Alfalfa Weevil Damage Complete For 2012

Concern about future damage caused by alfalfa weevils continues to exist, especially when larvae are still found in great numbers on the cutter bar of the swather. Rest assured the weevil larvae are at the end of their feeding period in North Dakota.

The next stage of the weevil larvae is the formation of pupae or cocoons. This is a non-feeding stage. Adult weevils emerge from the cocoon in one to two weeks and then move to sheltered areas under other plant residue. There they go through a period of inactivity. As the temperatures begin to drop in late summer or early fall, adults will resume activity and begin feeding on foliage. However, such feeding is non-economical. The adults will over winter in shelterbelts, ditches, other areas of heavy plant growth and in the crowns of alfalfa plants.

No one can predict the possibility of damage again next year. Weevils do have natural enemies and winter conditions may affect survival. The best approach to prevention is timely scouting next spring.

In the spring, the adult beetles feed for a few days and mate. The females lay their shining, oval, yellowish eggs in the stems of alfalfa. They first make cavities in the stems with their beaks and in these insert from one to as many as 40 eggs. Each female weevil lays anywhere from 100 to 800 eggs during the spring. The weevils fly actively on calm days above 70 degrees. Upon hatching the larvae are nearly white but soon become green with a prominent, white, mid-dorsal stripe. They feed on the interior of the stalk for 3 to 4 days then make their way to the opening leaf buds at the tips of the plants where they feed for some time. There are three to four instars over a period of 25 to 58 days. Most of the larvae become full-grown about the time of cutting the first crop. With this in mind, scouting for the presence of weevils should start in early May.

Tree of the Week

I recently visited a site where there were many, many green ash trees. It definitely was a monoculture as there was no other tree species in site. Unfortunately, approximately one half, my best estimate, of the green ash were either dead or struggling to maintain life. Now, green ash is a very hardy tree native to North Dakota. So, it can withstand a lot of hardship including drought.

It did not take long to conclude this population of green ash is being devastated by ash borers. Now I next call attention to you readers that these borers are not the same as the Emerald Ash Borer which we have been reading so much about in recent years. However, in this situation the outcome is about the same.

I call this to your attention because the first question most people ask after they are encouraged not to plant green ash or American elm is “What is a good substitute? After all, both are native to North Dakota and both have proven to have good drought resistance”.

My response to an alternative for both of these “loved” trees is common hackberry. In this area most common hackberries may not grow as large as American elm but they have similar form and perform well on a variety of soils. Its tolerance to drought is not at the level of green ash but it should do very well where occasional water supplementation is available or on sites where other plant growth is held to a minimum.

The leaves of common hackberry are bright green on the topside and a paler green on the bottom. In the fall the leaves generally have a yellow color. They do have a pitted fruit that is round and about ¼ inch in size. When young, the tree tends to have a ragged appearance but does broaden and arch at the top as it matures.

Give common hackberry serious consideration when you are thinking about green ash.