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Insect reproduction is typically spread out in time and space, making estimates of lifetime reproductive success quite difÞcult under natural Þeld conditions. Such estimates are, however, critical to providing rigorous tests of theory for reproductive behavior. One approach to circumventing this problem is to study insects whose lifetime reproduction is compressed in space and time, and whose ovarian dynamics (oogenesis, oosorption, and oviposition) are simpliÞed. We investigated the gall midge Rhopalomyia californica Felt (Diptera: Cecidomyiidae) as a possible candidate for an herbivore with such a simpliÞed pattern of lifetime reproduction. We demonstrate here that R. californica matures all of its eggs before adult eclosion (it is strictly proovigenic), and it does not resorb eggs, even when deprived of oviposition sites. Observational and experimental studies in the Þeld demonstrate that this midge typically completes its entire lifetime reproduction in a single day: females usually emerge at dawn, mate, and after a posteclosion period of resting, they initiate a sustained period of active oviposition during which most eggs are laid over a 4 Ð5-h period. Mean longevity of adult females is very short, consistently 1 d and only 5Ð 6 h on clear and warm days. Two key aspects of senescence are experimentally demonstrated: an increase in mortality rates and a decrease in oviposition rates with age. The life history traits associated with the ephemeral adult stage of R. californica are consistent with a syndrome that has been characterized recently for many proovigenic herbivorous and parasitic insects. Proovigenic species, including R. californica, may be particularly valuable as model systems for studies of lifetime reproductive success in nature.

Pesticides targeted at pest species have often been demonstrated to have strong adverse effects on the survival of biological control agents in short-term laboratory bioassays; however, studies examining the influence of pesticides on the actual reproductive success of biological control agents in the field are rare. Because natural enemy reproduction is often directly tied to biological control success, effects of pesticides on reproduction are of central importance. Here we use a new technique to examine the influence of sulfur, a fungicide widely used in grape production, on the reproductive success of Anagrus erythroneurae (Hymenoptera: Mymaridae) and Anagrus daanei (Hymenoptera: Mymaridae), egg parasitoids of the grape leafhopper, Erythroneura elegantula (Homoptera: Cicadellidae). Sulfur has previously been shown to be highly toxic to Anagrus spp. in short-term laboratory and field bioassays, creating the expectation that sulfur should also reduce Anagrus reproductive success in the field. Surprisingly, in two studies, the first comparing the oviposition success of Anagrus collected live in paired sulfur-treated versus untreated vineyards and the second comparing the lifetime reproductive success of Anagrus collected at the end of their lives in unpaired sulfur-treated versus untreated vineyards, we found no effect of sulfur on parasitoid reproductive success. In this system, traditional short-term assays of laboratory toxicity do not appear to predict effects on parasitoid reproductive success, suggesting that demographic approaches to assessing the disruptive effects of pesticides may have an important role in designing IPM programs.

We examined the toxicity of a fungicide, sulfur, to the egg parasitoid Anagrus erythroneurae (Hymenoptera: Mymaridae) Trjapitsyn and Chiapini and the vineyard leafhopper pest Erythroneura elegantula Osborn (Homoptera: Cicadellidae) and tested whether or not the use of sulfur in the field affects biological control of E. elegantula. Using field cage bioassays, we demonstrated that sulfur is toxic to adult A. erythroneurae parasitoids, but not toxic to adult E. elegantula leafhoppers. We nonetheless found in a field experiment that sulfur produced no changes in rates of parasitism or E. elegantula egg density, and generated only a very small increase in the density of E. elegantula nymphs. These results suggest that sulfur, although toxic to A. erythroneurae, is not highly disruptive of E. elegantula biological control in vineyards. Our results suggest that simple bioassays of acute toxicity may not accurately predict the impact of agricultural chemicals on biological control.

Control of the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), with the entomopathogenic nematode Heterorhabditis marelata Liu and Berry (Nematoda: Heterorhabditidae) was examined in the laboratory and in potato fields in north central Oregon. This research tested the hypothesis that varying nitrogen fertilizer levels would affect foliar alkaloid levels, which would stress the host, and allow increased nematode reproduction and long-term control of the CPB. Laboratory results indicated that nematodes tended to reproduce more readily in CPB fed on potato plants with high levels of fertilizer. Field trials tested CPB population responses to four treatments: application of nematodes vs. no nematodes, with application of low vs. high rates of nitrogen fertilizer. The higher nitrogen application rate increased field foliar levels of the alkaloids solanine by 35%, and chaconine by 41% over the season. Nematodes were applied twice during the season, causing a 50% reduction in adult CPB populations, and producing six times as many dead prepupae in nematode-treated soil samples as in the untreated samples. However, no reproducing nematodes were found in the 303 dead prepupae and pupae collected from nematode-treated plots. Nitrogen fertilizer levels, and their related alkaloid levels, did not affect nematode infection rates or reproduction in the field. Foliar alkaloid levels of plants from the growth chamber were 3-6-fold as high as those in the field, which may explain the variation in nematode response to nitrogen applications to host plants of the CPB. Heterorhabditis marelata is effective for controlling CPB in the field, and does not have negative non-target effects on one of the most common endemic CPB control agents, Myiopharus doryphorae (Riley) (Diptera: Tachinidae), but the low rate of nematode reproduction cannot be manipulated through alkaloid stress to the beetle. Until H. marelata can be mass-produced in an inexpensive manner, it will not be a commercially viable control for CPB.

For more than 80 years, ecologists have debated whether reproduction by female insect herbivores and parasitoids is con- strained by the time needed to find hosts (time limitation) or by the finite supply of mature eggs (egg limitation). Here we present the first direct measures of permanent time limitation and egg limitation and their influences on the cost of oviposition and lifetime repro- duction for an insect in nature. We studied the gall midge Rhopa- lomyia californica, which neither matures nor resorbs eggs during the adult stage. By sampling females soon after their death and correcting for predation effects, we demonstrate that females lay a large pro- portion of their total complement of eggs (multiyear mean: 82.9%). The egg supplies of 17.1% of females were completely exhausted, with the remaining 82.9% of females being time limited. As predicted by theory, we estimate that even though egg limitation is a minority condition within the population, egg costs make a substantial con- tribution (57% of the total) to the cost of oviposition. We conclude that insect life histories evolve to produce a balanced risk of time and egg limitation and, therefore, that both of these constraining factors have important influences on insect oviposition behavior and population dynamics.