North Carolina Pest News – July 12, 2013 (Field & Forage Crops)
NORTH CAROLINA COOPERATIVE EXTENSION SERVICE
North Carolina State University * College of Agriculture & Life Sciences
Department of Entomology * Box 7613 * Raleigh, NC 27695
NORTH CAROLINA PEST NEWS
Volume 28, Number 14, July 12, 2013
CAUTION: The information and recommendations in this newsletter are applicable to North Carolina and may not apply in other areas.
FIELD AND FORAGE CROPS
From: Jack Bacheler, Extension Entomologist
Insects and Soggy Cotton
Excessive moisture has been and continues to be an issue in much of North Carolina’s cotton acreage, impacting both the cotton crop itself as well as complicating field access for needed inputs like herbicides, growth regulators and nitrogen. The short term outlook doesn’t look any better. Unfortunately, most of our cotton insect pests don’t mind this excessive moisture a bit. All of the succulent wild and cultivated host vegetation could translate into a more prolonged insect season, especially in view of our generally late cotton crop that could be attractive to pests for a longer period. With our generally weak root systems in many areas of the state, we will also be more susceptible to drought if conditions turn dry.
Plant Bug Update
Some areas of the state report low square retentions rates, though in some cases with plant bug levels only about half or less of the 8 per 100 sweep threshold recommended for pre-blooming cotton. This appears to be more common in fields with saturated soils, with square loss due at least in part to excessive moisture. In other areas, mostly in the far eastern region of our cotton production area, plant bugs are prevalent – sometimes to the tune of 2-fold or more the threshold level. Once cotton has begun to bloom, a 2.5 foot black beat cloth (either purchased or homemade) is the sampling device of choice. The black background is better for spotting the small bright green nymphs than the white sweep net (see video – https://www.youtube.com/watch?
Stink Bugs Coming to a Cotton Field Near You?
Stink bugs also appear to be particularly abundant on both wild and cultivated hosts such as field corn, with brown stink bugs presently being particularly common. Despite some persistent anecdotes to the contrary, remember that stink bugs, especially the adults, are not regarded as a significant pest of cotton until the first appearance of new bolls during the initial week of bloom, as documented in this research article: http://ipm.ncsu.edu/cotton/
Stink Bug App Now Available!
We have a new web-based stink bug decision aid app posted on our Cotton Insect Corner web site: http://ipm.ncsu.edu/cotton/
Although spider mites most often thrive under dry conditions, we have had several calls this week about building mite levels from wet areas of the state (I guess that doesn’t rule out much). In one case, the light stippling leaf dots and reddening had progressed to the point of lower leaf defoliation with mites on most plants. That’s not a bad definition of a threshold level of mites.
Cotton and Soybean Scouting Schools
Dominic Reisig and I are scheduling annual soybean/cotton scouting “schools”. Each school will cover both cotton and soybean insect ID, biology, crop damage, scouting procedures and the use of correct thresholds. There will be both an indoor and a field component of these schools. The present line-up is:
From: Dominic Reisig and Jack Bacheler, Extension Entomologists, and Alejandro Del Pozo-Valdiva, PhD Student, Department of Entomology
Insecticides for Plant Bugs
I’ve received many calls about the presence of nymphs in soybeans that are not being picked up in the sweep net. This is not surprising, since populations of nymphs in nearby fields can be anywhere from mostly large to mostly small. Small nymphs are green, round, and are less likely to land in the net except in dense (read treatable!) populations. Entomologists are aware that the sweep net “undersamples” nymphs compared to adults. The sweep net threshold, then, is calibrated for this undersampling. You will begin to pick more nymphs up as they become larger. Moreover, there are some things you can do to improve your sampling technique for this insect as discussed below.
As an aside, if you have a field where you are going to apply herbicide or fungicide and kudzu bugs are present at levels you think might hit one nymph per sweep in the future (above photo represents a several nymphs per sweep situation), you might want to tank mix in the insecticide to eliminate the nymphs. Keep in mind that small nymphs are not causing as much damage as large nymphs and the soybean plant has an amazing ability to compensate. Kudzu bug reduces number of seed and seed size, but not pod number. Therefore it might pay to wait and see what happens. Also remember that a spray at R2 or R3 will kill all beneficial insects, potentially flaring worm populations. Our first major corn earworm moth flight and egg lay into soybeans generally happens in the end of July/beginning of August. It almost never pays to tank mix and insecticide automatically hoping to kill pests that are there. You will kill what is there, but you will also set yourself up for future problems.
