Efficacy of combining Beauveria bassiana and local diatomaceous earths against two coleopteran stored product insect species

In this study, the efficacy of combining local Beauveria bassiana isolates withdiatomaceous earths collected from Turkey against Rhyzopertha dominica andSitophilus oryzae adults was investigated, together with the importance of ambientconditions on applications. Each experimental unit was set using a 50 ml centrifuge tubecontaining 40 gr wheat kernels and 20 individuals. Diatomaceous earth and fungus weremixed into wheat prior to the experiments. Clean wheat was used for control units andexperiments were conducted inside humidity chambers at 25±1ºC and 65 % relativehumidity in darkness with five replications. Mortalities were assessed after 7 and 14days. After removal of the live ones they were kept for 65 days to determine the numberof F1 progency produced. B. bassiana 5-4 and six diatomaceous earths, one of which isa commercial product, were applied at 300 ppm concentration to R. dominica adults. Alldiatomaceous earths caused low mortalities and the highest mortality was due to isolate5-4 (42% and 69% on 7th and 14th days, respectively). The highest mortality (92% on7th day and 99% on 14th day) amongst combined applications was caused by 5-4+AGN. As a result of single applications to S. oryzae adults, the highest mortality wasdue to a local diatomaceous earth ACN (42% and 69% on 7th and 14th days,respectively). The highest mortalities on 14th day amongst combined applications werecaused by 5-4+ACN and 5-4+AGN, which were statistically different from the rest. Inthe second set of experiments, the concentration of ACN was kept constant at 300 ppmand its combinations with three B. bassiana isolates at three concentrations (200, 250,300 ppm) were tested against R. dominica and S. oryzae adults. Statistical differencesamong the treatments were not recorded for R. dominica. However, ACN+1-1 wasfound to be the most effective against S. oryzae, and reducing concentration of thefungus mostly reduced the efficacy of the combination. Moreover, it was noticed thatcombining ACN with isolate 5-4 reduced progency production of S. oryzae adults morethan the others. The final set of experiments was carried at two ambient temperatures (25oC and 30oC) and three relative humidities (45, 55 and 65 %). Mainly, theeffectiveness on R. dominica adults did not change at tested relative humidities at bothtemperatures. For S. oryzae adults, the effects of the ACN combinations with both fungiat 25oC decreased at 65% relative humidity. At the same relative humidity, a similarreduction was recorded for combination of ACN with 5-4 at 30oC. In conclusion,depending on the pest species, combined applications can have higher efficacies thansingle diatomaceous earth or fungus applications. However, appropriate selection ofboth diatomaceous earth and fungal isolate is important for a higher efficacy of acombination. The effect of ambient conditions on combined applications is mainlyparallel to the effect of the ambient conditions on the most effective component on themortality of the targeted species.