Impacts of granivorous and frugivorous arthropods on pre-dispersal seed production of western juniper (Juniperus occidentalis)

Authors: DIMITRI, LINDSAY - University Of Nevada; Longland, William - Bill; Tonkel, Kirk; Rector, Brian; KIRCHOFF, VERONICA - University Of Nevada

Submitted to: Arthropod-Plant Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2018
Publication Date: 3/2/2018
Citation: Dimitri, L.A., Longland, W.S., Tonkel, K.C., Rector, B.G., Kirchoff, V.S. 2018. Impacts of granivorous and frugivorous arthropods on pre-dispersal seed production of western juniper (Juniperus occidentalis). Arthropod-Plant Interactions. 12(3):465-476. https://doi.org/10.1007/s11829-018-9603-3
DOI: https://doi.org/10.1007/s11829-018-9603-3

Interpretive Summary: Western juniper is a native tree that has been expanding its range into shrub lands and thereby reducing the amount of forage plants available for wildlife and livestock. New juniper seedlings must establish for this expansion to progress. However, even though it is known that most of a crop of juniper seeds can be heavily damaged by seed-feeding insects, there have been no comprehensive studies of insects and other minute seed-eaters that feed on western juniper seeds, and may thus play an important role in combating its ongoing expansion. Here, we document that 4 different groups – 3 insects and a species of mite – can cause substantial damage to western juniper seed crops. We describe characteristics of juniper trees that seem to influence the degree to which their seeds are vulnerable to damage by juniper seed and juniper berry feeders. We also describe symptomatic damage that can be seen on juniper seeds and juniper berries to indicate the presence of these insects and mites.

Technical Abstract: Arthropods impact seed production in various juniper species, but effects of pre-dispersal seed predation are generally unknown for arthropods that feed on western juniper (Juniperus occidentalis). From 2009 to 2013, we quantified impacts of three arthropod granivores and a frugivorous insect on western juniper seed production at two N. California field sites— Madeline and Shinn Peak. Insect larvae were genetically identified using the COI barcode region. Seed damage by all arthropod taxa varied both spatially and temporally. Juniper berry mites (Trisetacus quadricetus) had the greatest effect on reducing seed production at Madeline, and granivorous moths (Periploca spp. and Argyresthia spp.) had the greatest effect at Shinn Peak. Three findings supported the predator satiation hypothesis, which suggests that unpredictable masting events overwhelm responses of seed predators. First, estimated berry production had significant negative effects on the proportion of seeds damaged across sites by a chalcidoid wasp (Eurytoma juniperina) and by granivorous moths at Shinn Peak. Second, seed damage by granivorous moths was significantly reduced in a mast year of juniper berry production. Third, number of seeds per berry positively affected seed damage across sites by granivorous moths in all years except the mast year as well as damage by a frugivorous tortricid moth (Henricus infernalis) at Madeline. Distance to neighboring trees had positive, site-specific effects on damage by granivorous moths and Henricus, perhaps due to limited movement of parasitoids that attack these insects. Our results suggest that arthropod damage can significantly impact seed production of western juniper.