To investigate how parasitoid effectiveness relates to the availability of different food sources, we released the solitary braconid parasitoid, Microplitis croceipes, in a corn earworm infested cotton patch with either extrafloral nectar, whitefly honeydew, sucrose, or no food sources available. Extrafloral nectar is nectar produced outside of the flower; in cotton it is secreted from extrafloral nectaries located in the largest midribs under the leaves, on the squares between bracts, and at the bases of bracts. Cotton extrafloral nectar is available during most of the growing season. A nectariless cotton variety was used for the plant patches with sucrose, honeydew, or no food. We compared behavior and performance of individual wasps among the different treatments.
Results revealed that wasps feeding on extrafloral nectar were retained much longer in cotton patches than wasps that were not able to feed. And more importantly for biological control, longer patch retention resulted in higher parasitization. Feeding on sucrose had a similar effect, but only very few wasps were able to find this food. Surprisingly, retention and parasitization in patches with whitefly honeydew was low and comparable to patches without food. We observed that the quality of honeydew rapidly decreases when it dries up on the leaves. This, together with factors such as relatively low quantity per site, haphazard distribution in the patch, and low detectability probably accounted for such poor performance.
These experiments indicate that important attributes of an ideal food source in the field are: high food quality, high food quantity per site, high food detectability, and high predictability of the food location. All these attributes combined will retain parasitoids in a target area and maximize their parasitization effectiveness. Only nectar sources possess all these qualities. Therefore, more attention should be given to integrate plants with floral and extrafloral nectaries in agricultural systems. Because of the low nutritional value of sucrose, spraying sucrose, another alternative for retaining beneficial insects, should only be used as a back-up therapeutic when the availability of plant nectar is low.
- J. Oscar Stapel and Anne Marie Cortesero,visiting scientists, Coastal Plain Experiment Station, Tifton, GA
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