Cotton Insect Situation No. 11 July 5, 2001 Blake Layton Extension Cotton Entomology Specialist Area Specialists: Dr. Gordon Andrews Dr. Scott Stewart Dr. Mike Williams Cotton Insect Hotline: 1-800-445-4931 Crop Situation: The Weather and Crop Report for the week ending July 1 indicated that 30% of the Mississippi cotton crop was setting bolls. This is only 1% ahead of the five-year average for this time. Seventy seven percent of the crop was rated as "good" or "excellent". The week of June 25 through July 1 averaged 4 degrees cooler than normal and statewide rainfall averaged 0.12 inches below normal. Boll Weevil Eradication Update: During the week of June 21-27, 97.5% of all Mississippi cotton fields were free of boll weevils. A total of 193 fields, or 13,064 acres, were treated that week. This brings the year to date total to 939 treated fields and 67,290 cumulative acres treated with ULV malathion. Bollworms in Bt Cotton: Once Bt cotton begins to bloom, it is much more susceptible to damage by bollworms than it is before bloom. This is because the blooms provide a site where toxin expression is lower, allowing young larvae to survive and reach a size and age where they can damage bolls. Overall, most varieties of Bt cotton are pretty effective against bollworms, even when they are blooming, but under heavy pressure enough larvae can survive to cause yield loss, unless supplemental treatments are applied. When scouting Bt cotton for bollworms, it is important to check for larvae in white blooms, inside closed red blooms, and under stuck bloom tags. Treatment is recommended if counts reach or exceed four larvae greater than ¼ inches long per 100 plants. Pyrethroids are very effective against bollworms in Bt cotton and are usually the most economical treatment. Cotton Aphid Survey Summary: During the week ending July 3, aphid populations continued to be low in all of the 18 fields involved in our statewide survey (See Table 1). However higher aphid populations, in the range of 10 to 25 per leaf, were observed in some fields that are not part of the survey. Table 1: Summary of Cotton Aphid Survey for the week ending July 3, 2001. Region Avg. No. Aphids Per Leaf No. Aphids/Leaf in Highest field No. Fields in Bloom North Delta 2.50 4.95 5 of 5 (NAWB = 5.6) South Delta 1.04 2.60 6 of 6 (NAWB = 6.9) Hills 1.44 4.95 0 of 6 (10.5 nodes) Scouting for Aphids and Deciding When to Treat: Scouting for aphids is a two step process. The first step is simply to watch for the presence of aphids, as indicated by spots of honeydew coated plants. Once aphids are detected in a field it is time to make more specific counts. This usually is done by randomly selecting plants, pulling the 5th leaf below the terminal and counting the number of aphids present. Because aphid populations can be quite high, scouts will need to carry a note card and pencil to write down the number of aphids for each leaf. After checking 10 to 20 plants, a calculator can be used to determine the average number of aphids per leaf. This is not as time consuming as it might sound, and it provides a definitive number that can be used to compare aphid populations from week to week. Most importantly, it takes the guesswork out of aphid treatment decisions. If you send three scouts into a field to "rate" aphid levels as low, medium, or high, you may get three different answers. But, if you send three trained scouts into a field to count aphids, their numbers should be in close agreement. One of the main points to keep in mind when counting aphids is that the goal is to get a count of the average aphid population for the field as a whole. It may be tempting to select plants from those heavily honeydewed spots, but this will bias the count. One of the best ways to avoid this bias it to have a pre-determined sampling plan, such as taking 50 steps and selecting the plant nearest your right knee, then taking another 50 steps to choose the next plant. When aphid populations are high, it is possible to reduce sampling time by only counting the number of aphids on one half (or even one quarter) of the leaf and multiplying by 2 or 4 to get the number of aphids per leaf. Because aphid populations can build to relatively high numbers before they require treatment, aphid treatment decisions can also be made on more subjective observations. These are definitely quicker and easier to make, but the information they provide is not as accurate as that provided by specific counts. Specific counts also make for better records and communication. There is no doubt what "45 aphids per leaf" means, but what does "aphids heavy" or "aphids building" mean? Based on subjective observations, treatment for aphids is recommended when the following conditions exist simultaneously within a field: There are isolated spots of plants that are covered with honeydew and heavily infested with aphids. Aphid numbers are increasing on remaining plants. There is no indication that the aphid fungal disease is present. For a threshold based on specific counts, treatment is recommended when counts reach 50 to 100 aphids per leaf and there is no indication that the aphid fungal disease is present. Note that for either threshold there is the implication that the presence of the fungal disease usually negates the need to spray. This fungal disease is the most effective and persistent treatment we have for aphids, so once it begins to spread insecticide sprays are not needed. Insecticide of the Week: Trade Name and Formulation: Denim 0.16EC Common Name: Emamectin Benzoate Producer: Syngenta Chemical Class: Avermectin Target Pest(s): Tobacco Budworm, Beet Armyworms Use rate range: 0.0075 to 0.01 lbs ai/acre (1 gal. to 21 acres to 1 gal. to 16 acres) Toxicity (rat/oral LD50 of technical product): 76 mg/kg Denim does not yet have a full, Secton 3, label, but it does have a Section 18 Emergency Exemption that allows use against tobacco budworms in non-Bt cotton or against beet armyworms this year. This is another Syngenta product, but it was developed by Merck. Cattlemen are familiar with a product known as Ivomec, which is used for control of both internal (worms) and external (lice, mites, flies) parasites. Denim belongs to the same general class of chemistry, the avermectins. The production of Denim is rather interesting. Syngenta also produces a