Beet armyworms and tobacco budworms occurring in GA and SC peanut fields, and corn earworm moths are starting to show up in VA
We are getting reports of infestations of both beet armyworm and tobacco budworm in peanut fields in Georgia and South Carolina. These are pretty early reports and could signal trouble to come. Although we get both species in this area of Virginia, it has been several years since we have experienced severe infestations of either.
Beet armyworms seem to be cyclic, occurring only once every few years, with no pattern that we understand. Moths migrate into the area from the south and are particularly attracted to pigweed species. Often the caterpillars will be found feeding on pigweeds on field edges, or wherever the weeds are growing. They feed on pigweed plants then move to nearby crop plants. So peanut (cotton or soybean) fields bordered by a lot of pigweeds or with a lot weeds in the field are particularly vulnerable to beet armyworm infestations.
Tobacco budworm is known to be an occasional pest of cotton, peanuts and soybeans in this area. Caterpillars are easily confused with corn earworm, a different but related species. Adult moths of these species look very different, but the caterpillars are almost identical except for features that require some knowledge and good magnification. We separate these species holding large sized caterpillars under a microscope and inspecting the shape of the mandibular ridge (the inside ‘jaw’ of the mouthparts). Obviously this is not something that can be done in the field.
Budworms are not a big threat to peanuts or soybean, as they are leaf feeders, only, and it takes a lot to do economic damage. They do no more damage than corn earworm, the more common of the two species. In peanut and soybean fields, both budworm and corn earworm can occur at the same time, but typically corn earworm is the dominant pest making up 70 percent or more of the total. In cotton budworms can be troublesome as they feed on young developing squares. Another difference between these species is the timing of the infestation. In some years we see budworm infestations early, before corn earworm. This seems to be the case this year, at least in states south of us.
Corn earworm moths are starting to show up in VA. We are catching 10-20 per night in our local pheromone traps which isn’t a lot in comparison to what we will see later in the season, but enough to result in some possible small outbreaks of worms.
Beet armyworm, tobacco budworm, and corn earworm are strictly leaf feeders in peanut so any decision to treat a field should be based on 1) the number of worms per row foot, and 2) the degree of defoliation compared to the total foliage of the canopy. The threshold for ‘worms’ for this time of year is 4 per row foot (total, all species). If you sample using a drop cloth, after slapping plants on both sides of a 3 foot long cloth you would need to see at least 12 worms. The likeliness of actually finding this many worms is very low&that is&it is pretty rare to find threshold levels of worms in any peanut field.
But is always pays to be vigilant. Check fields periodically throughout the season for pests. If after checking you find at or near thresholds of any pest, select a product that fits the need. If for example you do find a field at threshold for a combination of beet armyworm, tobacco budworm, or corn earworm pyrethroids would not be the best choice. These species would be controlled better with non-pyrethroids like Steward, Belt, Prevathon, or Blackhawk.

I have seen reports of mixed species worm populations in some peanut fields in both South and North Carolina. Populations include corn earworm (may be some tobacco budworm also, but you cannot tell these species apart without some experience and good magnification of the mandibles/jaws), beet armyworm, and fall armyworm. Although I have not heard of worms in Virginia peanut fields, we should be on the lookout for them.

With these mixed species worm complexes it will be pretty important to know which species are in your fields—so—how good are your worm ID skills? We have a good insect ID guide available that shows the characteristics that most easily distinguish these different worm species. We will be glad to mail some to you, let us know, or you can access the guide on the web at: http://pubs.ext.vt.edu/444/444-360/444-360.html. Knowing the species is important in choosing the right insecticide—if control is needed (see below).

The next important question is—should I treat for worms? In my experience, many growers treat fields that are not at any risk to yield loss—that is, they ‘jump the gun’. It takes a lot of leaf feeding for worms to do economic damage to peanuts, and I have seen only a handful of cases where this has occurred over the past many years. This will be especially true this year when peanut canopies are very large, so are able to withstand even more leaf ‘tattering’ with no negative consequence. But if you do determine that a field is in danger, feel the urge to spray, or just love killing worms—better not rely on a pyrethroid or you may not be satisfied with the results. Corn earworms have become a little more difficult to control with pyrethroids in recent years, and beet armyworm and fall armyworm have never been very susceptible to pyrethroids. You will need to turn to some of the non-pyrethroid options like Steward EC @9.2-11.3 oz/acres, Belt SC @2-4 oz/acre, or Blackhawk (was Tracer) @1.7-3.3 oz/acre.

