Cowpea Aphid in Alfalfa
We started to see some cowpea aphid in one of our alfalfa experiments. We received some reports of cowpea aphid populations in a small number of fields in central Arizona, as well as from alfalfa production area along the Colorado river. Cowpea aphid is sporadic in the region with numbers peaking from October to January in the desert. It seems cowpea aphid populations tend to appear earlier in the alfalfa production areas of the southwest low desert over the last few years. This aphid has an extensive host range, including beans and other legumes, cotton, and weeds. Beside feeding on the plant sap, this aphid injects a powerful toxin into the plant while feeding and, when their numbers are high, this can stunt or even kill the plants. Cowpea aphid produces a considerable amount of honeydew upon which sooty mold can grow; reducing photosynthesis and hay quality, and makes the alfalfa sticky, which causes problems with harvest.
There are few varieties of alfalfa that are resistant to aphids in general. Economic thresholds have not been developed specifically for this pest. Treatments may be necessary if large numbers of cowpea aphids are present. Until economic thresholds are developed for the cowpea aphid, use the following thresholds which were developed for the blue alfalfa aphid:
- 10 to 12 aphids per stem when plant height is under 10 inches, or
- 40-50 aphids per stem when plant height is over 10 inches.
Proper irrigation scheduling will eliminate stress on the plant that makes it more susceptible to aphid infestations. It is crucial to check fields for natural enemies, which can keep the populations of these insects in check. We noticed higher percentage of parasitism on this aphid in some fields. Border harvesting or strip cutting and altering harvesting schedule can be important for preserving natural enemies. Strip cutting provides a refuge for natural enemies to help combat pests later in the season.
We have some information about efficacy of various insecticides against alfalfa aphids in general. CLICK HERE for one of my article on alfalfa aphids, and their management decision-making.
Related articles:
UC Pest Management Guidelines - Alfalfa - Cowpea Aphid
UC Pest Management Guidelines - Alfalfa - Harvest Scheduling and Harvest Impacts on IPM