Longevity of hymenopteran parasitoids in natural vs. agricultural habitats and implications for biological control

Agricultural habitats are frequently disturbed, and disturbances could have large effects on species in upper trophic levels such as hymenopteran parasitoids that are important for biological control. A strategy for conservation biological control is to provide a diversified agricultural landscape which increases the availability of resources such as sugar required by parasitoid biological control agents. Here, we ask whether parasitoids occurring in agriculture benefit from sugar resources more or less than parasitoids occurring in natural habitats surrounding agricultural fields. We collected parasitoids from agricultural alfalfa fields, field margins and natural prairies, and in the lab we randomly divided them into two treatments: half were given a constant supply of a sugar source to test their residual lifespan, and half were given neither sugar nor water to test their hardiness. Collected individuals were monitored daily and their day of death recorded. Parasitoids receiving a sugar source lived substantially longer than those without. Parasitoids collected in prairies lived longer than those from alfalfa fields in both the residual lifespan and hardiness treatments, with parasitoids from field margins being intermediate between them. Furthermore, the benefits of a sugar source to increase longevity was lower for parasitoids collected in agriculture than in natural habitats. This suggests that, even though parasitoid biological control agents benefit from sugar resources, their short lifespans make the benefit of sugar resources small compared to parasitoids that occur in natural habitats and have longer lifespans, and are adapted to consistent sugar sources.

Methods
Adult parasitoids were collected using sweep nets from alfalfa fields, margins of alfalfa fields, and prairies at the Arlington Agricultural Research Station, the West Madison Agricultural Station, and the Goose Pond restored tallgrass prairies; these sites were in Dane and Columbia Counties, Wisconsin, USA.

Samples were collected from 13 alfalfa fields, nine in Arlington Agricultural Research Station and four in West Madison Agricultural Station. Field margins were sampled next to seven of the sampled alfalfa fields in Arlington and all four fields in West Madison and five restored tallgrass prairies, all in proximity to the Arlington Agricultural Research Station.

Each individual parasitoid was then transferred to a 100 x 15-mm petri dish and mortality was monitored daily. All collected parasitoids on a given sample date were randomly divided between two treatments: half had a drop of honey placed in the middle-upper part of the petri dish and no water, and half had no honey or water. Petri dishes with wasps were kept under the same conditions in the lab at 25 ± 1 °C, 60% RH and LD 14:10h.