Effects of post-application irrigation and substrate moisture on the efficacy of entomopathogenic nematodes against western flower thrips, Frankliniella occidentalis

The efficacy of entomomatogenic nematodes (Steinernema bicornutum Tallosi, Peters and Ehlers and/or Heterorhabditis indica LN2 Poinar, Karunakar and David) against the soil‐dwelling life stages of western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) was assessed under different moisture conditions in a commercial plant‐growing substrate in laboratory experiments. In the first experiment, both nematode species were tested at substrate moisture ranges of 67, 78, 88, or 95% relative moisture content, that were maintained before applying the nematodes at 100 or 400 infective juveniles (IJs) cm^?2. In the second experiment, 10, 25, 50, 100, or 120 ml irrigation water, resulting in relative moisture contents of 72, 81, 90, 99%, or more than the saturation level of the substrate, respectively, was applied to the substrate. Heterorhabditis indica LN2 was applied either in 3 ml water and followed by irrigation, or by suspending the infective juveniles in the water amounts indicated above to apply the nematodes in higher volume. Results indicated that at the higher application rate, initial moisture content did not significantly affect the efficacy of H. indica LN2. On the other hand, increasing moisture content resulted in an improved efficacy of H. indica LN2 and S. bicornutum at lower and higher application rates, respectively. Similar thrips control levels of 44 and 60% at the lower and higher application rate of H. indica LN2, respectively, were obtained at 88% relative moisture content. In the second experiment, higher and statistically similar thrips mortality of 40 and 50% at lower and higher application rates of H. indica LN2, respectively, were obtained when the infective juveniles were applied in a high volume suspension of 100 ml, or when followed by irrigation with 25 ml water, resulting in both cases in 81% relative moisture content. Generally, efficacies of the nematodes for thrips control can be improved by using an appropriate moisture content and/or post‐application irrigation. Thus, the high nematode application rates required for successful F. occidentalis control can be partly attributed to substrate moisture content and/or post‐application irrigation.