SPATIAL AND DIEL AVAILABILITY OF FLYING INSECTS AS POTENTIAL DUCKLINGFOOD IN PRAIRIE WETLANDS

We examined spatial and diel availability of flying insects that are a critical food resource to young ducklings. We sampled insects in thre e native prairie wetlands on the Woodworth Study Area of south-central North Dakota. Insects were sampled with floating sticky traps within emergent macrophyte, edge, and open water microhabitat zones. Sampling took place from 12 June to 4 July 1995, a period that coincided with peak dabbling duck (Anas spp.) hatching in this region. Our sticky tra ps collected 28,527 insects and spiders totaling at least 32 families and 150 species. Chironomidae (Diptera) was the most abundant group, c onstituting 60% of the total insect count and 32.9% of the biomass (mg dry weight). Mixed-model ANOVA showed that a population of similar un disturbed wetlands may show differences in insect availability when co nsidering both time and space, primarily due to differences in communi ty structure among sites. In spite of these significant random effects , interactions between or among the fixed date, zone, and trap-height effects significantly influenced insect availability. Insects stratifi ed near the water surface in open water areas on all dates except duri ng cool, rainy weather (28 June); on this date, insects were virtually absent from open water. Vertical stratification of insects was less p revalent within and at the edge of stands of emergent vegetation, alth ough most insects were present in the emergent zone near the water sur face during inclement weather. ANOVA models from our diel study showed that a significant diel pattern in insect availability existed among zones, but this interaction also depended upon trap-height (chironomid biomass) or date (chironomid counts and biomass). Generally, more ins ect numbers and biomass were captured in and along stands of emergent macrophytes during the day but chiefly in open water near the surface at night. This diel-zone effect was especially apparent for large chir onomids, which were essentially absent during daylight but abundant in open water and edge zones during night. Daytime chironomids were smal l and predominantly trapped in stands of emergent vegetation. Our resu lts are consistent with previously documented brood foraging behavior and may indicate a trade-off between low energy foraging in the open a t night and potentially safer but less productive foraging in stands o f emergent vegetation during the day.