Age-Dependent Parasitism ofCulicoides variipennis sonorensis(Diptera: Ceratopogonidae) byHeleidomermis magnapapula(Nematoda: Mermithidae) and Considerations for Assessing Parasite Impact

Laboratory and field studies were conducted to determine whether all larval stages ofCulicoides variipennis sonorensisWirth and Jones were susceptible to parasitism byHeleidomermis magnapapulaPoinar and Mullens. All four larval instars (L1–L4) were readily attacked in the laboratory, although parasites took significantly longer to enter older hosts (average of 119 s for L1 to 199 s for L4). Field studies indicated that free-living adult nematodes, and presumably the preparasitic (infective) stages, were distributed above the waterline, where early-stage host larvae (L1 and L2) were most common. Dissection of field-collected hosts over a full host generation showed maximum parasitism was lower in L4 (40%) than in L1 (71%), L2 (64%), and L3 (64%) hosts. Second stage parasites were found only in L1 and L2 hosts. Absence of second stage parasites in L3 and L4 hosts showed that older larvae were seldom attacked in nature. The mature parasites emerge primarily from L4 hosts. Collecting L4 hosts in the field can significantly underestimate parasitism due to emergence ofH. magnapapulafrom L4 hosts prior to sampling or premature host mortality due to parasitism. Dissection of L3 hosts should result in the most accurate parasitism estimates, but these estimates still ignore some parasite-induced mortality in the early host stages.