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Field and Forage Crops |
The levels of migrating adult thrips seem to have declined in some areas this past week, especially where timely rainfall has occurred. However, damaging thrips levels are still present in many cotton fields in which the residue of the at-planting insecticide has significantly diminished (most often in April-planted cotton), or in most situations in which three weeks or more has passed after planting Gaucho or Cruiser-treated seed. Western flower thrips are still occurring in the vast majority samples sent to the Plant Disease and Insect Clinic at North Carolina State University, primarily from cotton fields which had already been treated with moderate rates of a foliar insecticide. A few samples, however, have shown very few western flower thrips.
In general, fields in which a 5 pound or greater rate of Temik 15G was used appears to be holding up well where soil moisture has been adequate. This was the case in a few early May-planted cotton fields in Craven County in which it was difficult to find thrips on May 30. In an at-planting insecticide test at Rocky Mount, on May 27 (3 weeks after planting) Temik at the 5 pounds of product rate showed lower thrips levels (9 per 5 plants) than Gaucho alone (181 thrips), Cruiser alone (108 thrips), Gaucho + 0.25 pound of active ingredient per acre of Orthene (72), Gaucho + 3 pounds of Temik 15G (31), Cruiser + 0.25 pound of active ingredient per acre of Orthene (15), or an Orthene spray at 0.25 pound of active ingredient per acre (22). This test shows the high thrips levels (and probable associated damage) with seed treatments if not sprayed inside of three weeks after planting when moderate to high levels of thrips are present.
Because western flower thrips are difficult to control with insecticides, apparently only the highest rates of Orthene (0.5 pound of active ingredient per acre or higher) have made much of a dent on these populations. Monitor has also shown some activity against western flower thrips in other states, but is very expensive at these high rates. Other foliar insecticides appear to offer limited activity against western flower thrips. Continuing to treat cotton fields with low or medium rates of Orthene or other materials will likely yield disappointing results where damaging levels of western flower thrips are present. Where possible, a foliar band can help keep the Orthene rate high at a more reasonable cost. Although usually not helpful in most insect control situations, a spray adjuvant may help reduce thrips levels.
If newly-emerging leaves look shiny and relatively straight and immature thrips are at or less than about one per true leaf, or if cotton has reached approximately the 5 true leaf stage with adequate moisture, spraying may not be needed. Also, in general, expect thrips levels to be quite a bit lower in strip-till or no-till cotton fields, so try to make treatment decisions on a field-by-field basis. Also, if history holds, levels of adult migrating thrips should be on the decline (not a day too soon).
The combination of extensive foliar insecticide treatments for thrips, along with the continuation of hot, dry weather, could be a recipe for increasing levels of spider mites on cotton. Some initial reports of spider mites were received in the past week from Northampton County. This is another cotton pest that is difficult to effectively and cheaply control with insecticides. As is the case with thrips, significant widespread rainfall across the state would greatly reduce the probably of mite damage in the coming weeks.
Thrips are abundant and in some situations causing severe damage to peanuts. There is a lot of variation from field to field as to the number of thrips and the amount of damage. Dry conditions have limited the effectiveness of at-plant, in-furrow insecticides in several locations. Peanuts in areas that have received some rainfall appear to be getting good control despite the relatively high insect pressure. There has been more phorate (Thimet) used this year to help with tomato spotted wilt virus and I have observed quite a bit of phytotoxicity this spring which is not unusual with the use of this product. Scout peanuts carefully over the next few weeks to determine the need for additional treatments in the form of foliar applications of insecticides. The threshold is to treat when more than 25 percent of the newly-opened leaflets show signs of thrips injury (wrinkled, scarred leaflets). This threshold is to prevent damage and yield loss from the physical damage done by thrips feeding and has nothing to do with tomato spotted wilt virus. If the damage is very slight then you may delay treatment and scout again in a few days. Be sure to look at newly-opened leaves as this provides the most current appraisal of what the thrips are doing in your field. Examining older leaves only provides a history of what the thrips were doing several weeks ago.
Western flower thrips are more abundant this year, but I do not suggest that we do anything differently. The increased abundance of thrips this year may also mean higher levels of tomato spotted wilt virus in peanuts. Remember that the only effective measures for the management of tomato spotted wilt virus occur prior to and at planting. Additional foliar insecticide sprays for thrips will, most likely, do little to reduce the level of virus. The less favorable the growing conditions, the more likely that thrips will do damage affecting yield. The best cures for thrips at the present time are good soaking rainfall and warm temperatures.
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Last modified on June 3, 2002 by Stephen J. Toth, Jr.