Compared to past years, we seem to be in a low cycle for many peanut pests. For example, not so many years ago it was pretty common to see rampant twospotted spider mite infestations turning large areas of peanut fields yellow, then brown. Dr. Rick Brandenburg at NCSU and I spent a lot of time in those years developing what at the time was the most effective management strategy: first miticide application at early detection followed by a second in 5-7 days. It took this two-spray system to break the mite egg-laying cycle and stop the infestation. Where have the mites gone? We see a few in fields every year but even in dry years we just do not encounter many really bad infestations. Let’s hope this holds because if we do start having problems, we have almost no control products available. The two general rule-of-thumb practices to lower risk to spider mites are 1) to use insecticides sparingly and only as needed, and 2) prevent mowing or burn down of weedy field edges during the high risk months of July and August.

Another pest that seems to be in a slack phase is the potato leafhopper. You can certainly find hoppers and some evidence of hopper burn in a lot of fields but it is pretty rare to see fields that are badly injured. During the heavy infestation years we ramped up field trials and were able to develop a decent plant injury threshold based on the relationship of percent of leaves showing hopper burn and pod yield. Results showed that yields began to be affected (reduced) when injury exceeded 20-25 percent of leaves. That is, some hopper burn is tolerable and will not cause any yield reduction, but there is turning point. Where are the badly injured hopper fields? Not easy to find these days and hopefully that will not change. Scout fields in July and August and tank mix an insecticide with a leafspot spray, but ONLY if plants are approaching the injury threshold and hoppers are active.

Southern corn rootworm (SCR) has received a lot of attention this summer because of the overall wet conditions that favor this soil pest. Many comments relating to SCR management were recently posted by Dr. Brandenburg and we are on the ‘same page’ so no need to restate. I do have a couple of additional observations. Even in recent moderately wet years we have not been seeing or hearing of much rootworm injury. And in general, we are not seeing as many of the adult cucumber beetles flying in and around peanut fields as we used to. During years with more consistent pressure, Dr. Brandenburg and I did a lot of work to establish the Southern Corn Rootworm Risk Index as a system to determine which fields are at risk. In normal rainfall years, it’s a pretty effective management system. Although we see some variability in its effectiveness, chlorpyrifos (Losrban) is still the only insecticide available if control is needed.

Corn earworm infestation intensity also varies from year to year with the general pattern of being worse in overall dry years and less of a problem in wet years. In my opinion however, corn earworm is only worth mentioning because many people spray for it, even though it is not an economic problem in peanuts. In the 25 years I have been working with the peanut crop, I can recall only ONE case where enough plant material was consumed by worms to result in a yield reduction—and that was in a dry year when the peanut plants had not developed a large canopy. In most years when worms move into fields with full-canopy, knee-high plants, they cannot eat enough leaf tissue to be a threat.

In summary, in general we seem to be in a ‘quiet’ period for peanut insect/mite pests. Pressure by mites, hoppers and rootworms has been generally low in recent years and so far, we have not experienced the burrowing bug or three-cornered alfalfa hopper problems that plague some Georgia and South Carolina growers. We have also been lucky with lesser cornstalk borers. Of course this could all change—in one year. So, feel lucky—but stay vigilant and question the need for each and every insecticide spray.

See below a note from Dr Rick Brandenburg at NCSU regarding southern corn rootworm in peanut. In general, Virginia has had less rainfall than what Rick is reporting from NC so our risk to rootworms is less, but his insigts are helpful.

From Rick Brandenburg, NCSU: The past week, I’ve received a lot of calls on a lot of different topics, but the most common question focused on southern corn rootworm. Many of the others were on worms in peanuts. The typical question, which was a very good one, focused on whether peanuts were at greater risk from rootworms this year due to the wet weather we’ve had recently and the fact that some of our peanuts are a little late. This is not information that shows up directly in the rootworm advisory, but the questions you ask show a great understanding of the index and the factors that affect rootworms.
Yes, the above normal rainfall is making rootworms more of a threat and even lighter, sandier soils could support rootworms with this kind of weather. So if you have a marginal field, the rains have probably moved the risk up a notch. We’ve got a lot of good looking peanuts out there and some of them are late. This lateness makes them at more risk from rootworms as well as the larvae prefer to attack the smaller, more immature pods. The word that I’ve been passing along is that if you have a good crop, the fields have been and remain on the wet side, and/or the crop is a little late, then the risk is higher than the index rating you will have. How much higher is a guess, but it is higher this year. Given the fact that the threat from spider mites seems minimal right now and that we’ve had quite a few leafhoppers out there, the use of Lorsban (chlorpyrifos) or a generic seems like a pretty good idea in many fields. Since it is about August 1, growers could consider going with a half rate.
Rootworm treatments are a bit of a guessing game each year, but the wet weather makes it a bit less, but no guarantees. I would hate to see some of these good peanuts we’ve got out there get dug this fall and have a lot of damage. That’s my personal judgment.

