MN Fruit & Vegetable IPM News - Vol. 1, No. 5

Plum curculio (PC)is a major pest of apples in Minnesota. Many commercial apple growers, in particular organic producers or those using soft pesticides are significantly concerned about this pest. PC is native to North America where it is a key pest of pome and stone fruits.

Biology

PC is a beetle in the family Curculionidae (snout beetles or weevils). Adults overwinter in ground litter or soil either within the orchard or in surrounding wooded lots. Overwintered adults emerge in the spring around apple bloom, and fly to apple trees to feed on apple buds, flowers, and young fruit. Adults are active when temperatures exceed 60°F. Developing fruit is most susceptible to PC injury after petal fall. The spring emergence of PC usually continues for about 6 weeks. After mating, females begin to lay eggs on the fruit. After hatching, the larvae continue to feed and develop in the fruit. Development is complete by late June or July when they emerge as adults. These summer adults feed on developing apple fruit, and overwinter in late summer. Only one generation occurs per year in Minnesota, as is the case in most other regions.

Damage

Fruit damage by PC occurs both during feeding and egg-laying activities. Feeding injury by the larvae could be in the form of surface scars (superficial stings), or may result in internal discoloration of the fruit if larvae gained deep entry into the fruit. Many fruits harboring PC larvae usually fall off the tree during the June drop. During egg-laying, an adult female PC cuts out a small hole in the fruit, deposits her eggs, and covers the hole producing a crescent-shaped slit just below the site where the egg was laid. The slit protects the eggs from being crushed by the developing fruit. Fruit damage is usually most common in border rows next to sites where adults overwinter.

Management

Monitoring: Various trap designs are commercially available for monitoring the immigration and within-orchard movements of adult PC weevils. They include circle (shown left), pyramid (shown below, left), and panel traps. Traps must be located in close proximity to woods in order to capture immigrating PC. Studies, conducted by researchers at the University of Massachusetts at Amherst on the efficacy of PC traps, have shown that some traps are more useful under certain situations. For instance, panel traps are useful for intercepting flying or immigrating PC, while circle traps are more effective for monitoring movement of adults from tree to tree. However, these traps are not at the moment effective for predicting PC injury. The development of an effective trap-based monitoring system for

PC is currently hampered by lack of effective and attractive lures. An aggregation pheromone (grandisoic acid) has been developed for PC, and various fruit volatiles are being tested alone or in combination for PC monitoring. Currently, work is continuing on efficacy testing of various fruit volatile compounds singly or in combinations for monitoring PC.

Chemical control: The objective of control is to prevent both feeding and egg-laying injury by adult PC weevils. Growers with a history of high PC pressure in their orchards should apply the first spray against PC at petal fall. Subsequently, PC movement, feeding, and egg-laying activities are highly dependent on the temperature and weather. A model has been developed by researchers at Cornell University ( Geneva, New York) to determine how many additional sprays will be necessary to maintain protective chemical residues to prevent subsequent damage through out the PC egg-laying period. This oviposition (egg-laying) model takes into consideration the effect of temperature on the activity and oviposition of the weevil. The model helps to determine the cut-off point for sprays after petal fall. One assumption of this model is that residues from control sprays after petal fall only need to be maintained on fruit and foliage until about 40% of the egg-laying period is complete. According to the model, this occurs at 340 DD (base 50°F) after petal fall. It is believed that after 40% of PC egg-laying is over, the movement of adult weevils into and within the orchard is greatly reduced. At this time, the resident weevil population in the treated or chard will not be able to complete the remainder of their normal egg-laying period having been controlled by insecticide residues.

Summary of PC egg-laying model ( Cornell University)

• Treat the entire orchard at petal fall using a broad-spectrum insecticide.

• Begin calculating the accumulation of degree day (DD) after petal fall treatment (base 50°F).

• No additional sprays are necessary if 340 DD accumulate with in 14 days of petal fall treatment.

For more information about pest management programs for MN apple growers, including PC recommendations, go to the web site listed above.

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ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@umn.edu
Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Division,jeanne.ciborowski@state.mn.us
Suzanne Wold-Burkness, Department of Entomology, University of Minnesota, woldx018@umn.edu

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Tuesday to Jeanne Ciborowski, 651-297-3217, jeanne.ciborowski@state.mn.us , MDA, 90 W. Plato Blvd., St. Paul, MN 55107-2094. You can access the Newsletter at the U of MN web site in htm. format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf. format at: www.mda.state.mn.us/biocon/fruitreports/default.htm

Partial funding for this publication is provided through partnership agreements with the Minnesota Fruit and Vegetable Growers Association and the United States Department of Agriculture – Risk Management Agency (RMA) and the RMA Community Outreach and Assistance Partnership Program. These institutions are equal opportunity providers.

DISCLAIMER

References to products in this publication is not intended to be an endorsement to the exclusion of others which may have similar uses. Any person using products listed in this publication assumes full responsibility for their use in accordance with current manufacturer directions.

Last Revised June 4, 2004.
The University, including the Minnesota Extension Service, is an equal opportunity educator and employer. ©1999-2004 Minnesota Extension Service, University of Minnesota. All rights reserved. Contact copyright@extension.umn.edu for information on reproduction or use of this material.