SELECTION OF ENTOMOPATHOGENIC NEMATODES (STEINERNEMATIDAE AND HETERORHABDITIDAE, NEMATODA) FOR THE BIOLOGICAL-CONTROL OF CRANE-FLY LARVAE TIPULA-PALUDOSA (TIPULIDAE, DIPTERA)

Larvae of the cranefly Tipula paludosa were collected in the field and used in laboratory bioassays to assess the control potential of entom opathogenic nematodes of the genera Steinernema and Heterorhabditis. P rior to the tests, the larvae were examined for infection and parasiti sm Few larvae were found to be infested by cephalic gregarines and mer mithid nematodes. One larva suffered from an infection with an iridesc ent virus. Up to 4 % of the tipulids collected in November and Decembe r were parasitized by dipteran larvae. The naturally occurring antagon ists were unable to suppress the population density below the economic threshold density. In preliminary tests, S. feltiae and S. anomali we re the most virulent nematodes while heterorhabditid nematodes and S. affinis never exceeded 20 % efficacy. When S. feltiae was repeatedly t ested, results were highly variable with a mean larval mortality of 50 %, ranging between 13 and 90 % (s = 26 %). In two rests, the number o f insects with nematodes in the haemocoel was assessed. Although an eq ual number of larvae was infested with nematodes, mortality was 27 % i n one and 65 % in the other test. Some tipulid larvae were able to sur vive nematode infestation by encapsulation of invading dauer juveniles . Field sampled larvae often show black melanizations on the cuticle, most likely as a result of a non-self contact, leading to an increased activity of defense mechanisms in the haemocoel. These larvae were si gnificantly less susceptible to nematode infection. A migration test s howed that Steinernema anomali exhibits a greater tendency to migrate downwards than S. feltiae. Other factors influencing nematode efficacy , like different larval stage or an increased susceptibility after mol ting, are discussed. On a 81 m(2) area, 0,5 and 1 Mio. S. feltiae per m(2) were applied ro control L4 larva of T. paludosa. An economically significant reduction of 53 % was only obtained in the plots treated w ith 1 Mio. dauer juveniles.