Mowing Versus Insecticide for Control of Alfalfa Aphids and their Differential Impacts on Natural Enemies

A study was conducted in a alfalfa fields on the loess plateau in northwest China to determine the impact of mowing and spraying imidacloprid on alfalfa crops on the population dynamics of aphids and their natural enemies. The results showed that mowing was very effective in controlling spotted alfalfa aphid, Therioaphis trfolii (Monell), and pea aphid, Acyrthosiphon pisum (Harris). Mowing in early June significantly reduced the average number of both aphid species in different seasons and kept them at low density. To a lesser degree, populations of their predators were also reduced under mowing. Consequently, the ratio of predators to aphids increased tremendously. A lower predator-aphid ratio occurred after spraying with imidacloprid, which led to an increase in the seasonal average number of both aphid species and a decrease in number of predators, including coccinellids, green lacewings and syrphid flies. The majority of these predators were killed by the insecticide, resulting in rapid recovery of the pests and severe damage to the crop. Based on the findings, it is recommended that the alfalfa crop should be harvested as early as possible if aphids are the only targeted pests, especially when the aphid populations peak near the harvest season. In seed fields, necessary chemical pest control should utilize highly selective insecticides. Sustainable pest control practices are important for agricultural stability. Our findings show that properly timed mowing of alfalfa fields is more effective at controlling pest aphids and maintaining their natural enemy diversity than the spraying of imidacloprid. This Insecticide had negative impacts on natural enemy composition and was only temporarily effective against aphids, leading to rapid rebounds in their populations. We suggest that toxic pesticides be avoided when possible to sustainably control pests.