Habitats and landscape interactions of tachinid parasitoids important in biological control of leafrollers (Lepidoptera: Tortricidae) in central Washington tree fruit

View the project final report

• 2009 annual report

Commodities

• Fruits: apples, peaches, pears, general tree fruits

Practices

• Pest Management: cultural control, field monitoring/scouting

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.

Introduction

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Surveys of parasitism in central Washington (V.P. Jones, unpublished), British Columbia (Vakenti et al. 2001, Cossentine et al. 2004) and Michigan (Wilkinson et al. 2004) demonstrate that the leafroller parasitoid complex is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.

References

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.