By Xander Rose
A second fruit fly monitoring season in my work in the orchards at Raintree has come to a close. Certain flies can be significant pests of orchard fruit, but they can also be monitored and controlled to minimize damage to fruit. As someone who went to graduate school to study insects (among other ecological, forestry, and agricultural topics), the scientist in me is super happy to be studying these and other insects at Raintree. The most wanted flies for us, in terms of significance in being a threat to fruit production, are apple maggots and Suzuki fruit flies.
Key to any success in controlling a pest is understanding its biology and natural history. Apple maggots (Rhagoletis pomonella) and Suzuki fruit flies (also known as spotted wing drosophila, Drosophila suzukii) are non-native to the Pacific Northwest. In the case of the apple maggot, some flies evolved to switch hosts from native hawthorn in northeastern North America to apples introduced by Europeans. This is a fascinating story of sympatric speciation, occurring in the last 200 years, that I would recommend the curious reader look into further.
Apple maggots are such important pests that the state of Washington has a quarantine law to protect the valuable apple orchards of eastern Washington from them. The apple maggot has figured out how to hide its larvae (maggots) away from predators in a large, protective food source. Flies emerge in the late spring as the ground warms from overwintering puparia (protective coverings of the middle life stage), mate, and then females lay their eggs into apples. Large, red apples seem to especially attract them on warm afternoons. The larvae develop in apples, which, being larger than hawthorn fruits, offer better protection from parasitoid predators like wasps. Adult apple maggot emergence is staggered through the summer and fall, meaning that flies emerge throughout the apple ripening season. The maggots mature in the fruit and then crawl out from ruined and rotting fruit that has fallen to the ground, going underground to pupate overwinter. This life cycle provides clues as to how to control apple maggot. It’s not by targeting the feeding larvae hiding in your apples! Perhaps the emerging adults could somehow be distracted or trapped? What of the ground fruit that may contain maggots about to crawl underground?
Fig 1. An adult apple maggot
Fig 2. An apple maggot larva in fruit
Photo credits: University of Minnesota Extension
With any pest, one should start by learning about the organism. What is it? How do you identify it? What does it feed on? How does it develop? Why can it be a problem? What can be done? These are all relevant questions that are worth asking, and answering! A good starting point is with information from agricultural extension services provided by land grant universities.
So, I needed to answer some of those previous questions for these insect pests at Raintree. Because the larvae are hidden in fruit and the pupae underground, control and monitoring are best conducted on the adults. They find bright red spheres (above) irresistible because they look like ideal, gigantic apples! Think of a red delicious. Putting a few traps covered in sticky paste on every apple tree (at about head height for checking and cleaning) will catch many of the adults. A yellow card as a backdrop to two red hemisphere that stick together makes these traps (called Ladd traps) all the more attractive. A pheromone bait put on the trap will further help draw the flies in. Checking the traps will inform flies’ locations and activity. Putting clear plastic wrap around the two red hemispheres of each trap makes refreshing them easier because the sticky stuff covered in bugs can be peeled away and then reapplied to a new sheet of plastic wrap. Other pesticide-free control options include kaolin clay and bagging fruit. Kaolin clay can be used to cover fruit, making it less attractive for egg laying. Bagging fruit when it's small with stretchy bags that are similar to pantyhose in terms of materials keeps the flies from laying their eggs. To make for bigger, healthier fruit, less habitat for apple maggot, and to make applying kaolin clay or using bags easier, fruit thinning of some fruitlets is highly recommended.
Fig 3. A Ladd trap used for trapping apple maggots
Fig 4. Other materials for a Ladd trap
Based on my research and experience, probably the most important control measure for apple maggot is simply picking up all the fallen apples. This should be done fairly promptly, depriving the mature maggots of an opportunity to crawl out into the soil. The apple "drops" need to be processed (into apple products for consumption like apple sauce or apple butter), disposed of (thrown away so that maggots will not pupate), or destroyed (by burning or thorough and complete "hot" composting). In order to kill maggots in a compost heat, I would think that the fruit needs to be put through a chipper and composted with plenty of sawdust. If the compost bin had a liner, then perhaps maggots couldn't escape.
So, apple maggot can be controlled. I have had success so far at Raintree. I'm encouraged by observing far fewer apple maggots so far this year in traps than I did last year. Having cleaner apples for cider, drying, fresh eating, and so many other good things is a great reward for all the hard work.
Spotted wing drosophila came over here from East Asia. They are generalist feeders on soft fruits. Unlike most fruit flies, female spotted wing drosophila can lay their eggs into the unbroken fruit skin. This means they can ruin otherwise untarnished fruit. Blackberries, blueberries, raspberries, cherries, and plums come to mind as especially vulnerable to Suzuki fruit fly. These flies do not overwinter underground as pupae the way the apple maggot does and freezing temperatures kill them fairly quickly. Therefore, they likely re-invade orchards from more sheltered spots every year. The maggots pupate in or on fruit. From egg to adult, the life cycle can take as few as seven days in optimally warm temperatures. The adults are attracted to rotting fruit smells. This life cycle shows that if you have an orchard in a place with cold winters, then this fly should be controllable. Where could the fly overwinter that is protected from freezing temperatures? Would picking up rotting fruit help reduce this habitat? Where might it reinvade from? How could you trap adult flies to reduce the population and fruit damage during the warm months of the year?
Fig 5. Adult Suzuki fruit flies (female on left, male on right)
The spotted wing drosophila can be trapped by hanging lidded plastic cups with holes poked into them up top and the bottoms filled with apple cider vinegar. A little dish soap will help to break the surface tension, so the flies don’t escape the traps. These traps should be hung in susceptible plants. For organic control, the more traps you hang the better. Traps can be moved around to cover susceptible fruit as it ripens. Just like with the apple maggot traps, these vinegar traps should be monitored for adult spotted wing drosophila. [Males have fairly distinctive spots on their wings and the flies can be identified through some study and maybe the use of a magnifying lens.] The traps should be emptied and refilled periodically. Thinning and picking up fruit and trapping adults will go a good distance towards having cleaner fruit.
Raintree offers pheromone lures for apple maggot. Apple maggot lures can be paired with Ladd traps, which can be purchased elsewhere, along with Tanglefoot or other similar sticky petroleum-based coating for insect traps:
We also offer pheromone traps for codling moths (a different insect pest on apples that may be relevant in your orchard and has its own life history). Codling moths lay their eggs on or near developing fruitlets. They eggs hatch and then the moth larvae crawl to the stem end of the fruit, where they enter and feed around the core. Unlike apple maggot, codling moths emerge as larvae before the fruit falls and pupate under bark on the tree or in other crevasses in the ground near the base of the tree.
Happy growing! Because, after all, change is the only constant. -Xander Rose
Agriculture and Natural Resources, University of California: Integrated Pest Management Program. 2019. “How to Manage Pests in Gardens and Landscapes: Codling Moth.” Accessed Nov. 7 2020.
Cornell University. 2017. “Spotted Wing Drosophila.” Accessed Nov 7 2020.
Hahn, Jeffrey, Grabowski, Michelle and Jill MacKenzie. 2018. University of Minnesota Extension Service. Accessed Nov 7 2020.
National Centre for Biological Sciences. 2017. “The apple maggot fly—how an altered sense of smell could drive the formation of new species.” Accessed Nov. 7 2020.
Oregon State University College of Agricultural Sciences. 2020. Accessed Nov. 7 2020.