Seed-feeding arthropods associated with western juniper
Western juniper (Juniperus occidentalis) has been expanding in its native range for more than 100 years in the western United States, and since this tree does not reproduce vegetatively, this expansion is necessarily attributable to seedling recruitment. Seed-feeding insects can greatly reduce the seed crop in any given plant species, so quantifying insect damage to seeds provides insight into the number of viable seeds available for dispersal. However, there has been no comprehensive study of the insect fauna associated with juniper since 1915. We have been collecting juniper “berries” (which are actually modified female cones) from 2009-2012 at two field sites in northeastern California, Madeline and Shinn Peak. Berries were collected from 20 trees in the fall and 10 trees in the spring each year. In the lab, 100 berries from each tree were randomly separated for dissection, and extra berries were placed in separate petri dishes to rear adult insects. Each berry was measured and weighed then carefully dissected, taking detailed notes including the presence of holes or other signs of insect damage, location of damage, the number of seeds and seed condition. Adults and larvae were described and preserved in ethanol for further identification.
Collections have yielded 37 different species of arthropods. Of particular interest to this study are the seed-destroying arthropods collected, which include species of Coleoptera, Lepidoptera, Hymenoptera, and Trombidiformes. Overall, we found that 16.5% of juniper seeds were eaten by insects and 4.7% were damaged by mites. This differed between sites, with Madeline having more mite damaged seeds and Shinn Peak having more seeds eaten by insects, and it was highly variable among trees, with some trees having more than 60% of their seeds damaged by insects. Over 500 adult insects were reared or dissected from berries and nearly half of them belonged to 3 groups- Periploca spp, Eurytoma sp. nr. juniperina, and a parasitic wasp in the family Pteromalidae. The former two species are, respectively, a moth and a wasp that feed on western juniper seeds, so it is likely that these two species are responsible for a significant portion of the seed damage observed. The plant mite Trisetacus quadricetus(Acari: Eriophyidae) has also been identified as a species destroying seeds at our sites. These preliminary results indicate that insects and mites can considerably decrease the number of viable western juniper seeds available for dispersal and thus seedling recruitment.