SEASONAL FREQUENCY AND POSITIONING OF PARASITIC MIDGES (CHIRONOMIDAE)ON PTERONARCYS BILOBA NYMPHS (PLECOPTERA, PTERONARCYIDAE)

Mature nymphs of Pteronarcys biloba collected from Catamaran Brook, Ne w Brunswick, between October 1994 and October 1995, were hosts to high numbers of parasitic chironomid larvae [Nanocladius (Plecopteracoluth us) undescribed sp., nr, branchicolus]. Nanocladius (P.) sp. has a uni voltine life cycle in Catamaran Brook, with emergence occurring nearly simultaneously with the stonefly host in late May and early June. The chironomid larva constructs a silken case on the stonefly nymphs and feeds on hemolymph by piercing the gill tissue or the intersegmental m embranes. Stoneflies were collected from different habitat types in 4 stream reaches from the headwaters to the mouth, and the position and number of attached chironomids was recorded for each nymph. The freque ncy and density of parasitic chironomids was not related to habitat ty pe, but was related to reach; significantly more larvae/host were foun d in mid-catchment reaches than at the headwaters or mouth (p < 0.05). No parasitized stoneflies were found in the headwater reach, but betw een 80 and 100% of mature stonefly nymphs collected from the mid-catch ment and mouth reaches were parasitized. Mean chironomid densities ((x ) over bar +/- SE) were 6.7 +/- 0.4 chironomids/mature host in the fai l of 1994 and 3.5 +/- 0.44 in the summer and fall of 1995. Both freque ncy and density of chironomids were highest on the oldest stonefly age class present; younger stoneflies were also parasitized, but at signi ficantly lower levels. Larval positioning on stoneflies differed with age of larvae; early instar chironomids attached mainly to the thoraci c pleura, just under the wingpads, but most migrated to femora by earl y fall (September), and overwintered on the femora.