Neochrysocharis okazakii(Hymenoptera: Eulophidae) as a Major Parasitoid Wasp of Stone Leek Leaf MinerLiriomyza chinensis(Diptera: Agromyzidae) in Central Vietnam

Liriomyza chinensisis a major pest of Welsh onionAllium fistulosumin Asia but little is known about the abundance of its natural enemies. A field survey was made to explore the major parasitoids ofL. chinensisleaf miner in central Vietnam. An eulophid parasitoid,Neochrysocharis okazakii,comprised more than 95% of parasitoids reared from leaf miner larvae collected in the onion field and 98.3% of leaf miner parasitoids found during searches of onion plants. The mean number of femaleN. okazakiion plants was greater in onion fields with a higher density ofL. chinensis, and, during searches, a greater proportion ofN. okazakiiwas found on onion with moreL. chinensismines, suggesting density-dependent parasitoid aggregation. Melanized dead larvae ofL. chinensiswere more frequently found in onion fields with more parasitoids, demonstrating that melanized leaf miners are a good indicator of parasitoid activity. Mean instant rate of host encounter byN. okazakiiin the field was estimated at 0.077, and the likelihood of a parasitoid finding a host increased with host density. Taken together, these results show thatN. okazakiiis the major parasitoid ofL. chinensis. Usefulness of this parasitoid in stone leek leaf miner management is discussed.

Foraging Behavior of Field Populations of Diadegma Spp. (Hymenoptera: lchneumonidae):Testing for Density-Dependence at Two Spatial Scales

The foraging behavior of populations of Diadegma spp. (Hymenoptera: Ichneumonidae) attacking the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), was studied in the field. The effect of host density on percentage parasitism was investigated at two spatial scales: that of the individual plant, as well as a “cluster of plants” at low, medium and high densities. Using binoculars, parasitoid searching was observed on host plants at different host densities over an 8-h period. Behavior was compared to that predicted under the Patch Selection Theory. Percentage parasitism was independent of host density at both spatial scales. However, the behavioral studies showed parasitoid aggregation behavior at higher host densities. The population displayed an imperfect preference for higher host densities in the first 4 h of foraging. In the second 4-h period, no preference was observed.