Bollworm treatments may be needed in Virginia cotton

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As of last week, we found bollworm eggs in cotton fields in numbers high enough to indicate that fields may need to be treated. The bollworm (= corn earworm) moth flight from corn is light and spotty, but it does not take many moths to cause concern in cotton.

In the old days before Bt cotton, we relied on the two spray system to control bollworms—the first spray applied at egg threshold followed by an automatic second spray 5 to 7 days later. This system worked very well to protect cotton from economic levels of boll damage.

With the advent of Bt cotton varieties (now TwinLink, BG2 and WideStrike) we found that even with these technologies, there were enough worm escapes (worms not killed by feeding on the plant toxins) to warrant a single insecticide spray. And—we determined that the best time for that single worm spray coincided with the second of the original two spray system—i.e., 5 to 7 days after the egg threshold.

Egg thresholds were found towards the end of last week in fields near the Tidewater AREC. The same is likely happening in fields across the region. So now is the time for applying a bollworm treatment. Based on our history of bollworm resistance to pyrethroids, the best control results will be obtained using a non-pyrethroid (e.g., Steward, Belt, Prevathon, Blackhawk). But we also recommend adding a pyrethroid to clean up any stink bugs that may be in the cotton. There are several pyrethroid options, and I would use the highest labelled rate for stink bugs—better kill and longer residual. Besiege is the only product that contains both the non-pyrethroid (= Prevathon) with a pyrethroid (= Karate). With all the other non-pyrethroids, you will have to do the tank mixing on the farm. And, it is past time to include any of the insecticides from the neonicotinoid class as they do not provide good control of either bollworms or stink bugs.

Late season Palmer amaranth

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I have received several calls regarding control of large glyphosate-resistant Palmer amaranth in soybean. In most cases, Palmer amaranth resistant to glyphosate is also resistant to the ALS-inhibiting herbicides (ie Harmony SG, Pursuit, Raptor, Synchrony STS, etc.). So these herbicides offer no help. Resistance to these two chemistries limits the chemical control options growers have left. In Roundup Ready soybean cultivars, the only postemergence options left for control of Palmer amaranth are the PPO-inhibiting herbicides (Cobra, Flexstar, Reflex, Resource, Ultra Blazer, etc.). When targeting Palmer amaranth with PPO-inhibitors, the weed must be small (less than 4 inches in height) to achieve adequate control. It is also important to remember PPOs can cause some soybean injury. In Liberty Link soybean, in addition to the PPOs, growers also have the option of Liberty to control Palmer amaranth. Again, small weeds are necessary to get the most out of Liberty. Once Palmer amaranth exceeds the 4 inch threshold, control by the PPOs and Liberty diminishes. Unfortunately, attempts to control large glyphosate- and ALS-resistant Palmer amaranth are often futile. Faced with this situation, it is necessary to physically remove Palmer amaranth. Prior to the appearance of a seed head, Palmer amaranth can be pulled and left in the field. Remember to shake dirt from the root ball to ensure the weed does not re-root. After seed head emergence, it is critical to remove Palmer amaranth from the field. Female Palmer amaranth are capable of producing over 1 million seed. Even while still green, some seed are viable. In the soil, Palmer amaranth seed remains viable for approximately 4 to 5 years. Assuming the seedbank is not replenished during this time period, daunting populations begin to decrease and management becomes easier. In problem areas, rotating to corn may be a good idea (atrazine and the auxin herbicides remain effective against Palmer amaranth). However, corn harvest occurs relatively early and Palmer amaranth can emerge following harvest and produce seed prior to a killing frost. Gramoxone applied after corn harvest (3 to 4 pints/A) has proven most effective controlling Palmer amaranth and ensuring the weed seedbank is not replenished in this situation.

Palmer amaranth seed produced by on female plant.

Palmer amaranth seed produced by one female plant.

 

Black light trap catches for the week ending August 6, 2015

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Corn earworm moth captures started increasing in area black light traps, ranging from 0.3 to 7.5 per night.  Brown marmorated stink bug nightly averages ranged from zero to 12.9.  Please see the data tables for specifics.  Thanks to the following for their reports this week:  John Allison, Scott Reiter, David Moore, Mary Beahm, Austin Brown, Janet Spencer, Keith Balderson, and Ames Herbert and his entomology crew. BLT_Aug_6

Soybean loopers are infesting soybeans in North Carolina

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I have gotten reports that soybean loopers are already infesting soybean fields in North Carolina in large numbers. This is basically about a month earlier than we normally see them in this area of the US. Loopers migrate from the south and typically infest fields in Virginia in early to mid September, but based on what is happening in NC, we should be on the lookout.

We are also in a high risk environment for having looper infestations. We usually have more problems with them in dry seasons, and they most often develop in soybean fields that have been previously treated with pyrethroids. Our current research is proving what we have been suspecting—that pyrethroids are non-selective insecticides which kill many of the natural enemy species that feed on looper eggs and small larvae—which if undisturbed, can prevent outbreaks in many situations. So previously treated fields are at the highest risk to loopers.

If we begin to hear about infestations in Virginia, we will post updates and include some recommendations in terms of thresholds and products that will work best. But generally, you cannot get control of loopers with pyrethroids—none of them—so you will have to turn to non-pyrethroid options, and there are several.

Looper

TriCor Receives Registration for use in Wheat and Barley

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The herbicide TriCor, a metribuzin product from United Phosphorus Inc., has received 24(c) registration (special local needs) for use in Virginia for control of Harmony and Harmony Extra (group 2 herbicides) resistant common chickweed. The supplemental label is here: TriCor DF Herbicide 24(c) VA label. This is good news for small grains growers.

