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| Figure 45. Broken cotton stem and frass due to infestation by a European corn borer larva (L. Jenkins). |
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| Figure 46. European corn borer larva infesting cotton boll (L. Jenkins). |
European corn borer is a recently recognized pest in cotton. Although low numbers have been observed for a long time in cotton stems (Figure 45), the insect has recently adapted to infesting cotton bolls (fruit) (Figure 46) and to frequently colonizing the crop. The most significant incidence of European corn borer infesting cotton has been in the Carolinas. However, the pest has been reported in many other cotton-growing states in the southern portion of its distribution area.
The E pheromone-responsive strain of European corn borer is the principal strain that affects cotton in the eastern Cotton Belt. However, the Z strain may be associated with cotton farther west. Typically, early infestations may occur in cotton stems during June and July, before large bolls develop. These infestations are from the second-generation larvae. Because of the plant's ability to adjust during early growth stages, damage is not usually significant. Second- and third-generation moths often emerge from alternate hosts (e.g., field corn) during late July through early September and move to cotton fields after primary hosts become unattractive. During this time, egg masses are laid deep within the plant vegetation, and larvae often feed on and within bolls, particularly the lower interior bolls. Damage may be significant because bolls in the lower plant positions are extremely important to yield and control of established populations is impractical. Larvae also may tunnel in stems and leaf petioles.
European corn borer larvae often first establish between the boll bract and outer boll wall where they graze on surface tissues of either structure. Young larvae (e.g., second instar) may penetrate bolls and, unlike bollworms (Helicoverpa zea), relatively mature, hard bolls may be penetrated by European corn borer. Tunneled bolls usually are destroyed by the maturing larvae or by boll rot fungi. Lint contamination that results from European corn borer feeding can adversely affect lint quality also.
