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Volume XV no. 10 August 10, 2001
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IN THIS ISSUE: Cotton Insects
The past week high temperatures have again been somewhat above normal. Low temperatures have been closer to normal. No significant regional rainfall has occurred, although, some spotty showers have provided relief for some fields. Insects have remained a minor threat to cotton in most fields. Boll weevil activity is picking up based on increased trap catches but thus far is still hard to find in fields. Whiteflies remain a concern in a few fields but thus far the area as a whole has avoided this problem insect. Both bollworms and beet armyworm problems are few and far between and aphids continue on hold for another week. Plant bugs need to be watched for a little while longer to protect bolls that are 10 days or younger. But the season as a whole is starting to wind down for insect problems. Really only about 3-4 weeks left for the later cotton.
The season is rapidly coming to a close for many cotton fields as far as insect problems are concerned. Luckily for us, most fields have not had much pest pressure this year and very few fields up to this point have been sprayed by producers. This is a blessing following such a severe pest year in 2000 and the poor economics of growing cotton this year. We are seeing more fields cutting out and even a few marginally irrigated fields with open bolls. For most areas in the High Plains, the probability of producing a harvestable boll from a white bloom on August 10th is below 50%---and even lower in the areas north of Lubbock. This means that protection of any size square is no longer warranted. But we do need to watch our small bolls for the threat of insect damage. Boll weevil activity was on the rise this past week as more 2nd summer generation boll weevil adults emerged from their hiding places and started to be picked up in traps. Thus far, field evidence of this increase in the form of damaged squares and bolls and red adult weevils in blooms has been scarce. There could be a few fields outside the three active eradication zones that may need treatment before treatments begin after September 1. After peak bloom but before cutout, treatment may be warranted if 25-30 percent of the squares examined while scouting are weevil damaged. As cutout approaches and squares become more scarce, switch to checking nickel to quarter-sized bolls. If 15% of these bolls are weevil damaged then control may be needed. If you are on a once-a-week monitoring program then don't wait for thresholds to be reached if you are already close. Otherwise the infestation will overshoot the threshold and probably will never be brought back under control.Bolls approach being safe from weevil damage when they are 15 days old. A good test is to try and cut the boll with a sharp knife. If you must resort to a sawing action then the boll is probably safe. Don't be fooled into thinking that older bolls are vulnerable to attack just because of the presence of weevil grubs. These weevils are the result of punctures that occurred several days ago when the boll was still soft. Trap catches are up this week but as a whole they are greatly reduced compared to last year at this time. Many of the trap increases are occurring along the Caprock and also up into Swisher County. To view the accumulative trap catches for the months of May, June or July, or to view individual weeks, look at the results of the Plains Cotton Growers, Inc./Texas Cooperative Extension GRID. I think you will agree that trap catches are way down this year both inside and outside active eradication zones. Both plant bugs and cotton fleahopper numbers are pretty high in some fields. But problems with fleahoppers are over for another season since we are no longer interested in protecting pinhead-sized squares. These fleahoppers will actually be helping us by acting as predators on some of our pests. Plant bugs on the other hand are a concern where their numbers are increasing. Most of the problem fields are still associated with favored alternate hosts such as peanuts and alfalfa.Plant bugs or western tarnished plant bugs can damage blooms and small bolls. I would certainly be concerned with protecting bolls for at least 10 days. Plant bug damage to bolls is characterized externally by small black spots. These spots represent attempts by plant bugs to penetrate the boll wall. You must split open the boll and look at the inside of the boll wall to see if there are any wart-like projections. These indicate successful penetration that would result in damage to developing seed and to lint. At this time, I would consider treating a field if my drop cloth counts exceed two adults or nymphs per 3 feet of row. Our best treatment last year was acephate (Orthene or Address). Most of the pyrethroids would be effective too, especially if movement into the field was heavy. But always remember to use pyrethroids with caution because of their aphid-flaring properties. Banded winged whiteflies are still with us but have for the most part stayed at levels that did not appear to be threatening boll filling. Many of the more severely infested fields are somewhat near a sunflower field - but not all. With more sunflower fields desiccated and harvested, further significant movements of whitefly adults may be arrested. Also if the predicted rains come to pass, we should see a marked, temporary reduction in whitefly adults but not nymphs. I still believe that most infested fields of concern have not reached levels to cause the same amount of stress that our 50-100 aphids per leaf threshold level does. A comparison of honeydew deposit levels appears to substantiate this.As more cotton begins to open, the threat of sticky cotton as a result of these whitefly infestations may become a reality. Luckily, the majority of fields in the High Plains have very little whitefly activity at this time. More information will be forthcoming in a future FOCUS issue. Thus far bollworm problems have been on the back burner. A few fields have been sprayed the last