidoptera: Tortricidae) complex in organically managed apple orchards in the north Okanagan valley of British Columbia. Pan-Pac. Entomol. 83:276–284.
Cossentine, J., L. Jensen, E. Deglow, A. Bennet, H. Goulet, J. Huber, and J. O’Hara. 2004. The parasitoid complex affecting Choristoneura rosaceana and Pandemis limitada in organically managed apple orchards. BioControl 49:359–372.
Jones, V., T. Unruh, D. Horton, and J. Brunner. 2006. Improving apple ipm by maximizing opportunities for biological control. Good Fruit Grower Dec.:1–8.
Jones, V. P., C. C. Eastburn, T. D. Wilburn, and J. F. Brunner. 2005. Instar-specific phenology of Pandemis pyrusana and Choristoneura rosaceana (Lepidoptera: Tortricidae) in Washington apple orchards. J. Econ. Entomol. 98:875–883.
O’Hara, J. E. 2005. A review of the tachinid parasitoids (Diptera: Tachinidae) of Nearctic Choristoneura species (Lepidoptera: Tortricidae), with keys to adults and puparia. Zootaxa 938:1–46.
Pfannenstiel, R., T. Unruh, and J. Brunner. 2010. Overwintering hosts for the exotic leafroller parasitoid, Colpoclypeus florus: Implications for habitat manipulation to augment biological control of leafrollers in pome fruits. J. Insect Sci. 10:1–13.
Pfannenstiel, R. S. and T. R. Unruh. 2003. Conservation of leafroller parasitoids through provision of alternate hosts in near-orchard habitats, in 1st International Symposium on Biological Control of Arthropods. USDA-Forest Service FHTET-03-05, pp. 256–262.
Vakenti, J., J. Cossentine, B. Cooper, M. Sharkey, C. Yoshimoto, and L. Jensen. 2001. Host-plant range and parasitoids of obliquebanded and three-lined leafrollers(Lepidoptera: Tortricidae). Can. Entomol. 133:139–146.
Wilkinson, T., D. Landis, and L. Gut. 2004. Parasitism of obliquebanded leafroller (Lepidoptera: Tortricidae) in commercially managed Michigan apple orchards. J. Econ. Entomol. 97:1524–1530.
1. Determine the types of habitats and the host species that are utilized by the parasitoids in the regional landscape and identify potential overwintering hosts.
2. Examine spatial and temporal parasitoid migrations of the flies around orchards.
Potential host larvae were collected and reared from extra-orchard habitats to determine parasitism. Collection efforts were concentrated in riparian habitats where the highest plant diversity occurs in our arid environment. When tachinid-parasitized hosts were collected, their potential suitability as overwintering hosts was determined given the criteria discussed in the introduction; overwintering hosts for N. pyste and N. erecta should overwinter as late instar larvae. Species-specific life history data from literature surveys and inference from relatives were used to determine the relevant life history of collected larvae. Seven sites were also established where host surveys took place every two weeks and traps were established to target tachinid flies to asses habitat utilization. Traps consisted of sticky cards baited with volatile attractant compounds (benzaldehyde, cis-hexanol, octyl-aldehyde), yellow sticky cards, unbaited clear pane intercept traps and a modified Malaise trap designed specifically for the project. Sentinel OBLR larvae were also used at these sites; larvae placed on apple foliage cuttings that were partially submerged in PVC tubes filled with water. Traps were monitored weekly throughout 2009. The seven habitats that were monitored were characterized as follows: wild rose (Rosa woodsii) dominated shrub-steppe, field crop dominated, high-elevation Douglas fir forest (Pseudotsuga menziesii), natural grassland, lowland riparian, arid sageland and low-elevation ponderosa pine forest.
To examine tachinid migration patterns, three apple orchards under soft pest management regimes with leafroller populations were selected to evaluate spatial use of orchards by N. pyste and N. erecta. Three evenly-spaced distances from the center of the orchard in each direction were sampled such that the orchard interior, the orchard edge and the extra-orchard habitat beyond the edge were represented. Three sampling methods were used in each of three regions: volatile attractants, passive pane traps and leafroller sentinels on foliage bouquets. Arrangement of the sampling stations was randomized within each region. We examined these trap catch data to determine whether there were differences in detection/trapping rates between the regions (distance effects) and to examine potential seasonal abundance relationships. These data were also used to assess the efficacy of different traps for monitoring the flies. Three apple orchards with established leafroller populations and cooperative managers were selected. These sites were monitored weekly and catch of Tachinidae was recorded.
1. In non-orchard sites, sticky cards baited with octyl-aldehyde caught the most Tachinidae (68.75%), followed by clear pane intercept traps (12.50%).
