Effect of four plant extracts on the mortality, population growth, and fluctuating asymmetry of Sitophilus oryzae

Abstract

AbstractManagement of storage pest insects relies heavily on chemical control, and there is a need to develop more sustainable management practices. Here, we evaluated the impact of 2% ethanol plant extracts of Ajuga reptans L., Ajuga pyramidalis L. (both Lamiaceae), Urtica dioica L. (Urticaceae), and Cannabis sativa L. (Cannabaceae) plants on mortality, population growth, and developmental stability [measuring fluctuating asymmetry (FA)] of the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae), a worldwide stored product pest. FA refers to small, random deviations occurring between the left and right sides of bilaterally symmetrical organisms; these deviations increase in response to environmental stress, making FA a reliable method to measure the impact of stress. FA was measured by means of geometric morphometrics, a method that allows for analyzing the whole landmark configuration of the insect, rather than taking single measurements. Extracts of the mentioned plants were used to treat maize (Zea mays L., Poaceae) kernels on which experimental populations of the rice weevil were grown, and we assessed mortality after 24–72 h, population growth after 30–90 days, and developmental stability after 90 days. Screening bioassays showed that S. oryzae adults were most affected by Ajuga extracts; Ajuga spp., especially A. reptans, significantly reduced population growth. In concordance, Ajuga extracts increased FA. The effects of U. dioica and C. sativa extract were less pronounced. None of the extracts significantly affected insect mortality. Overall, it can be concluded that A. reptans and A. pyramidalis are potential sources of bioactive compounds that may be further used for S. oryzae control. The results obtained confirm that variation in body shape asymmetry can be used as an indicator of population disturbance when insects are exposed to different types of stressors.