Irrigation affects insect number
Alfalfa grown for seed in southern Alberta can be damaged by several insect pests. Although alfalfa seed producers are aware of the possibility of damage by alfalfa weevil, plant bugs and Lygus bugs, they may not be aware that the number of insects is affected by the method of irrigation.
At the Agriculture Canada research centre at Lethbridge, insect abundance on alfalfa seed crops was studied under sprinkler and flood irrigation. Alfalfa weevil counts were higher under sprinkler irrigation. The weevil lays its eggs in the alfalfa stem about five to 7.5 cm above ground.
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Eggs need high moisture to hatch. In flood irrigated fields, where only one or two deep irrigations are applied during the crop year, moisture levels within the crop canopy may not be high enough to permit all alfalfa weevil eggs to hatch.
Number of pea aphids was also twice as high under sprinkler irrigation. These insects seem to prefer more humid conditions for longer periods under sprinkler irrigation.
Numbers of Lygus bugs and plant bugs were higher under flood irrigation. They are highly mobile insects that disperse readily when disturbed. Water moving gently in a flood irrigation system disturbs them very little compared with bursts of water from a sprinkler.
Beneficial insects were also affected by irrigation method. There are more ladybird beetles and damsel bugs in sprinkler-irrigated fields. The number of predator insects was probably related to host insect populations, rather than to irrigation method.
Seed producers might be able to reduce the likelihood of infestations of a particular insect species in their alfalfa crops by using one or the other irrigation method. They would also probably increase the likelihood of more pest insects.
– Agriculture and Agri-Food Canada