2. Although trap catches of Tachinidae from non-orchard habitats represented a diversity of species, none of the captured flies were N. pyste or N. erecta. This suggests that these flies do not utilize the non-orchard habitats we examined.
3. In the three orchards where traps were evaluated, unbaited yellow cards caught the most Tachinidae (50.48%), followed by sticky cards baited with octyl-aldehyde (21.90%). These species were exclusively N. pyste and N. erecta.
4. There were no clear trends in the spatial or temporal distribution of Tachinidae caught in the three sample regions, suggesting that the flies were relatively evenly distributed and they persisted throughout the season.
5. Only one of the sentinel larvae was ever parasitized by a tachinid (Nemorilla pyste). This occurred in one of the orchards. Most sentinel larvae were lost to predation, or else they prematurely pupated making them invulnerable to tachinid attack.
6. A novel design for an inexpensive Malaise trap was produced, and the design was published for the benefit of other tachinid workers.
7. Modified malaise traps were highly effective in capturing Tachinidae, however, out of 161 tachinid specimens recovered, only four specimens of the target species were recovered. These were taken from orchard peripheries; none were recovered from the non-orchard habitats.
8. The majority of Tachinidae from non-orchard habitats were recovered from the R. woodsii dominated habitat (59.63 %). This is an important habitat for overwintering of another leafroller parasitoid, Colpoclypeus florus on the alternate host Ancylis comptana, the strawberry leafroller (Pfannenstiel et al 2010).
9. The grassland habitat (13.67%) and field crop habitat (13.67%) had the next largest Malaise catch of Tachinidae.
10. The only alternate hosts that both flies were reared from was the western spruce budworm, Choristoneura occidentalis Freeman, which is a previously recorded host for both species (O’Hara 2005). Western spruce budworm is a very common species in the study region, and it occurs primarily on ornamental and wild fir trees. However, like C. rosaceana, C. occidentalis overwinters as a minute larva and probably cannot serve as an overwintering host for the flies.
Educational & Outreach Activities
1. Wiman, N.G. 2010. Malaise traps for Tachinidae on a shoestring budget. The Tachinid Times 23. Online: http:Tachinid Times vol. 23
2. Consulting entomologist at the Mendoza orchard farm walk. Farm walks (WSU Small Farms Team) highlight small farms and innovative agricultural practices. Biological leafroller management was discussed. Quincy, WA. 6 Sept 2010.
3. Wiman, NG and VP Jones. 2010. Bio-based leafroller management. Presentation to regional grower meeting for recertification credits. ~100 participants. Chelan, WA. 14 Jan.
4. Provided photos of Tachinidae for online guide to natural enemies in tree fruit. Online: http://entomology.tfrec.wsu.edu/VPJ_Lab/downloads/NaturalEnemies.pdf
5. Wiman, N.G. (In progress) Ecology of Nemorilla pyste (Walker) and Nilea erecta (Coquillett) (Diptera: Tachinidae), parasitoids of leafrollers (Lepidoptera:Tortricidae) in Washington tree fruit. PhD Dissertation, Washington State University.
We were unable to determine which, if any, alternate habitats are utilized by the tachinid parasitoids N. pyste and N. erecta. However, through the publications and extension activities that were completed with the funding of this project, we believe that we have raised awareness among growers of biological control of leafrollers by these flies. Publications will continue to emerge from this and ancillary projects as the dissertation listed in publications/outreach is completed. These data will demonstrate distributions and efficacy of these Tachinidae as natural enemies, as well as examine agricultural practices that conflict with goals of improving biological control and sustainability of leafrollers in tree fruit. Our results also provide a foundation for monitoring tachinid flies in the orchard, and specifically we found that an unbaited yellow sticky card was the best trap tested. These cards could be used by growers and managers to evaluate the presence of tachinid natural enemies in the orchard and make management decisions accordingly.
Economic analysis was not a part of this project.
Because of this project, growers have more access to information about these tachinid parasitoids, and it is hoped that greater awareness will influence management decisions to rely more on biological control of leafrollers even though we do not fully understand how this system works. Although we failed to accomplish some of our goals, we hope that by improving the tools used to sample Tachinidae in orchards and non-orchard habitats, we have set the stage for more successful research into tachinid overwintering dynamics in the future. Reducing insecticide use against leafrollers in tree fruit, particularly prophylactic use, has the potential to save millions of dollars in chemical costs and reduce environmental impacts of fruit production.
Areas needing additional study
More training on sampling and identification of tachinid flies is needed to increase use of natural enemies and to gain appreciation for the benefits of conservation biological control.