By C.J. Walke, Orchard Program Manager
Plum curculio (Conotrachelus nenuphar) is considered one of the most destructive insect pests in apple and plum orchards and has been referred to as the “Achilles’ heel” of organic orcharding because of the difficulty controlling this pest with cultural methods and materials approved by the Organic Materials Review Institute (OMRI). Plum curculio is a type of weevil (or snout beetle). Adults are about a quarter-inch long and dark brown in color. They overwinter as adults in forest leaf litter, usually at the edge of the orchard, but in polyculture orchards, like the Maine Heritage Orchard, where other trees and shrubs are interplanted to support ecosystem biodiversity, they may find sites to overwinter within the orchard.
In organic orchards, we typically try to control plum curculio by using a kaolin clay product named Surround, which is really a pest repellent, not an insecticide. Adults emerge in spring and by petal fall, females start laying eggs in newly set fruitlets. Surround is applied from petal fall to about July 1 here in central Maine, typically twice at petal fall to build up a good layer, and reapplied every 7 to 10 days to keep good coverage, especially after rain, which will wash off some of the clay. The kaolin clay particles are refined to no greater than 1.4 microns and flake off onto adults as they search for egg-laying sites, causing irritation and excessive grooming, and the adults usually move on to a more suitable site.
Surround works best when part of a trap crop or push/pull system, where a few (depending on scale) trees are left unsprayed and plum curculio adults will congregate on those trees. The fruit of these trees are essentially sacrificed, where infected fruit can be collected and destroyed, or a stronger organic insecticide can be applied, like PyGanic (labeled for weevils in general), to kill adults. An additional approach is to apply nematodes to the soil beneath these trap trees to kill larva in the soil. Managing the infected fruit is essential in this type of system so you don’t create a hotspot for plum curculio breeding and reproduction.
Multi-Cultivar Grafted Trees
Recent research in the Northeast has focused on grafting multiple apple varieties that are highly attractive to plum curculio, as well as some other pests, onto trees strategically located around the orchard perimeter. In 2023, Dr. Jaime Piñero from the University of Massachusetts Stockbridge School of Agriculture presented at the Maine Pomological Society summer meeting where he shared data from this research over the past few years.
Participating orchards have grafted multiple varieties onto these trap trees, using Red Astrachan, Yellow Transparent, Ginger Gold, Liberty and Wickson. Dabinett has also been used in some cases but has not shown to significantly attract plum curculio adults. Data collected from 2021-2023 shows that Red Astrachan, Yellow Transparent and Wickson provided the best results at attracting plum curculio adults. Liberty’s results were not that high, and some growers were concerned about using Ginger Gold because of its susceptibility to fire blight, a serious bacterial disease of apple and pear.
These trap trees are located around the perimeter of the orchard and spaced about 30 meters apart. Some of the research has incorporated pheromone lures to help attract plum curculio to the trees, as well as wintergreen oil, which is also highly attractive. Perimeter trees are chosen because they are the first trees plum curculio will reach as they move from the forest edge into the orchard. Trapping along the perimeter should prevent plum curculio from migrating into the center of the orchard, as long as they are managed. Some integrated pest management (IPM) orchards are using these trees to monitor for plum curculio activity and applying a perimeter insecticide spray once adults emerge as a more aggressive means of controlling this pest.
Practice in the Maine Heritage Orchard
These positive research results are compelling enough that we plan to incorporate multi-cultivar grafted trees into the Maine Heritage Orchard (MHO) in Unity in 2024. The first step is topworking (grafting) Red Astrachan, Yellow Transparent and Wickson onto trees around MHO to establish the trap trees, which should be completed by the time this article is published. The next step is to wait a few years for the grafts to grow, mature and start to flower and produce fruit, as it is the fruit plum curculio adults want to feed on and lay eggs into, not the leaves. While we wait a couple seasons for fruit, the trees will need attention to the grafts, strategic pruning and certainly branch training to get good tree structure.
Challenges are anticipated because MHO is not laid out in traditional “blocks” of a single variety with woods and forest around the perimeter. The orchard was built within an old gravel pit, so we have terraces, riprap rock waterways, a settling pond, woods inside the orchard as well as around the orchard, and sections that are essentially inaccessible to our tractor and spray equipment that we use as part of our pest management strategy. We need to be strategic in our selection for trap trees so we don’t create a breeding hotspot for plum curculio.
Obviously, this is a long-term project since we have a few years until the grafts produce fruit and then a few more years of implementation and data collection to evaluate the whole system. We plan to report back and share what we learn along the way regarding this unique approach to managing plum curculio organically in a non-traditional and diverse orchard setting. Patience is always part of the process when we grow tree fruit, knowing the trees we plant and tend to today are really for successive generations to harvest and enjoy.
This article was originally published in the summer 2024 issue of The Maine Organic Farmer & Gardener. Browse the archives for free content on organic orcharding and pest management. Subscribe to the publication by becoming a member!
