Sustainable Plum Curculio Control in Apple Orchards

New and innovative practices are required when trying to control pests and maintain quality in organic fruit production.

Objective: To control plum curculio in apple orchards with a trap crop of native warm season grasses planted around the perimeter of the orchard for overwintering of curculio. Fire (controlled burning of grasses) was used as bio-control of curculio during peak emergence.

Project Description and Results: The project goal was to produce an apple with minimum plum curculio damage without the aid of chemical pesticides. The first stage of the project was to clear unwanted and non-productive honey locust (Gleditsia triacanthos) and wild plum (Prunus americana, Marshall) thickets from the east and southeast orchard border and to replant the area with warm season (prairie) grasses and forbs.

This brushy habitat provides prime overwintering sites for adult plum curculio, Conotrachelus nenuphar (Herbst) that migrate out of the orchard in late summer. Replacing this habitat with a warm season prairie grass habitat with its high volume of biomass provides an alternative site for overwintering curculios that can be burned by timing their emergence with a degree day model. Clearing away the wild plum also eliminates an alternative wild host plant.

The establishment of this prairie would adjoin an already existing prairie on the southwest half and all of the west side of the orchard, making a unified border on three sides of the orchard. The fourth side is a paddock and a low mowed pasture.

An entomologist began the process of evaluating the orchard and surrounding prairie area for evidence of plum curculio overwintering sites. Random numbers were generated and coordinates were plotted for collection sites in the orchard and old prairie areas. Samples were taken with a Weedeater blower/vac (Wilson, et. al., 1993). A one square meter quadrate was placed over the generated coordinate and excessive vegetation was removed with shears. The "blower/vac" was fitted with a chiffon bag in the "snoot" to catch the fine plant material, soil particles and insects, which were bagged and labeled for sifting and identification.

The samples were taken on November 13 and 14, 1995. The new prairie was not sampled. (It was being cleared and replanted, and at this stage it was oat seedlings, dormant prairie seed, and straw.) The resulting samples were taken to a laboratory and sifted to determine possible overwintering sites for the plum curculio.

In the spring of 1996, all apple trees were given numbers and random numbers were generated. These selected trees were sampled for curculios after their emergence and migration (based on the degree day model for curculios) into the orchard. The trees were sampled by holding a cloth beating tray (3 ft. x 3 ft.) below a branch and striking the branch with a stick to dislodge curculios in the canopy. All curculios were captured, bottled and labeled. No burning of the prairie took place this year.

Apple production in 1996 was low with no saleable apple crop. All apples were collected and fed to livestock.

In the fall of 1996, samples were again taken from the orchard floor, the old prairie and newly established prairie using the same method of sampling described previously.

In the winter of 1997, before the beginning of the growing season (March 1st), a degree-day-recording device was readied and placed in the orchard at a central location. Degree days were then recorded for the entire growing season. Using a base of 40 degrees, the predicted emergence of plum curculios was 313 Fahrenheit degree-days. This was also an appropriate time for burning the tall grass prairie, which was done on April 19, with a previous midnight degree day recording of 310 degrees Fahrenheit. Following the burn, a second set of random numbers was generated to sample apple trees for the presence of curculios, which were again counted and labeled.

The initial sampling of trees in the spring of 1996 produced twenty adult curculios found on forty-eight trees. (Approximately one-third of these were non-bearing immature trees.) By extrapolation, 41.6 percent of the trees in the orchard were infested with curculios.

In the spring of 1997, the second year after clearing and replanting the east and southeast orchard perimeter to prairie, the old prairie was burned. Thirty-five trees were sampled. Non-bearing trees, which occupied approximately one-third of the orchard, were not sampled, since the previous years' sampling showed that non-bearing trees were not suitable sites for curculio oviposition. Five curculios were collected, reflecting a presence in the orchard of 14.3 percent.

Plum curculios were not found in ground litter samples from the orchard or prairie areas, in spite of intensive sampling. Many non-target curculionids were present however in the old prairie samples (one sample yielded ten), with very few collected from the orchard litter (one or two).

In 1997, the orchard produced a healthy yield for a five-year-old orchard. One of the primary indicators of curculio infestation is the abortion of the pome. Most of the apples completed the season, possibly resulting from lower curculio infestation in the orchard.

A thorough evaluation of the techniques employed in this study suggest that it will require several years of burning and sampling to determine whether it is an effective means of plum curculio reduction. The results of this study, while encouraging, are not to be considered as a conclusive proof that burning, in and of itself, is sufficient to lower plum curculio populations over time.

Discussion: Kelly notes, "I learned about insect populations by doing the sampling of the trees. Not just curculios fell into the beating tray. Many beneficial insects and arthropods such as spiders, lacewings, ladybug larvae, etc. also dislodged. The presence of these beneficials caused me to become more aware of insect populations and to be more cautions when using insecticides, albeit organic."

Kelly adds, "This project is another piece of the holistic pie in terms of viewing the farm aesthetically." Side benefits of the project included: additional wildlife habitat, forage opportunity, control of soil loss, addition to diversity, and the creation of an on-farm input.

Kelly says, "One of the best advantages of a long-term project is that it teaches you many new skills and raises your conscience." He noted that it worked out well to have help with the data collection and interpretation.

Additional Project Participants:
Doug LeDoux, Senior Research Assistant, University of Missouri, Columbia: served as entomologist for the project, planned the sampling procedure, oversaw the collection of data and interpretation, provided insect identification.

Keith Jackson, Wildlife Services Biologist and prairie specialist, Missouri Department of Conservation: Donated prairie grass mixture for project site.

Brown Brothers, LaGrange, MO: Donated tractor, disc, and operator and also slides of prairie burn.

William Casey, Farm Management Specialist, University of Missouri Extension, Kahoka, MO: Distributed project results through University of Missouri Extension.

Bibliography:
Wilson, S.W., Smith, J.L., Purcell, A.H. III, 1993: An Inexpensive Vacuum Collector for Insect Sampling/Ent. News 104(4): 203-208.