miticide known as Zephyr (abamectin), which has been sold for use in western cotton for a number of years (at a premium price). Zephyr is produced through a "biological fermentation" process, much like Tracer. Denim is made by chemically modifying the abamectin molecule to produce emamectin benzoate, which is then formulated into Denim. Although the Section 18 exemption is limited to tobacco budworms and beet armyworms, Denim is effective against most caterpillars, and should control any loopers or other armyworms that are present when an application is made against budworms or beet armyworms. Given the origin of Denim, one question that naturally arises is "does it control mites?". As of now it does not appear that mites will be listed on the Denim label, but there have been a few mite trials in recent years in which Denim showed mite activity. This bit of information may be useful if one is trying to choose a treatment to control tobacco budworms or beet armyworms in a field that also has a building spider mite infestation. But, keep in mind that Denim is not labeled for mites and the Section 18 does not allow use in fields where mites are the sole target. One of the key points to keep in mind when using Denim against tobacco budworm is that it takes the full rate of 0.01 lbs ai/acre (1 gallon to 16 acres) to do a good job. The trial results shown below compare the efficacy of Denim to that of Steward, Tracer, and Karate (rates are expressed as lbs ai./acre). 999 Small Plot TBW/CBW Trial, Winnsboro, LA The information in this newsletter is provided for educational purposes only. Mention of specific products or trade names does not imply their endorsement above other products that are labeled and recommended for a similar use. Northeast Counties - Mike Williams: We continue to see scattered rainshowers, which are really welcomed. They have really assisted the corn crop to mature and are doing a good job of supplying water to heavily squaring cotton. There is a little movement this week in the aphid populations with a few reports coming in that aphids are beginning to build in a few fields. The hope remains that this will be too few, too late to cause us much damage. We are still seeing tarnished plant bugs in pre-flowering cotton in numbers high enough to be sprayed. Trappers this week caught a total of 3 boll weevils, we've not confirmed them officially yet, but are reasonable sure that they are weevils. Mr. Jones captured one near Onward in Sharkey county and Mr. Wall took two from Lee county traps, one at Barnes crossing and another 1 mile south. We've always seen thunder showers flush weevils from overwintering and it looks like this might be the way things are happening again. The BWE traps have also identified the Barnes Crossing area as one troublesome for weevils. Other pheromone trapped pests are low. We are still awaiting the moth flight, which usually comes about this time of year. Avg. No. Moths/Trap/Week County Budworms Bollworms Beet Armyworms Boll Weevils Attala 14 7 1 0 Bolivar 36 15 9 0 Calhoun 2 1 4 0 Chickasaw 37 0 0 0 Choctaw 0 5 1 0 Clay 39 3 0 0 Coahoma 29 3 1 0 Hinds 0 4 0 0 Holmes 11 1 1 0 Humphreys 13 7 24 0 Issaquena 4 7 7 0 Lafayette 81 1 2 0 Lee 11 0 0 2 total Leflore 17 2 3 0 Lowndes 16 2 1 0 Madison 0 3 3 0 Monroe 41 6 1 0 Noxubee 0 1 4 0 Quitman 16 3 2 0 Sharkey 11 6 8 1 total Sunflower 26 14 12 0 Tallahatchie 44 20 3 0 Tunica 42 4 0 0 Union 16 6 0 0 Warren 4 4 0 0 Washington 5 5 0 0 Webster 2 0 0 0 Yazoo 58 16 25 0   Southern Counties - Don Parker: Several fields are flowering and the crop retention looks very good. Plant bugs continue to present a concern in several areas. Stinkbugs are beginning to show up in several fields but at low numbers so far. Aphid populations continue to build, but I am not aware of any treatable levels thus far. Overall, the crop is looking good and the insect pressure has not been severe. It would be nice if this trend would continue. Corn Corner - I have looked at several fields across the state and have not found treatable numbers of southwestern corn borers. That does not mean that no fields need treatment, but simply that I have not found fields needing treatment. There have been fields treated due to threshold densities detected in the field. The moth trap numbers have increased, and we should watch the corn very close for the next week. We do not recommend treatment based entirely on moth trap data. Moth traps simply provide information about when moths are flying and whether or not more moths are emerging (based on weekly comparisons). The moth trap numbers do not indicate the level of infestation (if any) in a cornfield. A crisis exemption permit has been granted for the use of Intrepid 2F (Rohm & Haas-Dow AgroSciences product) on corn to control southwestern corn borers. This product is a molting accelerator and has residual activity. Please be sure to obtain a label before applying this product and follow the label directions. Delta Counties - Gordon Andrews: One thing that has changed this week in delta cotton fields is temperature. It is hot. Plant bugs remain the main concern. Sub-threshold plant bug numbers combined with scattered stinkbugs have triggered several insecticide applications. Plant bugs have increased near corn in fields scouted this week. In fields where nature enemies of aphids are low, aphids are building. Winged aphids were seen in some fields where aphid population has been low. This probably means that some aphid populations are at a level where winged adult aphids are being produced in the vicinity. Some cotton fields have been reported to have treatable population of aphids. Salt marsh caterpillars are decreasing and caterpillars observed were mostly large and will probably pupate soon. Heliothine eggs have been hard to find. Some leaf rollers are being seen in non-Bt cotton. This is uncommon for this time of the season. Some striped blister beetles were observed in a cotton field. They were probably beneficial since they were feeding very little on cotton leaves but had almost defoliated the morning glories in the field. Cotton that was at node above white flower 5 (NAWF5) last week is at NAWF 5.73 this week after a timely rain. We are going to try to make as many of these new squares as possible.

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