White-fringed Beetles in Cotton
Keith L. Edmisten, Cotton Extension
|This season several growers have reported
unexplained wilting and eventual death of cotton seedlings in North Carolina. The
suspected pest is the larval stage of the white-fringed beetle. While these problems have
been isolated to specific fields, the symptoms have been reported in several areas of
North Carolina. These reports have not warranted treatment at this time and currently
there are no insecticides labeled for use on white-fringed beetles. This information is
intended to aid in recognition of symptoms and identification and to monitor the
occurrence in North Carolina over the next several growing seasons.
The white-fringed beetle is a species of weevil first introduced into the US through Florida in 1936 and has spread west to Missouri and north to New Jersey. White-fringed beetles were first reported in North Carolina in 1942. They are generally spread by transplants and agricultural implements and are often found in well-drained, sandy loam type soils. Their damage is usually very spotty, possible appearing in one field and not in an adjacent field. It has been reported to infest at least 385 plant species including tobacco, peanuts, soybeans, velvet beans, Irish potatoes, and strawberries. Plants with a smooth taproot and broad leaves are most commonly affected. Having a more fibrous root system, small grains are generally more tolerant to damage from the white-fringed beetle.
The white-fringed beetle adult is a weevil with a blunt, wide snout. The adult females (males have not been reported) reproduce parthogenetically (males are not required) and cannot fly. They are approximately 11 mm long and are darkish gray to grayish brown in color (Figure 1). Another characteristic of adults are two longitudinal stripes and a marginal, white band of hairs (Figure 2). Larvae progress through several instars and can be up to 13 mm in length. They have a legless, slightly curved, yellowish-brown, soft body with a light brown head.
They usually overwinter as grubs and begin to feed on roots of plants in the spring. After feeding on roots through May, most burrow down in the soil to pupate. By July, most have emerged as adults. While damage to the foliage by the adult form does not appear to significantly affect the plants, the damage to the roots by the larvae can cause wilting and eventual death of a cotton seedling. The damage is characterized by a frayed appearance on the roots just below the soil line (Figure 3). As opposed to the black cutworm, white-fringed beetles do not usually cut the root completely. Locating larvae consist of recognizing plants that are not dead but are just beginning to show signs of wilting (Figure 4) and digging around the roots for the larva.
In early June, plants from Union County were sent in for diagnosis. They had been found at various stages of wilt or death in the field and the roots had a knawed appearance just below the soil line. The damage appeared to be similar to what Dr. Keith Edmisten had previously seen in Alabama and what Dr. Ron Smith, the cotton entomologist at Auburn University, identified as white-fringed beetle. This cotton had followed by soybeans in the previous year with a winter cover crop of wheat. The land was not tilled. Around the same time in Greene County, several fields were reported undergoing unexplained stand reduction with plants wilting and eventually dying. One field was inspected and white-fringed beetles were found feeding on the roots (6-26-01). The extent of the damage was hard to estimate because plants were at various stages of decay and many had probably decayed beyond recognition. However, a rough approximation of dead seedlings was 5 %. The cotton was followed by sweet potatoes (a known host for white fringed beetles in the southeast) that were cut just after harvest and the vegetation had been left. Cotton was planted in what was described by the agent as semi-no-till conditions. The presence of vegetation in a field also seems to lend itself to survival and subsequent occurrences of this beetle. While similar damage was observed earlier in the other Greene County fields, the damage had ceased before the fields were inspected for the white-fringed beetles. It is also noteworthy that in northeastern North Carolina there was an abnormally high rate of what was described as vegetable weevils in seedling cotton.
The current status of the white-fringed beetle as a pest of cotton appears minimal. It probably has only affected a few fields in NC and it is uncertain as to whether this insect has ever been documented in cotton. Like all secondary pests however, growers and consultants alike need to be able to recognize the symptoms. With boll weevil eradication and Bt cotton, the reduction of insecticide sprays allows for potential outbreaks of secondary pests previously managed by spraying for key pests. Pyrethroid applications in July and August have most likely controlled white-fringed beetle adults and therefore larvae because the adults are not mobile. White-fringed beetles may become a more serious cotton pest as foliar insecticide applications are replaced by Bt technology.
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