Right now in the Central Valley of California, alfalfa leaves are being skeletonized by little green worms. They are about a fourth of an inch long and are proportionately plump. Much of the feeding is done at night if it doesn’t get too cold, although they were out in numbers last week (here in Fresno) in the middle of the day when the sky was overcast.
These little worms are larvae of the Egyptian alfalfa weevil, Hypera brunneipennis, and they’ve been in California and Arizona since the spring of 1939. How they got here is anybody’s guess. Originally they’re from the Middle East. But they seem to like America a bit more - at least they‘re a lot more abundant over here.
Lawrence Wehrle was the first to discover them. He was working on a farm at the University of Arizona in early April when he saw feeding damage on the fenugreek (a clover-like legume). He soon discovered the small worms responsible but supposed they were alfalfa weevil (Hypera postica) larvae. Alfalfa weevils were known to cause damage further north (having first appeared in the US near Salt Lake City in 1904) and Wehrle feared that they might have made it all the way to Arizona.
Unfortunately he was wrong. The sample he sent to Washington DC was identified as the Egyptian alfalfa weevil - a related species. This was unfortunate because the Egyptian alfalfa weevil has done a lot more damage than the alfalfa weevil in many parts of the Western US. Seventy years later, we still have a hard time keeping it under control.
That doesn’t mean there aren’t options. Fairly early on, entomologists discovered that if the first alfalfa cutting were done a bit earlier than normal (just before flowering) that some of the weevils (adults and larvae) would die. The larvae didn’t have anything left to eat and they ended up getting cooked in the heat without places to hide. Even some of the vulnerable newly emerging adults died too.
Some creative farmers went a step further and pulled a brush or wire drag through their fields. This not only dislodged any remaining larvae but it also kicked up a lot of dust that killed even more (dust is one of the oldest and most effective ways to kill many kinds of insect pests). Other ways to kill the weevils included spraying with lead arsenate or zinc arsenate. These chemicals, of course, are not available options today but other insecticides often work well. Sometimes it isn’t necessary to treat at all if the infestation is light and if the first cutting can be made soon enough.
The reason for this is that the weevils only go through one generation a year. This is not generally the case with arthropod species that become key pests. Often they become pests because they can go through multiple generations a year and several pesticide applications become necessary. With the Egyptian alfalfa weevil, the damaging larvae only occur in the spring. If they can be controlled then (now) they no longer present a problem.
But there are a few things to watch out for. To start with, it’s not a good idea to let the cut alfalfa sit around for very long. Windrows can hold moisture underneath where the larvae can survive and cause problems to new growth. Many of the little worms will pupate and ensure a healthy pest population the next spring.
Another thing to watch out for are the beneficial insects. They will be building up in the field about the time of the first cutting. If a late insecticide application is made, you might wipe them out and have a big aphid problem later on.
This doesn’t mean that beneficial insects will help you much with your weevil problem. They won’t. There is a little wasp that lays eggs in alfalfa weevil larvae and is pretty good at keeping their numbers down but it has a hard time with Egyptian alfalfa weevil larvae. For some reason, the haemolymph (insect blood) is able to prevent the little parasitoid wasps from developing.
Right now is the time to go take a look at your field. Chances are, you’ll find something out there enjoying a free lunch. Of course, depending on where you live, you might find other things instead. Alfalfa weevils are also active now. So are clover leaf weevils, clover head weevils and the lesser clover weevil. They all look pretty much alike. In fact they like to eat pretty much the same things - clover-like legumes. Let’s face it. This is a weevily time of year. We and our legumes had better get used to the fact.