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| Figure 44. Entrance of European corn borer larvae, two pods on left, and Helicoverpa zea, three pods on right (M. R. Spellman). |
Snap bean is an incidental host of European corn borer. The foliage of snap bean provides an environment suitable for an action site where adults rest during the day and mate at night. If a preferred host is not available to attract European corn borers, eggs will be laid on snap bean plants. Economic problems can occur before preferred hosts are available, after preferred hosts dry up and become unattractive, or when European corn borer populations are extremely high. European corn borer infestation of snap bean occurs primarily in late spring (before corn is available), in midsummer, or in late summer (in a late-planted crop). Egg masses are deposited on leaves about the time of blooming. The larvae hatch in about 4 days and disperse over the plant. First and second instars usually feed on leaf tissue, particularly new tissue. Second and third instars will feed on leaf petioles and in axils. Larvae may bore in stems and petioles before pods are present and then enter pods after they form (Figure 44). Holes made in pods by European corn borer are smaller than those made by Helicoverpa zea (sometimes called bean podworm) (Figure 44), another pest of snap bean in some localities.
The impact of infestation on growth and development of the snap bean plant is minimal. The main problem is larval contamination of pods used for processing. Tolerance of infested pods is quite low, and it varies among processors, ranging from 0.2 to 1 infested pod per 1,000 pods. A single sample of pods with an infestation at or above the tolerance level usually results in the harvest from an entire field or farm being rejected for processing. Tolerances are extremely low because of complaints often brought back to the processor when the consumer finds a larva in the canned or frozen green beans. Processors cannot afford any contamination.
