Snap bean growers in the Midwest and Great Lakes have three key pests to be on the lookout for this coming season.
Seedcorn maggot, potato leafhopper, and European corn borer have the potential to cause considerable damage to snap bean crops, but with proper management, including seed treatments and timely pesticide applications, adequate control can be achieved.
Brian Nault, Professor in the Entomology Department at Cornell University, addresses each pest including appearance, symptoms, and control measures.
Seed Corn Maggot
According to Nault, almost every acre of snap bean in the Midwest and Great Lakes region will be threatened by seedcorn maggot. “It’s pretty routine, however, some years may be worse than others,” Nault says.
In its adult form, the seedcorn maggot is a fly that lays eggs in the soil, which eventually hatch into small maggots. It overwinters, and adults tend to emerge in April and early May. There are normally three generations, the first of which is typically the most problematic.
They like cool, wet conditions and are most troublesome in fields that are planted in May or June because soil temperatures are cooler and there is more rainfall.
“Cool, wet soils will impede the seed from germinating as quickly, which means the seed is below the ground for a longer period, allowing for more time for the seedcorn maggot to attack it,” Nault says.
Seedcorn maggot feeds on the germinating seed and seedlings of snap beans, and if populations are high and the crop is not protected, they can kill both, he explains.
“More often than not, you will just see damage to that plant, which may delay its maturity by a week or two compared to all of the other non-damaged plants in the field,” Nault says.
There will typically be no true leaves, and the cotyledon leaves that do emerge will have a “snakehead-like” appearance. The plant will still produce some beans, but they will be much smaller at the time of harvest.
Unfortunately, there is no scouting that can be done, and seed treatments are the primary method of control for seedcorn maggot. The active ingredient thiamethoxam, which is found in Cruiser 5FS (Syngenta), is what is typically used as control. One of the added benefits of Cruiser 5FS, is that it also provides early season control of potato leafhopper.
As for cultural controls, Nault suggests to avoid plowing in a cover crop within seven to 14 days before planting.
The potato leafhopper is a very small light-green to lime-green, wedge-shaped insect that is approximately 1/8 inch long. They move around side to side, similar to a crab, Nault says.
They migrate from the southern U.S. every year and typically arrive in April or early May before snap beans are planted. There will typically be one generation present in alfalfa, and then the next generation will move onto nearby snap bean fields in June or early July.
The immature leafhopper typically feeds on the undersides of leaves, and depending on the pressure, you may see plants become stunted, eventually producing fewer market-sized pods.
You will eventually see what’s called “hopperburn,” which is a reaction of the plant from the saliva that’s injected into it as the leafhopper feeds. This will cause the outer parts of the leaves to turn brown or necrotic, and it may lead to cupping of the leaves.
Because Cruiser 5FS serves as an effective early control for potato leafhopper, many fields will not be sprayed for it as the season progresses, Nault says.
European Corn Borer
The European corn borer (ECB) is a pest that overwinters in New York and may show up as early as May, or as late as September. There are typically one to two generations per year depending on the race, and — in particularly hot years — there may be three.
Adults arrive and lay eggs on the leaves. When the eggs hatch, the larvae may feed on the leaves for a day or two and then penetrate the plant. The eggs will hatch and larvae will make tunnels and infest multiple plant parts.
“The main problem is that larvae may contaminate the processed beans. If larvae move out of the stem and into a market-sized pod, there is the potential that if that pod is harvested, it won’t be eliminated during processing and end up in a can or frozen bag of beans,” Nault explains.
For this reason, tolerance for ECB is extremely low for the processing industry. However, he says there have not been reports of ECB contaminated beans for at least five years.
Nault believes part of the reason for the limited presence of ECB in snap bean is the common use of insecticides to control it, and an overall suppression of the pest in the Midwest and Great Lakes as a result of the widespread use of Bt corn varieties.
Insecticides for use on ECB include Brigade 2EC (FMC), Hero (FMC), Warrior II with Zeon Technology (Syngenta), and Coragen (DuPont). While only one application of Coragen during early bloom is sufficient for ECB control, the other products may need to be applied during early bloom and again one week later.