Watching Alfalfa Insects
University of Minnesota Extension, Stearns County News
July 2, 2014
Source: Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties
Watching Alfalfa Insects
By Dan Martens, University of Minnesota Extension
FOLEY, Minn. (06/27/14) — There has been a noticeable amount of spraying for alfalfa insects over last 10 days or so. As farmers wait on the weather for finishing first crop harvest and starting second crop harvest, it makes sense to watch for insects in alfalfa.
Making strategic decisions about insect pest control based on established thresholds and diagnostic procedures is the best way to take care of the crop, the bank account, the environment, bees and other beneficial insects, preventing resistance issues, and reducing pressure for more regulations.
ALFALFA WEEVIL. Check new regrowth. Inspect 20 one-foot square areas and treat if there are 8 or more larvae per square foot or regrowth is suppressed by feeding. The larvae can be difficult to find. They might be 1/16 to 3/16 of an inch long, a dull creamy-greenish color with a black head. As the larva grow, they will have more of a pale green color with a white stripe down the middle of the back. In a taller crop, we might treat if we find 20 per sweep with a sweep net. We might treat if we find 30% of stems with signs of leaf feeding. Feeding is typically indicated by holes eaten out of the leaf and when severe, you might have just a net of leaf veins left in the leaf.
For all alfalfa insects it is important to consider the pre-harvest window for insecticides. If close to harvest, it could be better to harvest a little earlier if the weather allows and then watch the next crop. Spraying earlier or later in the day can reduce risk to foraging bees and other pollinators. Even when alfalfa isn’t blooming, clovers or other plants mixed in the field can be.
POTATO LEAFHOPPER (PLH) are small, bright light green and wedge shaped – larger on the head end and tapering to the end of wings on the tail end. The general threshold is finding 1 PLH per 10 sweeps with a 15 inch net for each inch of crop growth. When alfalfa is 3 inches tall, finding 3 PLH in 10 sweeps of a net is enough to merit control. At 10 inches, this means finding 10 PLH in 10 sweeps. With the most advance genetics for PLH resistance, the threshold might be 3 times the normal threshold. PLH suck juices from the leaves. This usually shows up first as a yellow v-shaped patch at the tip of the leaf, and eventually the whole leaf can turn yellowish-brown to purple in color. Extension pest management specialist says PLH hit in a big way starting last weekend in the Lamberton area.
PLANT BUGS look a lot like boxelder bugs in terms of body structure. There are smaller. Those called alfalfa plant bugs have more green color on them and can be 3/8 inch long. They are more serious. The other common type is called tarnished plant bugs - about ¼ inch long or so and colored more in shades of brown. Plant bugs cause a pinched or crinkled look in the middle of the leaf.
One threshold is to treat if there are 3 plant bug adults or nymphs per sweep on alfalfa less than 3 inches tall and 5 or more on taller alfalfa. If damage is found within 7 to 10 days of harvest, you might harvest earlier, or carefully consider the pre-harvest interval of insecticides. Another reference says to cut the threshold in half if the alfalfa plant bug is the dominant species or if potato leaf hoppers are also present. For example, if potato leafhoppers and plant bugs are each present at half their threshold, control efforts would be warranted.