two weeks but at levels generally close to my 10,000 per acre threshold. I expect some movement out of corn shortly but because corn acreage is down considerably, there may not be a big flush into cotton. And beet armyworms remain of little threat in spite of our earlier concerns. Maybe, just maybe we will escape their ravages this year. After all, earlier I did state that I had not seen back-to-back bad beet armyworm years in the 25 seasons I have been looking at cotton in the High Plains.Cotton aphids remain parked in most cotton fields. This is becoming a redundant statement week after week but since it is good news I will keep repeating it. Don't expect any big change in their status unless they respond to cool fronts that should become more common or unless more fields are treated with a pyrethroid for some other pest. The bad news is that beneficial insect numbers are down this week. This seems to be the trend with generally lower numbers of prey (food) available. This downward trend is also an artifact of the size of cotton we are checking. Two or three predators per plant were easy to find on 10 inch cotton but more difficult to locate on 2 ½ foot cotton. Besides, the increase in plant size increases the searching arena and decreases their effectiveness. JFL
Not much has changed in the last week and the spider mite threat is still with us. Southwestern and European corn borer (SWCB and ECB, respectively) are essentially done laying eggs. Emilio Nino, Extension IPM Agent for Castro and Lamb counties, found several closely spaced food corn fields with SWCB egg masses on almost every plant. These fields will have to be sprayed, and a miticide will be added to the tank. The question becomes why some fields get heavy egg laying pressure and some do not. There are many factors at work, but growers of food corn should consider helping themselves by planting some Bt corn too. Here is why. To begin with, Bt-corn is very lethal to both corn borer species and will kill almost all first generation larvae. Thus there will be fewer adults to lay eggs in July and August. Large (and even small) tracts of non-Bt corn can serve as a significant source of moths. These are the reasons EPA mandates a 50 percent non-Bt refuge when Bt-corn is planted. (Well, that is not entirely true. Most of the other places in the country only have a 20 percent refuge requirement. Ours is higher because we grow Bt cotton as well.) We usually do not spray first generation larvae. The adults that result from first generation larvae then fly, mate, and begin to lay eggs. In general, SWCB moths move locally, on the order of a few hundred yards to only a few miles. This means that their entire egg load will be laid fairly close to the field where they developed. Now think of a three-mile block of mixed cotton and corn fields. If there is no Bt-corn in the block, and first generation larvae are not sprayed, then there will be no shortage of adults to lay eggs for the second generation (depending on several other factors like initial numbers of overwintering insects, predation, etc.) However, if half of the corn fields in the three-mile block were Bt corn, and half were non-Bt corn, then half of the eggs from the overwintering insects would wind up on Bt-corn. These larvae would die, and the number of moths flying to lay the second generation would be reduced by half. (This assumes that all other things are equal and additionally, that moths can't tell the difference between Bt and non-Bt corn when laying eggs.) So why does this matter? It matters because growers who plant large blocks of food corn should consider planting some Bt corn too. Bt corn is a great insecticide and will reduce the number of moths laying eggs in July and August. (This is the place I am supposed to apologize to EPA for suggesting that Bt corn should be planted on more acres.) There may be something else going on as well. Moths are more sophisticated about where they lay their eggs than we usually give them credit for. Before laying eggs, moths touch the surface of the plant with their feet and legs. (Note to my morphology professors: don't take away my degree for using terms like feet and legs.) The feet contain receptors that can essentially "taste" the surface. If the surface is unacceptable for egg laying because it has either too much or too little of something, the moths move elsewhere to try again. If the surface is acceptable, eggs are laid. Plant varieties differ in the amounts of chemicals they produce in their tissues. This fact forms part of the basis for host plant resistance, a subdiscipline of entomology. Plant breeders intentionally raise or lower the concentrations of chemicals in plants to make them unacceptable or even toxic to some insects. In a practical sense, we can expect some varieties to be more attractive for egg deposition than some other varieties, simply based on chemical composition. The real picture is more complicated and depends on plant health, nutrients (fertility), growth stage, and other things. But in general, some varieties are simply more preferred as egg laying sites than are other varieties. It is possible that some types of food corn are preferred for egg laying over some types of grain corn (or vice versa), but do not misunderstand where I am going with this. I am not saying that food corn is necessarily more preferred than other corn. I am saying that some varieties of corn (food or grain) are more preferred. The fact is, much of our grain corn is Bt now and our food corn is not. We expect more corn borers in food corn simply because it does not have the Bt toxin in it. And because growers of food corn may devote all of their corn acreage to non-Bt corn, they may be increasing the number of moths in July over what they might have had if they planted part of their acreage to Bt corn. PP
Fields should still be monitored for greenbugs, spider mites, and headworms. Blooming fields are also at risk from the sorghum midge. I have visited a few fields with large numbers of false chinch bugs on some of the heads. We usually do not consider treatment until there are 140 - 160 false chinch bugs per head. Our publication, Managing Insects and Mites in Texas Sorghum, gives the details. PP
