To produce a healthy snap bean crop, you need to be wary of many pests. To help you zero in on the ones to be on the lookout for in the Eastern U.S. in the the coming year, two researchers provided information on some of the more troublesome pests and how to keep them in check.
Ric Bessin, an Extension professor in the Department of Entomology at the University of Kentucky, says spider mites, bean leaf beetle, and Mexican bean beetle are at the top of his list.
This pest reduces the vigor of plants when they feed. Hot, dry conditions favor spider mite buildup, so they’re most common in mid to late summer, Bessin explains. Large populations will cause leaves to appear stippled and bronzed.
To identify this pest, he says to look on the undersides of leaves with a 10x hand lens.
“Problems begin around field margins as [the mites] move into fields from weeds on the periphery, often near dusty roads,” Bessin explains.
To control spider mites, he says to “limit the use of unnecessary broad spectrum insecticide applications that reduce the number of mite predators.” Bessin encourages growers to use insecticides on an as-needed basis.
When using a miticide, he adds, oftentimes only a portion of the field needs to be treated.
“After using a miticide, the producer should re-evaluate the area in five to seven days to determine if a second application might be needed,” he says.
Bean Leaf Beetle (BLB)
Early in the season, BLB can attack emerging seedlings and young plants, feeding on the cotyledons and initial true leaves. The result, Bessin says, is a reduction in the vigor of plants.
Later in the year, a second generation of BLB will chew small, roundish holes in upper leaves, and the pest will feed on and damage the pods.
“BLB is a sporadic problem as regular sprays are not needed,” he explains. “Instead producers in our area should monitor their snap beans from emergence through the first trifoliate stage, then again in mid July and on. Look for numerous characteristic small round holes in leaves or holes in the pods. Use an insecticide if you see more than one BLB per 5 feet of row.”
Mexican Bean Beetle (MBB)
This pest feeds on the undersides of leaves, but only partway through the leaf, causing windowpane damage. Damaged areas turn brown and dry out and are easy to spot in the field, Bessin explains. The spiny, bright-yellow larvae of the MBB are only found on the undersides of the leaves and cause the same type of damage.
According to Bessin, the action threshold recommended is to keep the defoliation of snap bean to less than 20%. As with spider mites and BLB, MBB is a sporadic pest that does not require regular or scheduled sprays for control. Treat for this pest only as needed.
In Georgia, Stormy Sparks, a professor of entomology at the University of Georgia, lists multiple species of defoliating caterpillars, silverleaf whitefly, and lesser cornstalk borer for the fall snap bean crop, as the top pests for growers in his area.
One of the reasons caterpillars are on the list is because this type of pest is a problem year-round. To determine if any caterpillar species are in your snap bean fields, he suggests scouting with a sweep net or drop cloth.
“Typically treatments are based on both populations and defoliation,” Sparks explains.
“For caterpillars that only defoliate [and don’t attack pods], higher populations can be tolerated and it takes multiple caterpillars per foot to cause severe damage.”
Sparks tells growers to avoid reaching 30% to 40% defoliation. “For species like corn earworm, which can defoliate and will feed on pods, the threshold is much lower, in the range of one per 6 feet of row,” he explains.
The corn earworm is attracted to crops with open canopies where you can see the blooms.
“Good crop growth with a closed canopy reduces the potential for corn earworm problems,” he adds.
Most caterpillars in beans can be controlled with registered pyrethroid insecticides. “If beet armyworms are in the mix,” Sparks says, “you might need to go with one of several newer chemistries specific for caterpillars, such as Coragen (DuPont), Belt (Bayer CropScience), Rimon (Chemtura), or Blackhawk and Radiant (both from Dow AgroSciences).”
Silverleaf Whiteflies (SLWF)
SLWF is strictly a fall pest in Georgia. To keep this pest out of fall beans, growers often treat preventively at planting with a systemic insecticide (usually Admire Pro, Bayer CropScience), Sparks says.
Scouting consists of visual observation of adults per leaf, and nymphs are monitored to determine if and when reproduction is occurring and for insecticide selection. Foliar insecticides include Assail (United Phosphorus), Courier (Nichino America), Knack (Valent U.S.A.), Movento (Bayer), and Coragen (DuPont).
“All of these [products] tend to be more efficacious against immature stages than adults, which is a common problem with SLWF since we lost the use of endosulfan,” he says. “For example, Knack is primarily active against eggs and last instar nymphs, whereas Courier is primarily active against early instar nymphs. Proper selection and evaluation requires knowledge of how each product works.”
Lesser Cornstalk Borer (LCB)
LCB is an early season pest, pre-ferring hot, dry conditions and well- drained soils.
“LCB control requires preplant incorporation of insecticides, usually Lorsban (Dow AgroSciences) or Diazinon (Adama). The larvae are in the soil and enter the plant just below the soil line,” Sparks says. “They tunnel up the stem and kill small seedlings. Infested plants will have a silken tube attached where the larvae enter the plant. They actually move up and down the stem and into the silken tube.”
LCB stops being a concern once the plant stand is well established and the main stem becomes woody. Unfortunately, the presence of this pest often isn’t noticed until plants start to die.
“Once this starts, a lot of growers have tried a wide variety of treatments, and I have never heard of a successful legal treatment. It usually comes down to evaluating whether enough stand is left to produce the crop or replanting, usually with a preventive insecticide application,” Sparks explains.