ENTOMOPATHOGENIC NEMATODES - NATURAL ENEMIES OF ROOT-FEEDING CATERPILLARS ON BUSH LUPINE

A new species of soil-dwelling entomopathogenic nematode Heterorhabdit is hepialus killed up to 100% (mean = 72%) of root-boring caterpillars of a ghost moth Hepialus californicus in coastal shrub lands. When un checked, ghost moth caterpillars killed bush lupine, Lupinus arboreus. Here we describe this strange food chain. Although unappreciated by e cologists, entomopathogenic nematodes are widespread and probably one of the most important groups of natural enemies for underground insect s. The free-living infective juvenile (IJ) of entomopathogenic nematod es searches for host insects in the soil. A single IJ can kill a host, although several often invade together. After entering the host throu gh a spiracle or other orifice, the IJ regurgitates its symbiotic bact erium, Photorhabdus luminescens, which kills the host within 48 h. The bacteria digest the cadaver and provide food for the exponentially gr owing nematode population inside. The bacteria produce antibiotics and other noxious substances that protect the host cadaver from other mic robes in the soil. When the cadaver is exhausted of resources, IJs bre ak the host integument and can disperse. As many as 420,000 IJs can be produced within a large ghost moth caterpillar. Surface soil of the l upine rhizosphere is the primary habitat of IJs of H. hepialus. Attrac ted to waste gases emitted by insects, the 0.5-mm-long IJs can move 6 cm/day through moist soil. Prevalences of H. hepialus ranged from as h igh as 78% of rhizospheres in some lupine stands to almost zero in oth ers, but it was absent from no stand at our study site. Field intensit ies ranged from 0.003 IJs/cm(3) of soil to 7.5 IJs/cm(3), and correlat ed roughly with prevalences among sites. Few ghost moth caterpillars ( mean = 6.7) succeeded in entering lupine roots where prevalence of H. hepialus was highest, and this stand had lowest mortality (0.02) of ma ture bush lupine. In the three stands with lowest prevalence (mean = 2 %) of this nematode, many caterpillars (mean = 38.5) entered roots, an d lupine mortality was high (range = 0.41-1.0). Old aerial photographs indicate that the stands with highest recent nematode prevalence have had little or no mass die-off of lupine over the past 40 years. The p hotos depict repeated die-offs of lupine during the past four decades in stands with lowest recent prevalence of the nematode. This pattern leads us to entertain the hypothesis that the nematode affects vegetat ion dynamics indirectly through a trophic cascade. Dispersal of entomo pathogenic nematodes is little understood. We found that air drying of soil extirpates H. hepialus and speculate that this nematode is dispe rsed during the wet season in moist soil bits on the exterior of fosso rial insects and mammals. H. hepialus colonized some previously unoccu pied lupine rhizospheres during the wet winter-spring season and, obve rsely, became extinct from some rhizosperes as soil dried in summer. R oot-feeding insects have only recently been recognized as a force in c ommunities, and the regulation of these important herbivores is still largely an ecological term incognita. All evidence indicates that ento mopathogenic nematodes are found throughout terrestrial ecosystems, an d we propose that trophic chains similar to those described in this re port should not be uncommon.