When it comes to taking a bite out of profits, no insect poses a bigger threat to Arizona cotton growers than the lygus bug, which has topped the state’s list of yield-robbing insect pests for the past 13 years.
The plant bug reduces cotton production mainly by feeding on squares, which creates gaps in fruiting, forcing growers to lose valuable time by extending the production season. These gaps cut profits even more by disrupting the plant’s carbohydrate balance, resulting in excessive vegetative growth, making defoliation more difficult and expensive and reducing fiber quality, particularly high micronaire.
Properly implemented, action thresholds offer Arizona cotton growers a way to maximize profits when using insecticides to control lygus. By minimizing the total amount of chemicals applied, use of these thresholds can limit outbreaks of any secondary cotton pests when their natural predators are destroyed by the lygus insecticides.
These chemicals can also kill the insect’s natural enemies, if the materials are too broad spectrum or are applied at an inappropriate time. This enables lygus to bounce back more quickly than otherwise. Basing lygus control on thresholds also can help reduce or eliminate any resurgence.
Research-based thresholds for lygus bugs in cotton have been well-established for years and validated on commercial farms in Arizona. An action threshold provides an objective way to determine the most cost- effective time to apply an insecticide.
Peter Ellsworth, University of Arizona IPM coordinator and director of the Arizona Pest Management Center, defines this threshold by the total number of lygus (15) and the total number of nymphs (4) found in a 100-sweep sample.
This is commonly referred to as the 15/4 level. Both criteria — the total number of lygus and the total number of nymphs — must be satisfied in order to have reached the threshold, he emphasizes. For example, total lygus/total nymphs levels of 12/6, 17/1, 32/0, and 11/5 are all considered below threshold. On the other hand, 17/4 and 21/7 are above threshold.
“This means that there must be at least 4 nymphs present per 100 sweeps, along with enough adults to total at least 15 lygus per 100 sweeps,” Ellsworth says. “When you reach that level, you can assume that a spray will be most cost-effective if done within about two to three days. That allows for possible delays in application due to weather, irrigation and other factors.”
The total of each number in the threshold is critical. In the case of total lygus levels, this figure assures that the population is stable and large enough to consider action. Observing proper nymph levels insures that the insect is reproducing enough that you’ll be money ahead by spraying them.
Keep in mind that these action thresholds have been developed as reliable guidelines for growers and PCAs. The actual spraying decision, Ellsworth stresses, can and should consider other factors, like any other pests targeted by timing of the application and activity of the natural enemies of lygus.
“I’ve worked with some growers who have successfully adopted higher nymph levels in their thresholds without any sacrifices in production,” he says. “On the other hand, I’ve not seen a situation where more conservative or lower levels are economically justified in Arizona’s system.”
Making the best use of these numbers requires understanding the biology of lygus bugs and the dynamics of controlling them, he notes.
Insecticide applications should be aimed at the nymphs — the most damaging stage of the life cycle of lygus.
“The role of adults is to migrate by flying and to reproduce,” Ellsworth explains. “They spend much less time feeding than nymphs. Nymphs are born to feed. They suck on cotton squares, damaging the developing flowers and causing abortions. That’s all they do. They have no wings.”
Because lygus eggs are inserted into plant tissues, they remain safe from any insecticides sprayed on the plants. Also, none of the insecticides currently on the market are very efficient at controlling adults, Ellsworth says.
“When measuring thresholds, you shouldn’t think in terms of achieving a zero adult count,” he says. “That’s not a reasonable goal in our ecosystems, and it’s not a necessary one, either. A low count of adults might persist all season long and still not cause any economically-significant damage whatever.”
Another key to making the most of threshold guidelines in controlling lygus is to know when they are arriving in the field in significant numbers. That’s where sweeping the upper 12 inches of the plants with a net comes in. Even then, you have time to prepare for action. After lygus first invades a field, it takes about two weeks before the nymphs begin causing any real economic damage, Ellsworth notes. The eggs require about a week to hatch after they are laid. Then it takes about another week before the small nymphs have developed into larger nymphs that have larger mouth parts capable of damaging squares. Typically, in Arizona there’s no need to sweep cotton fields for lygus before squaring.
“Usually we don’t see threshold levels until we’re into the first flower and canopy closure is imminent,” he says. “You can find them in the field before that, but they’re mostly moving in and out a lot. Until the canopy closes and there’s a good amount of flowering, lygus isn’t comfortable with the high solar radiation and reflection off the soil.”
Light sweeping can start when cotton is 15 to 18 inches tall, Ellsworth notes, and should continue until the plants have grown to at least five nodes above white flower. He advises sweeping until the plant has become fully structured and is moving toward cut-out but not putting on much more vertical productive growth.
Determining which areas of the field to sweep should be based on specific conditions in each field, he says. To prevent deceptive catches of the insect, he suggests moving into the field at least 25 paces or so from the edge. This way, for example, you can avoid areas like the top end of a field where shorter-statured plants haven’t canopied over and might result in counts biased toward the low side. Similarly, avoid the tail end of the field or areas where water ponds, since the very rank plant growth in these places may attract more lygus than other areas that better represent the whole field.
Ellsworth recommends sweeping in each of four areas of the field. Pick a row in each area at random and take no more than 25 sweeps in each sampled area. Tally the counts from 100 total sweeps and compare to the threshold guidelines.
“You need 100 sweeps to be really certain,” he says. “If you’re getting lots of zero counts, don’t stop until you make all 100 sweeps so you can determine whether you’ve really exceeded the threshold. Otherwise, you might be unpleasantly surprised.”
It’s not unusual for the first set of 25-sweep counts to show zero nymphs, Ellsworth notes. For example, a PCA might find only two or three adults in each of the first two or three 25-sweep samples, incorrectly conclude that the field is safe, and discontinue taking samples — tallying just six or seven adults and no nymphs in 50 or 75 sweeps. However, the last 25-sweeps sample could easily contain the all-important four nymphs and as many adults in it.
“On the other hand, when populations crest over the threshold pretty quickly and significantly, you can take shortcuts,” he says. “For instance, you might go from catching no nymphs in 100 sweeps last week and to catching as many as 5 or 10 nymphs and several adults in the first 25-sweep sample you take this week. Under such a scenario, there’s no need to make the remaining 75 sweeps, because the threshold has already been satisfied and you’ll need to take action.”