EFFECT OF SOIL MOISTURE ON THE PERSISTENCE AND EFFICACY OF HETERORHABDITIS BACTERIOPHORA (RHABDITIDA: HETERORHABDITIDAE) AGAINST ANASTREPHA LUDENS (DIPTERA: TEPHRITIDAE) LARVAE

The efficacy of Heterorhabditis bacteriophora (Poinar) infective juveniles (IJs) was evaluated against third instar Anastrepha ludens (Loew) (Diptera: Tephritidae) under laboratory conditions in a sandy clay soil at various levels of soil moisture. Three experiments were performed in which the efficacy of the IJs against A. ludens was estimated, i.e., (a) at 6 different levels of soil moisture, (b) in soil that was allowed to lose moisture over a 15 day period, and (c) in soil with an initial moisture content of 16% and in which moisture loss was partially mitigated by adding water at 5-day intervals. In the first experiment, the greatest A. ludens mortality (80%) was observed in soil with 18% moisture (-63.1 bars), although this was not significantly greater than A. ludens mortality at 21% moisture (-20.4 bars). At 24% soil moisture (-7.70 bars), percentage of mortality of A. ludens declined to about 50%. Likewise insect mortality was substantially lower at soil moisture levels of 15% (-240.1 bars) and 12% (-1,232 bars) and very much lower (about 16%) at 9% soil moisture (-10,147 bars). In the second experiment, as soil moisture declined from 16% to less than 10% over a 15 day period, the infectivity of IJs, as indicated by A. ludens larval mortality, progressively declined from more than 55% to less than 10%. In the third experiment, in which moisture loss was partially mitigated by adding water at 5-day intervals, the decline in infectivity of IJs was gradual up to 21 days, but decreased thereafter. We conclude that soil moisture levels must be carefully considered when applying H. bacteriophora IJs to control A. ludens under field conditions, because soil moisture has a marked effect on the efficacy of IJs for the biological control of this pest.