The Use of Ephemeral Plant Parts on Small Host Plants: How Depressaria leptotaeniae (Lepidoptera: Oecophoridae) Feeds on Lomatium dissectum (Umbelliferae)

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Abstract
Small or ephemeral plants on which a phytophagous insect could not complete development on a single plant part occur throughout grasslands, steppes, deserts, tundras and forest understories. D. leptotaeniae (Lepidoptera: Oecophoridae) uses several parts of its seasonally-restricted herbaceous host plant. L. dissectum (Umbelliferae), as plant parts change in availability over time. In observations of larvae on tagged plants, larvae fed initially in the umbel sheaths with enclosed floral buds and then later in the flowering umbels. Larvae beginning development early in the season and those on plants with at least several umbels often completed development by feeding on flowers alone. Larvae beginning development later and those on plants with few umbels completed development by feeding, in addition, on leaves. In laboratory trials using excised plant parts, larvae pupated at significantly heavier pupal weights when fed either umbel sheaths with enclosed floral buds or flowering umbels than when fed leaves. In field trials, however, larvae restricted to a single umbel or a single leaf with excess quantity of food showed no difference in weight at pupation. Umbels and leaves become essentially equal resources for later instar larvae developing on plants with only 1 or 2 umbels, as umbels pass into the stage of setting seed. N levels in plant parts were consistent with the pattern of larval feeding: floral buds > flowers > leaves = immature seeds. The combination of laboratory trials, field trials and field observations suggests that variation in the use of plant parts by D. leptotaeniae larvae is related to the nutritional value of different plant parts at different times, the availability of umbels in the floral bud or flowering stage, and when larvae begin development. The specialization of other species of genus Depressaria on different parts of other plant species emphasizes the importance of feeding site selection behavior in successful colonization of hosts by parasites.