Hylobius abietis is the most important pest of reforestation in Northern Europe. Weevils develop in stumps of felled conifers and emerging adults feed on and kill young trees. In trials conducted in each of 3 years, entomopathogenic nematodes were applied around pine stumps containing late instar weevils. Since these immature weevils are sedentary and occur within stumps at varying depths in soil, it is predicted that nematodes with a cruise foraging strategy, such as Heterorhabditis spp., should be most eVective. The three commercially available species used in our trials, H. megidis, Steinernema feltiae, and S. carpocapsae, have cruise, intermediate and ambush strategies, respectively. We also included the indigenous species H. downesi and a strain of S. feltiae isolated from an Irish coniferous forest. Heterorhabditis downesi suppressed emergence of adult weevils in 2 of 3 trials; and commercial S. feltiae and H. megidis NLH85 in 1 trial each. Stump excavation showed that H. downesi parasitized 55-63% of developing weevils; this was matched by H. megidis NLH85 and S. carpocapsae in 2 and 1 years, respectively. A higher proportion of larvae (46%) than of pupae (32%) or callow adults (30%) were parasitized by nematodes. All nematodes, including S. carpocapsae, parasitized weevils 40-49 cm from the bole of the stump and 30-39 cm below soil level. We conclude that heterorhabditids, particularly H. downesi, have greatest potential against pine weevils in stumps, but that a reputation as an ambush forager should not exclude a species such as S. carpocapsae from trials against sedentary subterranean insect pests.

Hylobius abietis develops in stumps of recently felled coniferous trees, and adults emerge to feed on and kill young seedlings. Entomopathogenic nematodes applied to stumps containing late instar larvae and pupae can reduce the number of adults emerging. We tested the feasibility of reduced application rate and volume during a broader window of application in Weld trials in 2004 and 2005. Application at the standard rate of 3.5 million infective juveniles per stump suppressed the number of adults emerging by up to 79-85% relative to controls for Heterorhabditis downesi, 57-64% for Steinernema carpocapsae and 51-56% for S. feltiae. When the application rate was halved, only H. downesi gave a signiWcant reduction in emergence (75-79%). Nematodes applied in April, May or June signiWcantly reduced weevil populations, and application of nematode-killed insects in May was as eVective as an aqueous suspension. A fourfold reduction in application volume from 500 to 125 ml per stump did not aVect the percentage of weevils parasitized, and stump excavation revealed that even at the reduced volume, both S. carpocapsae (ambusher) and H. downesi (cruiser) parasitized weevils 40-49 cm from the bole and 40-49 cm below soil level. A higher percentage of larvae (60%) were infected than either pupae (36%) or callow adults (18%). Our trials show promise for expanding the use of entomopathogenic nematodes against pine weevil, as the standard application rate can be halved, and the volume reduced to 25% of the standard without signiWcant loss of eYcacy, and there is a wide window of opportunity for application, even when soil temperatures are as low as 9°C.