Unlike cotton where thrips pressure has peaked, numbers are still increasing in peanut. It is hard to compare the numbers we get in these two crops as we sample very different amounts of plant material. In cotton we sample whole plants, but in peanuts, only unopened terminal leaflets…where thrips concentrate their feeding. In untreated peanut plots, counts have gone from zero (May 17), to 7 (May 24), to 18 (May 30) per 10 leaflet sample. The result is not pretty with damage ratings reaching 5 to 6 on the 0-10 scale. Treatments are not keeping up, generally, and few are holding plants below a 2.0-3.0 on the scale which is where I think we need to be for plants to achieve potential yields. There are a few decent treatments and we plan to share these results in more detail at the June 5 tour, and at winter meetings.

The next two weeks should be key weeks if indeed we are going to see caterpillars in the peanut crop; so keep your eyes open. The last week or so has been good to many of you with some much needed rains. This really helps with the spider mites. In fact, when we get two consecutive days of cloudy weather and temperatures below 90 F, that really helps a fungus that attacks mites and knocks populations back. However, mites still need to be watched carefully given the hot summer and the forecast for the next week. As we move further along into August you can feel more confident applying a compound like Brigade or Danitol for mites and caterpillars with less concern about a rebound of the mite population. Comite is an effective mite product but can be hard to find in certain areas. The later we get in the season, the more practical it is to spray once for mites and then take a wait and see approach and hope you get favorable weather.

Caterpillar sprays should be considered only if thresholds are met. In fields with normal or better leaf canopies, treat if caterpillar populations reach 4 per row foot; and this increases to 6 per row foot towards the end of August. Many peanut fields are treated when populations are well below threshold, a practice that does not reap any benefit, and could increase the risk of other problems. If plants have poor canopy growth, you may be justified in using a lower threshold, but again, it is important to at least try to determine the number per row foot and abide by the thresholds. Another consideration is what species of caterpillars are present. They all count equally in the threshold, but if fall armyworms are in the mix, a non-pyrethroid may be needed to achieve good control.

Early August is the time of year we commonly see populations of corn earworms and other caterpillars in peanuts in Virginia and North Carolina. Recent light trap catches would indicate we should see some worms in peanut over the next couple of weeks. Two very appropriate questions have been asked in recent weeks and a quick review of possible answers to these questions is in order.

The first question focuses on the issue of insecticide resistance in corn earworm populations in peanuts. There is some indication that pyrethroid resistance among corn earworms, including the ones we find in peanut fields, is a real concern. However, the pyrethroids cost less than other options and for many growers are the preferred means of control. In addition, pyrethroids are not the best choice for controlling tobacco budworms or fall armyworms in peanuts and control is often less than satisfactory against these pests. The second question focuses on which product to select if the field is also infested with spider mites.

In our opinion, 100% control should not be an overriding goal when treating for caterpillars in peanuts. Good control of corn earworm can still be obtained in most situations with the high rate of pyrethroids. Guidelines for helping determine which type of product to select in peanuts this year might include the following:

1.Very high populations of worms. If populations exceed 8-10 worms per row foot, then the use of a non-pyrethroid insecticide might be a better choice.

2.If populations of tobacco budworm or fall armyworm make up more than 25% of the population, then the use of a non-pyrethroid insecticide might be a better choice.

3.If you understand that you do not need to obtain 100% control of caterpillars to preserve yield, then the use of a non-pyrethroid insecticide might be a better choice.

4.If spider mites are present, then Danitol or Brigade should be used for caterpillar control.

Guidelines are subject to specific local conditions, but in general we believe that pyrethroids still hold value for caterpillar control in peanuts this year. The recent rains certainly helped with the spider mite situation, but temperatures this week will drive populations back up. If caterpillar and/or spider mite populations increase in the coming weeks, we will update you on any changes in our recommendations.