Full-season soybeans are all large enough to sweep for kudzu bug. Double-cropped beans planted in stubble are difficult to sweep, but are not attractive to kudzu bug as seedlings. Sweeping is still an excellent technique to estimate the abundance of small nymphs, which blend in with soybean stems and are difficult to see. By the time nymphs are large enough to see up and down the stem, yield may already be compromised. You should be able to prevent this by spraying when you hit the one nymph per sweep threshold. Keep in mind that most fields will have kudzu bugs at some level. As discussed in the first paragraph, even if you can see insects when you peel back the canopy, you might not have a threshold-level population or you might start picking them up as they get older. Kudzu bugs take a long time to develop relative to other insects, so this will not happen overnight. Some tips for using the sweep net are to:
1) Take 15 to 20 sweeps per sample away from field edges. Ideally, keep the same number of sweeps per sample and per field to compare them.
2) Try to get the net as low as possible between sweeps to hit the middle portion of the plant. More than a half of the insects are located this section of the plant. I like to imagine that I’m stripping the bugs off the main stem, which hopefully buries my net pretty deep in the canopy.
3) Sweep at a comfortable pace that you can maintain throughout the sampling bout.
4) Kudzu bug is more active from 11 a.m. to 2 p.m., resting near the bottom of the plant in quiescent times in the morning and late afternoon and evening. To facilitate capture and reduce the chance of underestimate adult population, scout during the midday period above.
5) Look closely at the bottom of the sweep net because nymphs cluster in this section of the net.
For both visual samples and sweeping, you can check field edges to see if the bugs are present. However, this, insect congregates much more heavily on these edges. The insect is also attracted to structures with height, often aggregating on taller plants, fence posts, or volunteer corn plants in a field. Treatment decisions should be based on field interiors of average sized plants. Start sampling at least 50 feet into the field, being sure to visit several parts of the field.
From: Steve Koenning, Extension Plant Pathologist, and Jim Dunphy, Extension Soybean Specialist
Current Status of Soybean Rust in North America: July 12, 2013
Asiatic soybean rust was confirmed this week in a soybean sentinel plot in Pearl River County (Poplarville, Mississippi), which is located in southwest Mississippi. The incidence was low, though the infected leaf had numerous pustules. This does not put rust any closer to our North Carolina soybeans, although the recent find of rust in Elmore County, Alabama, put rust closer to soybeans in the western part of our state. The closest confirmed rust to our North Carolina soybeans is approximately 355 miles from Charlotte, 620 miles from Elizabeth City, 445 miles from Fayetteville, 220 miles from Murphy, 485 miles from Raleigh, 550 miles from Washington, 460 miles from Wilmington, and 415 miles from Winston-Salem, North Carolina.
We do not recommend spraying soybeans that have not started blooming with a fungicide to control Asiatic soybean rust. Such pre-bloom applications have seldom improved yields. Once soybeans start blooming, we would recommend spraying if rust has been confirmed within 100 miles of the field.
Soybean rust occurrence and northward movement has been greater, for this time of year, than in past years. Though rust has not spread far northward at this time, most of our moisture has come from the Gulf of Mexico and traveled across areas where rust is active. There is a slight possibility that rust has spread more than we know at this time.
Other Soybean Foliar Diseases
On another note, frogeye leaf spot has been confirmed in the state already, and the wet weather we’ve been experiencing is conducive to its development. It has been long enough since we’ve had a serious outbreak of this disease that we have quite a few good varieties that are susceptible to the disease (42 of the 90 varieties in the “Top Ten” list in the January 2013 version of North Carolina Soybean Variety Information (CS-SB-25). The same publication has a list of varieties with resistance to the disease. The disease can limit photosynthate production and yields enough that we recommend scouting for the disease now, even though it usually does not show up this early.
Other diseases that may impact soybean with extended wet weather include target spot on susceptible varieties, Anthracnose, and brown spot.
The current status of soybean rust in the U.S. can always be found at http://sbr.ipmpipe.org/cgi-
Resources for Soybean Rust in 2013
Some sources for more detailed information on Asiatic soybean rust are listed below:
USDA Soybean Rust website: http://www.sbrusa.net/
North Carolina Agricultural Chemical Manual: http://ipm.ncsu.edu/agchem/
Plant Disease and Insect Clinic Update
Corn, cotton, small grain and soybean disease samples submitted by county agents are still free of charge in 2013.