Answer: The worms are plum curculio larvae, says Jerome Hull, Extension horticulturist at Michigan State University in East Lansing. When nighttime temperatures reach 55_F, adults begin emerging and the females lay eggs on the undersides of developing fruit.Usually the hatching larvae cause the fruit to drop. However, a second generation in August can cause deformities and scarring of developing fruit, explains Hull. The telltale sign of plum curculio damage is a small, crescent shaped cut on the fruit where the female inserts the egg. You can check for plum curculio by placing a sheet under your plum tree and shaking the limbs. The small, dark, lethargic beetle will fall onto the sheet. The plum curculio is hard to control without using chemicals, says Hull. One spray of Imidan after petal drop and another in mid August should do it. There are no effective biological controls available yet, and cultural controls give limited results.
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