Diatomaceous earth outperformed horsetail powder in laboratory assays on the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera, Bostrychidae)

This study aimed at comparing the insecticidal and repellency activity of a commercial diatomaceous earth (DE) formulation (Silicosec®, Biofa AG) and a horsetail powder - Equisetum telmateia (Ehrh.) (Equisetales: Equisetaceae) against the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae). These substances were mixed with soft wheat grains at concentrations of 1000 and 2000 ppm and tested on unsexed adult beetles of mixed ages. Insecticidal assays were conducted at three exposure intervals (2, 7 and 14 days), after which the wheat samples were destructively sifted and mortality was recorded. The highest mortality was achieved with the DE, and the percentage of dead beetle significantly increased with exposure interval and concentrations. On the other hand, treatments with the horsetail powder did not significantly affect beetle mortality, regardless of the dose and exposure interval. Repellency tests, which were run according to Mohan and Fields’s method, showed a higher activity of both doses of the DE in comparison to the horsetail powder. The difference between the two application doses of the DE was statistically supported only at 24 h of exposure, whereas no differences were detected between the two doses at 48 and 72 h. Based on our findings, the horsetail powder seems not to be a reliable alternative to DEs for the control of R. dominica . Considering both efficacy and cost-effectiveness, the 1000 ppm DE dose could represent the best compromise for the management of the lesser grain borer in commercial applications.