Orchard Pest Thresholds

Tachinid flies and their larvae parasitize some orchard pests. English photo

By C.J. Walke

The term “threshold” is used in Integrated Pest Management (IPM) strategies to describe the level at which pest pressure and crop damage have reached the point where plant health will start to decline and/or crop damage will result in reduced yields. Typically these pest-specific thresholds are tied to the balance of the economic value of the crop versus the expense of the management action required to keep the pest in check. So while standing in the orchard, we debate: Will the expense required to manage a pest exceed the value of the loss in yield? Or what level of pest pressure can we accept and still attain a fruitful harvest?

When growing tree fruit or other food crops, we need to expect that those plants are going to attract other critters in the farm or garden ecosystem that also want to consume that food crop for their own growth and reproduction. As organic farmers and gardeners, we need to understand the life cycles of these pests, learn to anticipate their arrival, and monitor their activities to determine what level of damage they may inflict upon our crops. When we start to notice pest populations on the rise and crop damage becoming visible, we’ve reached a certain “threshold” where we may need to take more aggressive action to limit the damage, but we can employ many cultural methods before getting to that point.

As certified organic growers, we need to observe the standards of the National Organic Program (NOP) and use them as a guide in our management practices, specifically NOP §205.206 Crop, Pest, Weed and Disease Management Practice Standard. Here we will find that certified organic producers need to take a three-tiered hierarchical approach to decide how to manage pest, weed and disease problems. The Practice Manual from MOFGA Certification Services (MCS) outlines this approach in a clear manner, using three levels:

Level A – Prevention/avoidance of the problem is the required first line of defense. Crop rotation, cover crops, sanitation and varietal selection are examples of level A approaches.

Level B – If level A methods are not adequate, producer may use mechanical or physical methods as well as non-synthetic or natural control materials. Examples of level B approaches include row covers, mulching, cultivation, flaming and mowing.

Level C – If level A and B methods are not adequate, producer may use an OMRI- or MCS-approved pesticide. NOTE: A pesticide application log must be maintained and be made available to the inspector.

Now that we have the terminology and our paperwork is in order, how does all this actually manifest in the orchard? From here we’ll look at a few of the major pests of apples and establish an approach according to these three tiers.

Level A – The primary activity we can employ in the orchard to prevent or avoid pest issues is to cultivate diversity in both the plant and animal realms. Beneficial insects themselves need food sources and shelter, and we can grow or encourage various flowering plants (Queen Anne’s lace, yarrow, sweet cicely, cilantro) with varying bloom times throughout the season to provide food and shelter. By cultivating plant diversity, we support beneficial pollinators (mason bees/blue orchard bees) and predatory insects (hover flies/sweat bees). We also encourage parasitoid wasps (Ichneumonidae) and parasitic flies (Tachnidae) that lay their eggs inside or on the surface of pests. When eggs hatch, the resulting larvae feed on that pest, while the beneficial adults also feed on numerous larvae themselves. These activities help our orchard efforts but usually are not enough on their own, so we move into the next level.

Level B – Here we begin to employ different types of traps to target specific pests – mostly internal fruit feeders – and to monitor population levels in relation to established thresholds for that pest. Just before apple bloom begins, we can hang white sticky cards to monitor the presence of European apple sawfly (EAS), a small, reddish-bodied fly that lays eggs at the base of the flower. When those eggs hatch, the resulting larvae eat a winding trail across the fruitlet, then burrow in toward the seeds. The white of the card mimics the color of the apple blossom, and the flies land on the card and get stuck. The threshold for this pest is an average of three flies per trap when checked twice a week.

Later in the season we expect the arrival of apple maggot fly (slightly smaller than a housefly in size), whose egg laying in developing apples leaves an oviposition sting scar on the surface of the fruit, but whose larvae tunnel throughout the apple, creating a trailing, brown mess. Red ball traps coated with Tanglefoot can be hung by July 1 to monitor populations of this pest, as the females are attracted to the red color that mimics a ripening apple. Small containers of apple essence attractant can be hung near the ball trap to lure females toward the trap. The threshold for this pest is an average of one fly per trap per week for an unbaited trap or an average of five flies per trap per week on a baited trap.

Level C – Once a pest has reached the determined threshold level, an OMRI-approved pesticide may be needed to keep the population from increasing and crop damage from spreading. The trick with EAS is that the adults are out during bloom, and we never want to spray or apply materials that may interfere with pollination or negatively impact pollinators or beneficial insects. This is where the three-tiered approach doesn’t apply just to a single growing season, especially with perennial crops, because I may have reached the threshold for sawfly, but I don’t want to affect beneficials, so next season I’ll apply Surround (an OMRI-approved kaolin clay product) at early pink bud stage to repel sawflies before the blossoms open.

This is also a good example of how these three levels of activity are not independent of one another. I am cultivating diversity year-round in the orchard (Level A), then hanging white sticky cards before bloom in May to monitor EAS populations (Level B), with a possible Surround application before bloom based on last year’s monitoring (Level C), while also picking up drops throughout the summer to prevent mature EAS larvae from exiting the fruitlet, pupating in the soil and returning the following season for a new round of pest pressure (Level A).

When we look at orchard pest thresholds that have been determined through scientific and economic research, the actual pest population levels for these thresholds is very low. In my experience growing tree fruit in Maine, if you have bearing apple trees and hang white cards and red ball traps, you are going to quickly catch European apple sawfly and apple maggot fly adults in numbers greater than the established thresholds. Orchard pest dynamics are complex, with local nuances varying for each grower. This is where education holds the highest value, so that we are aware of and prepared for the challenge ahead rather than reacting to it and being a step behind or a day too late.

C. J. Walke is MOFGA’s organic orchardist. You can address your orcharding questions to him at 568-4142 or [email protected].
 

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