ISSUE 4 May 24, 2001
COLD TEMPERATURES, INSECTS, AND INSECTICIDES
A popular set of questions this week has been what might be happening with flea beetles in canola, cutworms in row crops, how are the temperatures affecting their activity, and if we apply an insecticide at low temperature, what might happen.
First, how are the temperatures affecting the insects? The flea beetles become active at temperatures of 57EF. Activity increases with warmer temperatures. In North Dakota we expect most of our field activity at temperatures of 68EF and higher. We are entering the point in May where flea beetle activity typically increases as beetles disperse from their overwintering habitat. At temperatures lower than mentioned above, the flea beetles tend to sit idol on plants and in soil cracks. They are not very mobile. This tends to concentrate feeding damage to field edges and small patches. If they do move, they tend to walk. They fly when temperatures are warmer and the day is sunny. Cooler temperatures along with moisture will favor the young canola plants. They continue to grow and are not subjected to heat and moisture stress. True leaves continue to develop, though at a slower rate. So, reduced feeding and no heat and moisture stress favors canola growth. At the same time, feeding by flea beetles is reduced, and their mobility will be limited.
Some Dingy cutworm activity was reported from the central valley and in central ND in row crops. In general, these cutworms are more likely to be found in areas where there was a legume grown last year or where heavy crop residue was present. These cutworms overwinter as partially grown larvae. As soil temperatures warm in the spring, they become active on the surface. We expect feeding when soil temperatures are in the 50+EF range. On May 22, soil temperatures in the eastern half of North Dakota were in the upper 40's to low 50's. Soil temperatures were slightly higher in the west. Marginal for much active feeding. Combine this with nightime lows around 40EF, the cutworms will stay quiet, waiting for some warmer, drier times.
Regarding insecticides and temperature. The flea beetles and cutworms are both treated most frequently in the region with pyrethroid insecticides. For the flea beetles in canola, the insecticide is Capture. For the cutworms the products include Asana, Ambush, Baythroid, Capture, Pounce, Scout Xtra, and Warrior, depending on the crop.
In general, permethrin (Ambush, Pounce) most often has a negative temperature toxicity. This means that as temperatures decrease, the toxicity level of the insecticide increases.
Many of the remaining pyrethroids have a different molecular trait that results in the reversal of this relationship. They more often display a positive temperature toxicity, or increased toxicity with increasing temperature. However, there are exceptions. The temperature effect on their toxicity varies by insect species and by a specific active ingredient tested. Sometimes the result is reversed, or a negative relationship occurs.
Why this variation? Suggested explanations are due to different chemical structures of the separate pyrethroids, or the ability of the insect to detoxify the compound which may change with temperature, or the activity level at the target site in the nervous system.
Most of the pyrethroid-temperature interaction studies that have been published evaluated the effect within a temperature range of 60E to 98EF. The low temperature is below the current daytime temperatures in much of the state at the time of this writing. By the time you read this, it should be warmer (we hope!).
So, should we be treating during colder temperatures? The preference would be to wait until temperatures are above 60EF, particularly with the newer pyrethroids. Keep in mind the target pest, as well. The insect needs to be active in its habitat to either be contacted by the spray, come in contact with a treated surface or ingest treated plant material. If cooler temperatures are reducing the feeding and general activity of the target pest, they may not come in contact with sufficient insecticide to achieve adequate control.
LEAF INJURY IN SWEET CLOVER
Sweetclover weevil has been reported from southwestern ND. The adult weevils can be found feeding on the foliage in established sweet clover fields. These dark gray snout weevils (1/4 inch long) chew crescent-shaped, jagged notches in leaves. The damage is usually easier to find than the small weevils.
The weevils overwinter in plant residue in and around sweet clover fields. When temperatures warm to 40o F in the spring they begin feeding. The weevils can fly and may move among sweet clover fields. If weevils locate field sites where new sweet clover seedlings are emerging, their feeding can be very destructive. The results are thin or completely destroyed stands. To reduce the risk of sweetclover weevil damage in new sweet clover plantings, sow new stands as far away as possible from second-year stands.
Damage to second-year stands occurs as clover starts to grow. Heavy infestations can thin and stunt these stands at this time. It is not usually necessary to control weevils in second year stands. With favorable weather, the established clover will outgrow the damage. If clover fields are cut for hay or silage, consider a shallow cultivation after hay removal. This operation will destroy this year's new weevil generation which will still be in the top soil layers as the larval or pupal stages.