ISSUE 10 July 5, 2001
THISTLE CATERPILLAR IN SOYBEANS
Thistle caterpillar has been quite active in the region since late May, feeding on a number of crops in locations where Canada thistle were established. We are entering the second generation and some different crops are being affected. Currently, soybean appears to be the most common complaint. Here are some comments about management.
The caterpillar is known as the thistle caterpillar, the adult is the Painted Lady butterfly. It is easily recognized by the prominent, stout spines on the body. We are in the second generation, but there is a wide range of larval sizes. Caterpillars can be found from ½ inch to 1 ½ inches. They are not difficult to see in the field. Look for the feeding damage on upper leaves. Leaves are often webbed together and the caterpillar is resting inside.
In general, treatment of soybean is recommended when there is 25 to 30% defoliation prior to bloom; 20% after bloom or pod set. This normally requires about 4 to 8 caterpillars per row foot. If plants are still small, then best judgement may have to be made. One reference suggests a treatment guideline of 3 larvae per row foot in young soybeans, another when damage is severe and plants are being killed.
While inspecting some soybean fields on 7/3/01, the populations in the field were definitely greater the nearer one was to the Canada thistle patches. The farther away from the thistle, the lower the numbers of caterpillar.
Insecticides that mention thistle caterpillar under soybeans include: carbaryl (Sevin XLR), Pounce, and Warrior. Other insecticides that are effective against caterpillars in soybean are: Asana, Lorsban, Penncap-M, and Scout X-tra.
The thistle caterpillar feeds on a wide variety of plants. Their preference may be thistles, but they will readily feed on soybeans, sunflower, and many vegetables. In sunflower, the threshold is 25% defoliation provided that most of the larvae are still under 1¼ inch long. If the majority of the larvae are 1.25 to 1.5 inch long, most of the feeding damage will have already occurred and treatment is not advised.
The only insecticide that includes thistle caterpillar on the sunflower label is Warrior. As with soybeans, there are other insecticides approved for application that control caterpillars. Those insecticides include Asana, Baythroid, Lorsban, and Scout-Xtra.
WHEAT MIDGE UPDATE
Degree days have reached the point where peak emergence would be expected in the northwest areas of the state where midge populations were the greatest last season. The next 14 days in these areas will be important for monitoring any wheat that is heading in order to make treatment decisions. Emergence should be complete in the southeast quarter of the state. Scouting newly heading wheat in those areas should continue for another 5 to 7 days to be sure no midge are present.
WHEAT MIDGE NIGHT SCOUTING REPORTS
McLean County - Roseglen to Raub - Fields were scouted the night of 7/3/01. Scouting conditions were ideal for egg laying by female wheat midge – calm, near 60NF, and crop in the susceptible stage (heading). The majority of the fields in the heading stage were at economic threshold for both durum (1 per 7-10 heads) and HRSW (1 per 4-5 heads). However, one earlier planted field in the flowering stage was not at economic threshold. This area was rated as moderate pressures based on the fall soil larval survey.
Ward County - Minot to Carpio - Low numbers of wheat midges (<10) were found on white plate traps at the NCREC in Minot since June 29, 2001. Most of the small grain crop was not in the heading stage last week. This week crop is starting to head to flowering. Night scouting on 7/4/01 revealed very low numbers of wheat midge per head, ranging from 1 per 20 to 1 per 50 heads, or none. Scouting conditions were good. Fireworks were nice! This area was rated as low pressures based on the fall soil larval survey.
Mountrail County - Makoti - Low number of male and female wheat midge have been detected on white plate traps set-out on June 25, 2001. However, the majority of the small grain crop was not in the heading stage last week. Night scouting reports will be available from Wheat Midge Scouting Clinic on 7/6/01 at the Plaza Elevator at 8:30PM.
Pierce County - Rugby - Grower commented that few wheat midge were found in wheat fields on 7/2/01. This area was rated as very low pressures based on the fall soil larval survey.
Janet J. Knodel
Area Extension Specialist Crop Protection
North Central Research and Extension Center
OTHER WHEAT INSECT ISSUES: ARMYWORM AND APHIDS
Entomologists are accused of not reporting much good news. Well, the aphid populations in small grains are still small. Though wheat farther north in the state is still at vulnerable stages, it does not look like there will be many aphids in the region that could move from the more mature wheat to the younger wheat later.
Armyworms on the other hand are still a concern. Look for young larvae in the canopy of small grains. Leaf notching from the caterpillars feeding may be the only sign of infestations. Look for larvae at ground level if suspicious feeding is seen. Small (<½ inch) to medium (< 1 inch) armyworms are being reported. In general, the populations found or reported have been below the threshold of 4 to 5 larvae per square foot. Scout now in order to make a treatment decision that might coincide with your fungicide decision.
CORN BORER EMERGENCE, SOME FEEDING IN SOUTHEAST
Corn borer emergence is picking up. Peak emergence should occur within the week in our southern counties. Peak egg laying occurs about within 10 days. Scouting for corn borer should begin in seriousness by about July 10.
There have been reports of some corn borer feeding in the counties of Ransom and Richland. The two-generation corn borers that occur at low levels have been active since about June 10. Black light captures have been light, but the traps are better indication of general activity, not so good for assessing the size of the population.
ALFALFA INSECTS IN MISSOURI RIVER AREAS
Several calls earlier this week on feeding damage in alfalfa fields. Based on the description of damage, it sounds like two insects were feeding. Initial descriptions of fields described them as having a greyish or whitish cast. The larvae of the alfalfa weevil were causing some ragging of leaves will create this effect. In addition, some leaves were found that were white, but were not ragged up. In those cases, it is believed that alfalfa blotch leafminer larvae had been mining those leaves. Confirmation of this is pending.
For more information on these two insects, refer to the NDSU Crop and Pest Report, No. 5 from May 31.