Insect Threats to Alfalfa

The alfalfa weevil, Hypera postica, is the major yield-reducing insect pest attacking alfalfa in Arizona and western U. S. during the winter and spring season. The damage caused by this insect coincides with the prime cuts of hay that are considered the main feed for dairy cows in Arizona. Because of this economic consequence, growers and agricultural professionals were asking for accurate economic thresholds to inform their decision for control measures against this serious pest.

The University of Arizona Cooperative Extension Field Crops IPM Program conducted multi-year research to investigate alfalfa weevil thresholds. Our results indicated that an action threshold of an average of 2-4 large larvae per sweep justifies the application of control measures. This threshold can be dependent on hay price and the cost of treatment.

Sweep net samples should be conducted in at least four areas in the field (five sweeps per area). It is important to use the proper monitoring methods for alfalfa weevils to determine their threshold levels. Using the right pattern for sweep net sampling is equally important. After many observations of sweep net sampling techniques in alfalfa, we’ve noticed that different “swing patterns” resulted in different insect pest counts. The economic thresholds we use to monitor and justify treatments for alfalfa insect pests are based on 180-degree sweeps. Using other sweeping techniques may give different results and may lead to either spraying too late or unnecessary spraying. As stated earlier, this threshold is based on counting large alfalfa weevil larvae.

Alfalfa varieties with rapid spring growth may be more tolerant of weevil damage and early harvesting can mitigate damage. It is important to note that surviving larvae under windrows may reduce subsequent alfalfa growth.

It is also critical to correctly distinguish alfalfa weevil larvae and adults from other insects that could be in the field at the same time of year.

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.

Cutworms are frequent pests in the low desert of Arizona. The granulate (Feltia subterranea) and the variegated (Peridroma saucia) cutworms are the two most common species in alfalfa.

Full-grown caterpillars are about 1.5 to 2 inches long and vary in color and patterns. Larvae frequently roll into a C-shape when disturbed. Cutworms feed at night and hide during the day in soil cracks and under debris.

Cutworm populations may develop in weedy areas and migrate into seedling or mature stands.

Established fields are damaged when cutworms cut off new growth or feed on the alfalfa foliage. Established alfalfa fields can be severely injured when cutworms cut off new shoots at or below ground level following harvest. The pest often goes undetected after cutting and hay removal, but the problem becomes apparent when the field is irrigated and there is little or no regrowth.

Seedling alfalfa stands can be severely damaged by cutworms cutting the seedlings off at or just below the soil surface. Cultural control is important in managing cutworms in alfalfa, especially for new stands. Cutworms are most injurious in fields with high plant residue. Tillage helps to limit cutworm populations; seedlings in well-tilled fields, especially when there is an interval between crops are less likely to have cutworm problems. Keep the field and field edges weed-free. Flood irrigation can drown many cutworm larvae. Flood irrigation during the day will attract many birds that prey on the cutworms as the advancing water forces larvae from hiding. When damage is severe in seedling fields, apply an insecticide bait.

Treatment guidelines have not been established in Arizona. Check for cutworms by looking under duff and carefully digging to a depth of 1 inch in loose soil near alfalfa crowns. When cutworm numbers exceed one or two per foot of row or severe damage is apparent, it may be necessary to treat. If treating with baits, apply baits in late evening or at night when cutworms are on plants.