If we presented awards for Most Evil Insect Pest, plum curculio could definitely contend for the grand prize. Both the adults and larvae of this weevil attack apples, cherries, peaches, pears, plums, and quinces! For a long time, the only organic controls for this pernicious pest included shaking them out of trees and faithfully destroying the fallen and damaged fruit that house larvae and the leaf litter that adults overwinter in.
But an emerging control technique gives hope to organic growers. Researchers at the University of Missouri found that the nontoxic kaolin clay product Surround WP provides substantial control of plum curculio on apples.* Surround forms a thin clay barrier around the fruit that repels adults and prevents them from depositing eggs in the fruit. The researchers began spraying Surround on the trees at petal fall and continued applications until one week before harvest. The study found that a heavy, consistent coating of Surround provided the highest level of control (with numbers of insect-infested fruits down by an average of 79 percent).
While Surround WP (also sold as Surround at Home) is available to the home gardener through a variety of mail-order sources, it can be expensive. Andy Thomas, the research associate who ran the University of Missouri study, suggests looking into a local source, such as an orchard- or farm-supply store, to avoid shipping costs. Determining what level of damage you find acceptable and then experimenting with rates and frequency of application can also reduce costs. Trees coated with Surround do look a bit strange, but the dramatic reduction in plum curculio damage may be worth odd white trees.
Although Surround is a promising addition to an organic pest-control plan, it must be combined with sanitation and proper tree maintenance, cautions Thomas. "Surround is not a panacea; it is one of many tools an organic gardener needs. Weak trees are more susceptible to damage, so the bottom line is it's important to keep your trees healthy." *Surround can be sprayed on all fruit trees, including plum and pear.