Just because their damage isn’t readily visible doesn’t mean soil insect pests can’t considerably lower peanut yields. In fact, their subterranean nature means they can cause more direct yield loss by feeding on peanut pods and kernels, says Avanava Majumdar, entomologist with the Alabama Cooperative Extension System.
Timely detection, correct identification, and good management all are necessary in controlling these pests, said Majumdar during the recent IPM Web Conference sponsored by the Alabama Cooperative Extension System.
“Among the dozen peanut insect pests that feed on various parts of the plant, about half the insect species are associated with soil and occur sporadically in all peanut production areas of Alabama,” says Majumdar.
Peanut pests that live in soil generally feed as immature insects or as caterpillars, grubs or nymphs, he adds. “Very commonly, the adults are not threatening to the crop. The distribution, relative abundance, and seasonal activities of insects are largely moderated by specific soil conditions. Some soil insects that attack peanuts have unique life cycles characterized by multiple host ranges, excellent mobility and host-finding abilities, and prolonged development period during unfavorable conditions,” he says.
Out of the 27 species of burrower bugs that are known to be crop pests, only six species have been reported from peanuts in several states, says Majumdar, and the predominant species causes direct kernel feeding injury.
Burrower bugs, he explains, are small black or brown insects that are extremely mobile. Research from South Carolina and Texas indicates two peaks in population — May to June and again in July to August. “Burrower bugs have been held under control by conventional-tillage practices and the common use of soil insecticides. However, in conservation-tillage systems, burrower bugs can become a sporadic pest that feed on maturing peanut kernels late in the production season.
“Adults and nymphs have piercing-sucking mouthparts that are inserted into maturing kernels during feeding. Peanut kernels develop light yellow to dark brown lesions as a result of feeding, and this damage is called ‘pitting.’ Extensive feeding causes loss of kernel weight and decreases the number of sound kernels. Research indicates that 30 percent kernel feeding is associated with a 1.2 percent increase in damaged kernels. Direct examination of pods is the most reliable method of sampling,” he says.
Peanuts strip-tilled into corn or wheat stubble can have increased risk of feeding injury from burrower bugs, says Majumdar, and fall tillage before planting could be effective against the pest. Applying granular insecticides at pegging also is effective.
There are several species of whitefringed beetles that can occur simultaneously in peanut fields, he says. Immature stages of the insect can feed on more than 350 different host plants that include peanuts, cotton, cowpeas, alfalfa, cole crops, okra, beans and various grasses and sedges.
“Whitefringed beetle is a major insect pest of sweet potatoes in central and north Alabama where feeding injury occurs late in the season. Adults emerge from the soil at the beginning of May and emergence could continue throughout the production season. The soil-dwelling grubs are most injurious to peanuts,” says Majumdar.
For prevention, growers should limit planting leguminous crops like peanuts and soybeans in heavily infested areas. There are no labeled insecticides for controlling this insect. “Some broad-spectrum pre-plant insecticides, already labeled for use in peanuts, should be broadcast and mixed with the top 3 to 4 inches of soil for some control of this insect.”
Southern corn rootworms, he says, are present throughout the United States and feed on a variety of crops. This pest has more than 200 host plants which include cultivated crops and weeds.
“Southern corn rootworm is a sporadic insect pest of peanuts in Alabama,” says Majumdar. “Larva can make one or more holes on the side walls of a peanut pod in order to access the kernels. Soils with high organic matter and those that hold moisture have been associated with infestations in peanut fields probably due to increased survival of the immature stages under those conditions.
“Feeding activity of beetles may destroy seedlings in some situations, but leaf injury caused by beetles late in the season is generally nonthreatening to the crop. Larval infestations in soil should be thoroughly scouted during the pegging and pod development stages.”
Crop injury from this pest could be severe on plants grown in sandy soil and infestations are aggravated in hot, dry weather, he says. Larval feeding injury on the plant roots aids in the transmission of diseases, such as Southern stem rot and also moderate aflatoxin levels in peanuts.
Several species of wireworms can occur in peanut fields, says Majumdar. “Wireworms may have life cycles ranging from three months to two years. Some species can live up to nine years in soil. Wireworms over-winter many inches below the ground which makes insecticidal control very difficult.”
Wireworm larvae start feeding on the underground parts of peanut plants in May, he says. Wireworms are attracted to the carbon dioxide and heat given off by below-ground plant parts. This attraction also depends on the nutritional status of the insect.
“These insects often become a problem when peanuts follow sod. Thus, wireworms may attack the peanut crop early in the season causing major stand loss. Wireworms make large entry holes in the pods to feed on the seeds. Pod injury symptoms from wireworms may look like Southern corn rootworm injury, but wireworms are known to make larger entry holes in the pod than rootworms.”
Maintaining a good crop rotation is critical to reduce infestations of this pest, says Majumdar. “Very few chemical insecticides are available for wireworm management in peanuts. Some broad-spectrum insecticides can provide limited early season control of wireworms. Several species of wireworms also have a tendency to avoid treated crop seeds and this is cause of concern for plant protection specialists.”
Cutworms, he says, are migratory insects that can damage foliage as well as peanut pods, and several species are found across the state.
Cutworms, says Majumdar, feed on more than 60 different host plants and are considered one of the primary threats to peanut production in certain areas. In Alabama and elsewhere, cutworms feed on a variety of cultivated food crops as well as turf grasses and wild plants.
“Depending on the species, cutworms can over-winter in the egg, larval or pupal stages in soil. For example, dark-sided cutworms over-winter as eggs that hatch in the spring resulting in delayed crop damage. Black cutworms over-winter as pupae in an earthen cell and moths emerge from soil using subterranean galleries that open to the soil surface.”
Cutworm feeding activity, says Majumdar, is influenced by prevailing weather conditions, especially rainfall. “Cutworms generally cut off young plants at the soil level and some species can climb plants to access the soft tissues. Of concern is the habit of injuring more plants than what cutworms actually eat. This is especially true for black, bronze, clay-backed and dingy cutworms. Their feeding may be intense in short straight rows within the planted crop making symptoms appear in patches.”
Feeding injury at the peanut pod stage is characterized by the presence of a large entry hole in the middle of pods and complete absence of kernels. According to some reports, peanuts that have been dug and inverted may also be targeted by the cutworms, although yield losses are insignificant in this case.
“Look for caterpillars during dusk and dawn by turning up about 1-inch of soil beneath cut plants and along the affected row. Check for plants that have been dragged a short distance to a feeding hole. Insecticidal control is effective when caterpillars are small. High mobility of cutworms within soil and an extended life cycle limit the efficacy of chemical control measures.”
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