Research by Drs. Scott Hagood and Michael Flessner indicate that TriCor at 2 oz/a results in excellent common chickweed control when applied in fall or spring, with no significant injury to wheat. However, wheat injury can be a concern. Using crop oil concentrate or vegetable oil surfactants with TriCor increases the risk of crop injury, as well as when applying with fertilizer in combination with TriCor. Growers should select wheat and barley varieties that are tolerant of metribuzin if planning on using TriCor as certain varieties are more sensitive to metribuzin than others. Seed dealers may have information regarding metribuzin tolerance. Additionally, preliminary nonreplicated wheat variety sensitivity research by Drs. Wade Thomason, Carl Griffey, and Michael Flessner is included in this publication: https://pubs.ext.vt.edu/CSES/CSES-129/CSES-129-PDF.pdf. See pages 99 to 101.

Always make sure to read and follow the product label. Also note that TriCor is the only stand-alone metribuzin product legal for this use.

Example of wheat variety differences in sensitivity

Example of wheat variety differences in sensitivity to metribuzin. Thomason 2015.

Annual Corn Earworm Field Corn Survey–2015

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Annually, we conduct a survey to estimate Helicoverpa zea (corn earworm) infestation levels in field corn in mid- to late July. Corn is considered a nursery crop for corn earworm, allowing the pest to complete a lifecycle and then move on to other crops such as soybean, cotton, and peanut in August. Over 30 years of data show that there is a linear correlation between the infestation level in corn and the amount of soybean acreage that gets treated with insecticide for this pest.

To conduct the survey this year, the number of corn earworm larvae found in 50 ears of corn was recorded in 5 corn fields in each of 25 counties, totaling 6,250 ears and 125 fields sampled. When fields were known to contain Bt or non-Bt corn, this was noted. Otherwise, samples were considered to be random and assumed to be representative of the actual Bt/non-Bt composition in each county. Age of corn earworms, or if they had already exited the ears, was also recorded (data not shown). We greatly appreciate the help of Virginia Cooperative Extension Agriculture and Natural Resource (ANR) Agents, Virginia Tech faculty and staff, summer agricultural assistants, and interns in this effort. These cooperators are acknowledged in the attached Figure. We also would like to thank the many growers who graciously allowed us to inspect their fields for corn earworm larvae.

Results of the survey are provided in the Figure (CEW_survey_2015). Statewide, approximately 17.5% of ears were infested with corn earworm. This is even lower than the numbers reported in 2014 (20%) and 2013 (18%)—both years with very few cotton, peanut or soybean fields being infested with treatable numbers of worms. Regional averages for 2015 were 4% infested ears in the Northern Neck, 13.4% in Mid-Eastern, 9.5% in South-Central, and 27.7% in the Southeast.

So far, all indicators point to another year with few fields reaching thresholds. We will continue monitoring and posting updates.

This survey is intended to be a representative sample, not a complete picture. We always recommend scouting individual fields to determine exactly what is happening in terms of corn earworm as well as other pests and crop problems. Also, please check the black light trap reports on the Virginia Ag Pest and Crop Advisory and other reports posted weekly to keep up-to-date on the insect pest situation.


Update on soybean insect pests in Virginia

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In mid July we initiated our annual soybean insect pest survey, our black light trapping program, and our corn earworm pyrethroid resistance monitoring program. Following are short updates.

BMSB (brown marmorated stink bug): So far this summer, only a TOTAL of 4 BMSBs have been found in soybeans—one in a field in Amelia Co. and 3 in a field in Bedford Co. (see the attached BMSB occurrence map). This breaks all records for the fewest for this time of year. Was it the cold winter, or those unusually hot days in June (BMSB does not do well when temperatures exceed the mid 90s)? We will continue posting our field survey results but all indicators point to a season with very low risk to BMSB infestations in the Virginia soybean crop.  BMSB_map_30_July_2015

Kudzu bug: As of the end of July, kudzu bugs have been found in soybean fields in 21 southern/eastern counties (see the attached kudzu bug occurrence map)—but in all cases, these have been adults only, and at very low numbers. Most counts reported by our scouts were 1 or fewer adults in 15 sweeps, and occasionally 2 per 15 sweeps. These numbers are way too low to be a threat to fields and DO NOT warrant treatment. These adults represent the first generation that will set up the nymphs that could cause concern later, but only if numbers increase a lot. Our best guess based on what we are seeing in fields now, and the lower number that successfully overwintered compared with last year, is that not many, if any, fields that will reach the nymphal threshold.  KB_map_30_July_2015

Corn earworm moth activity: We are seeing an increase in CEW moth activity just in the last week and what we think is the beginning of the flight from field corn. Our local pheromone traps have gone from just a few caught in a week, to averaging in the teens and twenties. We are just beginning to see moths fly up in crop fields as we walk the rows, and some local growers are seeing the same. On a separate advisory we have posted the results of the annual corn earworm field corn survey—which showed a statewide average of only 17.5% ears infested. This is even lower than the numbers reported in 2014 (20%) and 2013 (18%)—both years with very few cotton, peanut or soybean fields being infested with treatable numbers of worms. So far, all indicators point to another year with few fields reaching thresholds. We will continue monitoring and posting updates.

Corn earworm pyrethroid resistance: To date we have tested 255 moths (see the attached line graph). We started the season with about 15% survivorship at the end of June and as of the end of July are up to 40% survivorship. This level, this early in the season is cause for some concern. We will be posting updates often.  Vial_tests_30_July_2015