Insects of alfalfa in Manitoba with particular reference to Lygus spp., Adelphocoris lineolatus (Hemiptera: Miridae) and Acyrthosiphon pisum (Homoptera: Aphididae) and their natural enemies
Uddin, Md. Jashim
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The study was conducted from 1999 to 2001 to determine the most suitable method to be used for sampling insect pests on alfalfa, to determine the seasonal occurrence of common insect pests and natural enemies, to assess the roles of common natural enemies on insect pests, and to find out ways to increase the benefits from insect natural enemies in relation to the production practices of alfalfa in Manitoba. Among different sampling methods, sweep-net sampling was found to be the most consistent and reliable method of sampling most insect taxa, although beat tray sampling provided better estimates for Lygus spp. nymphs. It was found in the present study that Lygus spp. Hahn, Adelphocoris lineolatus (Goeze) and Acyrthosiphon pisum (Harris) were the most prevalent insect pests. Carabids, chrysopids, coccinellids, Nabis spp., Orius spp., spiders and opilionids were the most commonly occurring predaceous taxa. Circumstantial evidence from the field study and evidence from the laboratory study indicate that coccinellids play important roles in population growth of the above pests on alfalfa. Field and laborarory studies also provided evidence that carabids, coccinellids, chrysopids and nabids alone or together as a guild can suppress these pest populations on alfalfa. Parasitoids appeared to be important, as they killed a considerable percentage of Lygus spp. and A. pisum, but not of A. lineolatus. Aphidius ervi Haliday was the most prevalent aphid parasitoid. No control measures were required in hay fields, as swathing hay suppressed pest populations below damaging levels, partly through causing Lygus spp., and sometimes A. lineolatus, to move into adjacent seed fields. Conversely, insect pests were more numerous in seed fields, although these fields usually received chemical insecticides as pest control measures. Whether populations of natural enemies can be enhanced in seed alfalfa fields was examined, and it was found that nepetalactone, which is an aphid sex pheromone component, can be used to enhance natural enemy populations, and the benefits thereof, in seed fields. Successful rearing of Chrysoperla carnea (Stephens) and Chrysopa oculata Say larvae by using Megachile rotundata (Fabricius) (leafcutting bee) prepupae as the chrysopid larval diet expanded the use of leafcutting bees and provided a basis for inexpensive culture of chrysopids. The potential of insect pests and their natural enemies for management in connection with the current alfalfa production practices are discussed.