This time of year alfalfa can really be hit hard by weevils. Here in the Central Valley we have to deal with the Egyptian alfalfa weevil (Hypera brunneipennis). These pictures are from about a week ago on a plot that has had a heavy infestation the last few years. As you can see the feeding damage can be significant.This first picture is of a mature larva - the stage that does most of the damage. Smaller larvae don't eat nearly as much as the larger ones. This particular individual is about a quarter of an inch long.
It surprises me how much of a problem these weevils have become. They are really not that hard to control if you disrupt their life cycle with an early cutting. This only needs to be done once (on the year's first growth) because the insect only goes through a single generation a year. Here in the Central Valley, a cutting around the middle of March would kill most of the population. A good pyrethroid will do a good job too if you decide to wait and make the first cutting with a more mature crop.
The Second picture is of the cocoon. When the larva gets as big as it is going to get, it will find a secluded place on the plant (sometimes on the ground) and encircle itself with a silky protective covering. You can see that it isn't as tightly constructed as the cocoons of other insects but it does the job at keeping most predators and parasitoids away.
This last picture is the mature adult - no doubt out looking for a mate