Augmenting Entomopathogenic Nematodes in Soil from a Florida Citrus Orchard: Non-Target Effects of a Trophic Cascade

Abstract

Laboratory experiments were conducted to study non-target effects of augmenting entomopathogenic nematode (EPN) communities in soil. When raw soil from a citrus orchard was augmented with either 2,000 Steinernema riobrave or S. diaprepesi, fewer EPN (P ≤ 0.05) survived if the soil had also been treated with 2,000 S. riobrave 7 d earlier (i.e., two augmentation events rather than one). EPN survival was unaffected by treatment (P ≤ 0.05) in soil that was air-dried to disrupt antagonist activity prior to the experiment. When S. diaprepesi, S. riobrave, Heterorhabditis zealandica or no EPN were added to raw soil and S. diaprepesi was added 5 d later, the survival of both S. diaprepesi and of total EPN was greater (P ≤ 0.05) in soil that received no pretreatment than in soil pretreated with S. riobrave. Pretreatment of soil with H. zealandica or S. diaprepesi had less or no affect on survival of S. diaprepesi or total EPN. When nematodes were recovered from soil and placed on water agar, the number of S. diaprepesi that were killed by endoparasitic and trapping nematophagous fungi was greater (P ≤ 0.05) if soil was pretreated with steinernematid species than if the soil was not pretreated or was pretreated with H. zealandica. The adverse effects of pretreating soil on EPN survival were density dependent within a range of pretreatment dosages (20-100 IJ/cm2 soil surface), and the treatment effects required more time to become evident at lower than at higher dosages. These experiments suggest that non-target effects of augmenting the EPN community in soil vary among EPN species and have the potential to temporarily reduce EPN numbers below the natural equilibrium density.