The past week high temperatures have again been somewhat above normal (August temperature graph). Low temperatures have been closer to normal. No significant regional rainfall has occurred, although, some spotty showers have provided relief for some fields. Reference ET values have been averaging about 0.25 to 0.30 inches per day across the area. Heat unit accumulation has been averaging 19, 20, and 21 per day at Halfway, Lubbock, and Lamesa, respectively. The seasonal total from May 1 continues to be considerably above the long-term average as Halfway, Lubbock, and Lamesa are at 15, 20, and 21 percent above normal. The current seasonal totals are 1623, 1836, and 1954 at Halfway, Lubbock, and Lamesa. Moisture stress. Fruit shed is underway in some fields that can't keep up with crop moisture demands. Normally a boll will be retained once it reaches 10-14 days after bloom. Even though the boll may still be retained by the plant, it will likely be smaller due to moisture stress and have shorter fiber length. Many deficit irrigated pivot fields have soil profiles that are getting depleted of moisture. We would like to target the soil profile to be nearly depleted as we enter harvest aid season. One should keep the field with reduced stress at least until the final bloom to be taken to the gin becomes about a 10-14 day old boll. This will reduce the likelihood of small bolls shedding due to water stress. Fiber length is generally determined during the first 25 days or so in the life of the boll. This indicates that small amounts of irrigation should be applied to carry the boll through the important length development phase. After that, late bolls can handle considerable stress. For a boll set on August 10th, it is apparent that the field should have reduced amounts of water stress probably at least through the end of the month, unless rainfall is obtained to offset irrigation needs. Otherwise moisture stress could limit quality of the uppermost bolls. A rod probe or other tool may be useful in determining the amount of moisture remaining in profiles in fields. Bacterial blight found near Muleshoe. We have recently observed some irrigated fields in the Muleshoe area which are infected with bacterial blight. Control of this disease was once a major objective of cotton breeding programs. Many older varieties are resistant due to genes, which had been identified and incorporated into breeding lines. More recently, most transgenic varieties have not generally been as resistant as their conventional recurrent parents have. Leaf symptoms are angular, dark, shiny spots, which follow the outline of the cells, hence the name Angular Leaf Spot. Symptoms on bolls appear as small and waxy-looking, sunken, rounded to irregular, watery lesions. As the infection progresses, the lesions will enlarge and may blacken. Once the carpel wall of the boll is breached, secondary microorganisms can colonize the boll. Subsequently, the lint may be discolored, resulting in staining and thus lower grades. This disease can be very devastating to susceptible varieties given the correct environmental conditions. These bacteria may originate from debris of diseased cotton plants or planting seed. Plants may get infected when bacteria from infected plants are carried by insects or when infested soil gets splashed up onto leaves, bolls or other plant parts. Bacteria may enter stomata on the leaves, or wounds caused by insects, hail, blowing sand, equipment, etc. Boll infection often results in contamination of seeds, which may then carry the bacteria to the developing seedling the next year. Acid delinting of seed has been useful to reduce the carryover to the next generation of plants. Only disease-free fields should be used as seed blocks. Burial of infested debris should help reduce potential for problems in the next crop. Countdown after cutout. Some fields are as much as 2 weeks ahead of normal in terms of development. We have developed a table that indicates where we are as of August 9 (Table 1). It is based on actual Lubbock 2001 heat units from August 1 through 8, and from that point forward, it uses the 30-year long-term average for each day. For example, the table shows that for a field that reached cutout (here defined as 4 NAWF) on August 1, we should obtain 350 heat units by about August 18 IF we have normal temperatures. For the 450 total, it should be around August 24. For cutout at August 10, we should obtain 350 heat units by August 29, and 450 heat units by September 5. This table also indicates the likelihood of obtaining maturity of late season bolls. For more information see last week's issue of Focus. RB
Table 1. Heat unit events based on date of cutout (4 NAWF) and actual Lubbock August 1-8, 2001 temperatures with subsequent long-term average values for the remainder of the season.
Angular leaf spot is causing defoliation of cotton near Muleshoe. The bacteria which causes this disease (Xanthomonas campestris pv. malvacearum) has been seen throughout the High Plains since 1997. It probably spread during the relatively wet growing season in 1997 and has occurred sporadically during July or August since then. Diagnosis of the pathogen is confirmed when angular leaf spots are seen on the top of the leaf and wet spots are seen on the underside of the leaf. Defoliation can be a problem with this disease, but the greatest losses occur when the bacteria causes a boll rot. The bacteria needs rain or moisture to spread from plant-to-plant. The more it rains, the worse the problem. While no research has been conducted on the effect of LEPA versus spray irrigation, it is expected that spray irrigation would aggravate the problem worse than LEPA. However, it appears that rainfall is necessary to start the disease. Table 2. Cotton varieties tested and their rating for resistance to bacterial blight.
*Paymaster 2200RR is less susceptible than most varieties in this list, but more susceptible than those on the partial resistant list.
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