Figure 1. Caught red handed: two adult plum curculio beetles feeding on an apple (left), and a plum curculio larva inside a peach (right). Photo credit: J. Obermeyer
The plum curculio (Conotrachelus nenuphar) is a small “snout” beetle (also known as a weevil – one of my favorite beetle groups!) that will feed on apples, peaches, pears, plums, and cherries. Both adults and larvae have chewing mouthparts and can damage fruits; adults by feeding and laying eggs in newly set fruit and larvae by feeding internally within fruits, causing premature fruit drop in some cases. Studies have reported that if plum curculio is not managed effectively, up to 85% of fruit may be damaged at harvest! So, in this issue, I wanted to highlight a bit about the biology and behavior of plum curculio and provide some helpful tips on management.
Plum curculio overwinters in the adult stage in ground litter or the soil, and when spring arrives, they migrate to trees and are most active from petal fall through first and second cover. The adult beetles can walk and fly and are primarily active at night when temperatures are 70°F or warmer. Diagnostic symptoms of plum curculio in the orchard include: 1) physical distortion of newly set fruit, called “cat-facing”, caused by adult feeding damage, 2) appearance of “c-shaped” or “crescent-shaped” wounds on fruit, caused by egg laying, and 3) premature fruit drop, caused by larvae as they feed within the fruit. Aside from losing fruit prematurely, plum curculio can also hurt your bottom line when injury caused by adult feeding or egg laying leaves scars that make fruit unmarketable.
Figure 2. Symptoms of plum curculio egg-laying damage to young fruit (left) and mature fruit that did not drop from the tree (right). Photo credit: J. Obermeyer.
So, what steps can you take or add to what you are already doing to improve management of plum curculio in your orchard?
#1: Use cultural control strategies
Plum curculio is native to North America, so it is adapted to survive and feed on other non-cultivated hosts in the plant family Rosaceae. Beyond your orchard, you can reduce plum curculio success by removing unmanaged or wild fruit trees, which provide alternative food sources and mating sites for adult beetles. Within your orchard, you can prune fruit trees during the dormant period to create an environment that is less favorable for the adult beetles during the growing season.
#2: Monitor adult activity
You can fine-tune the start of your monitoring and management efforts to coincide with key periods of plum curculio activity using degree-day models.
Tracking degree days (the accumulation of temperature units, based on average daily high and low temperatures) can help us predict the biological activity of insects more reliably than calendar dates, because insect development and activity are dependent on temperature. Researchers have identified the lower developmental threshold for plum curculio to be 50°F, which means any time the average daily temperature is greater than 50°F, degree days are accumulating, and this insect is active and developing towards the next life stage. Based on research in the northcentral region, there are key degree-day time points that are associated with plum curculio activity. For example, using a start date of March 1st to begin tracking degree days, researchers at Michigan State found:
At 275 DD50: plum curculio females will likely have mature eggs and will be ready to lay eggs in developing fruits.
At 340 DD50 after petal fall: 40% of egg-laying by plum curculio will be complete.
Along with tracking the accumulation of degree days, you can monitor adults using plum essence or benzaldehyde lures and pyramid traps (beginning at bloom), as well as beating sheets, which are placed beneath the canopy while you shake or beat branches with a stick or similar. Place pyramid traps on the borders of your orchard, or where damage has been observed previously, to get the best sense of pressure this year. Use beating sheets earlier in the morning when adults are less active and easier to dislodge. Beating sheets are also effective because adult weevils are notorious for playing dead (ha!), so they will readily fall from the branches and land on the sheet, where they will be easier for you to spot. Last, but not least, monitor traps several times a week (the more frequently you can check the better!), and once fruits begin to set, monitor them regularly for signs of feeding or egg-laying injury.
Step 3: Consider using biological control agents
There are naturally occurring predatory and parasitic insects, fungi, and insect-parasitic nematodes that attack plum curculio. These biological control agents can be thought of more as a long-term, sustainable approach to reducing plum curculio populations and associated damage. So, in other words, these are not agents that will reduce populations quickly, so you will want to plan ahead and allow time for these organisms to do their work! More recently, the success of insect-parasitic nematodes against plum curculio has received more attention. These nematodes only attack insects (not plants!), and when applied to the soil at the base of trees in the orchard, insect-parasitic nematodes are available in high densities to attack plum curculio larvae as they leave dropped fruit and enter the soil to pupate. By reducing the number of larvae and pupae surviving each season, there will be fewer adults that overwinter, and therefore fewer adults to emerge the following spring! There are several factors that impact the success of biological control agents, so these strategies may not work equally well for everyone, but they can be a valuable complement to chemical control strategies.
#4: Apply chemical control strategically
The adult beetles are the main target of insecticide applications. Time insecticide applications at the petal fall through first and second cover stages of tree development to kill adults before they feed and lay eggs. Typically, an insecticide application timed at petal fall will be sufficient to manage plum curculio. Several classes of insecticides, including the neonicotinoids, pyrethroids, diamides, and carbamates have good to excellent efficacy against plum curculio. However, keep in mind the use of pyrethroids at petal fall are not recommended because they kill off predatory mites, leading to potential outbreaks of pest mites like European red mite and two-spotted spider mite. Also, take care if using Sevin (active ingredient carbaryl) against plum curculio within 30 days of bloom because fruit thinning may also occur. To get the best out of your insecticide sprays, time them carefully using degree days if possible, consider residual activity and whether one insecticide spray can target other insect pests at the same time as plum curculio, and be mindful of which insecticides are labelled for use on stone or pome fruits, because some are labelled for use on one crop group but not the other.