Document: MIC-3-48-40

Highly skewed sex ratios of tephritid flies reared from two high-altitude Erigeron (Asteraceae) species: Why do female flies predominate?

LEIGH, M.J.* ^1,2 and D.W.INOUYE ^1,2

University of Maryland, College Park, MD 20742, USA ¹
Rocky Mountain Biological Laboratory, Crested Butte, CO, USA ²

Abstract:
Pre-dispersal seed predation can strongly affect the reproductive output of plants. This is particularly true for composites (Asteraceae), whose tightly clustered ovaries present a concentrated resource for seed predators. Composite plants host a variety of fly species, including Tephritidae and Agromyzidae, that spend their entire larval stages within a single flowerhead, feed on ovaries and developing seeds, and leave as adults after pupating. Two years of data on flies emerging from Erigeron flowerheads at several sites near the Rocky Mountain Biological Laboratory (Colorado, 2,800 m) reveal strong skewing in the sex ratios of adults for two tephritid fly species, Neotephritis finalis and Tephritis signatipennis. When flowerheads were collected from E. elatior (n=25) and E. speciosus (n=41) at two high-altitude sites in 1998, virtually all of the N. finalis emerging from these flowerheads were female (72 females, 0 males; and 85 females, 5 males, respectively). However, sex ratios were not skewed for N. finalis from other plant hosts at those sites, nor were T. signatipennis encountered there. In 1999, collecting was expanded to include more sites and plant species across a broader range of elevations. Practically no N. finalis (n=3) were found at the two high-altitude sites, while T. signatipennis occurred there in significant numbers (n=156). At all of the sites, only female T. signatipennis were found on E. elatior (226 females). Although some male T. signatipennis were reared from E. speciosus flowerheads, the sex ratios at all sites were still strongly female-skewed (5 males, 77 females). The occurrence of skewed sex ratios on only Erigeron species suggests some plant-related factor may be responsible, such as secondary plant compounds that mimic insect sex hormones, or some factor that induces higher male mortality. The virtual disappearance of N. finalis from the 1998 high-altitude sites, and the appearance of T. signatipennis in 1999, suggests that skewed sex ratios can have significant effects on the population dynamics of the flies, which may, in turn, have an effect on the reproductive success of their host plants. If this effect is plant-induced, it may represent one more plant defense against herbivory.

Keywords: sex ratios, predispersal seed predation, Tephritidae, Erigeron, secondary plant compounds, herbivory, plant defenses

This abstract is being presented at: 3:30 PM in session:
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