Alfalfa fields in western South Dakota are looking green and healthy with little or no pest insect activity. However, Anitha Chirumamilla, SDSU Extension entomology field specialist, warned alfalfa growers not to be complacent when it comes to scouting fields.
"Except for few spots in the northeast, much of the state has warmed up enough for alfalfa weevils to become active," Chirumamilla said, encouraging alfalfa producers to monitor for them.
Chirumamilla's warnings should be appropriate for other areas of the nation where alfalfa is grown and in typical stage of growth. Most likely other production areas have progressed without cool weather concerns slowing pest pressure. As noted, four or more generations of pests can attack alfalfa in a growing season.
Monitoring for weevils is being carried out every 10 days in many alfalfa fields. There are signs of weevil activity beginning. She said, "It looks like the on-off cold weather had slowed down the activity of insects along with the growth of plants."
Besides spotting weevil activity in isolated fields, several yellow butterflies, called alfalfa butterflies were seen flying in alfalfa and weeds on field edges. While these butterflies are pretty and pleasing to eyes, Chirumamilla said it is important to know that their larvae, known as alfalfa caterpillars feed on alfalfa leaves.
The butterflies are pale to bright yellow or orange in color with black wing margins and two dark spots on the front wings. The caterpillars are velvety green with a white stripe running on each side of the body. Unlike alfalfa weevil larva, Chirumamilla explained that alfalfa caterpillar feeds on entire leaves including the veins and mid-rib. "Larvae are very sensitive to touch and roll-up when disturbed or dislodged from plant. The best time to see them actively feeding is early in the morning," she said.
There are four to seven generations occurring through the crop season of May through October due to which different stages of the insect - egg to adult - can be seen at any time in the season. "Scouting for these caterpillars should be done using a sweep net and chemical spraying should be considered only if the number of larvae exceeds 10 per sweep," Chirumamilla said.
The other insect that was seen actively running in alfalfa fields was a lygus bug, commonly known as a tarnished plant bug. "It is a true bug about 0.1 to 0.25 inches in length, oval-shaped, and green to brown in color," Chirumamilla said.
She said it can be easily recognized by a distinctive triangle on the back and large bulging eyes. Nymphs are green and similar to adults but lack wings. They look like large aphids from a distance. Both nymphs and adults of lygus bugs have piercing and sucking mouthparts allowing them to feed on plant sap. The saliva of these bugs also causes necrosis of plant tissue at the feeding spot. Lygus species are severe pests in seed alfalfa and many other field crops but not a serious concern in forage alfalfa.
Chirumamilla said that neither the alfalfa caterpillar nor lygus bug are considered serious pests of forage alfalfa. "Presence of these might actually help feeding the natural predator and parasitoid populations," she said. "However, alfalfa caterpillars can be damaging in case of high populations. Scouting frequently with a sweep net and keeping an eye on insect species and populations is always a safe and secure practice."