Apple maggot pupae in yard waste piles can be killed by insulating and heating the piles, according to a preliminary study, but experts say the results, while promising, require more research to ensure the pests will not survive to infest commercial apple orchards.
Apple maggot began appearing in the Northwest in the 1980s, which is when state officials began imposing quarantines to prevent it from infesting commercial orchards.
The pest has proliferated in backyard fruit trees in urban areas that are now looking to compost yard waste in a bid to recycle more and save landfill space.
Washington officials issued permits to several companies allowing them to transfer yard waste to composting facilities in pest-free zones, despite concerns from the industry that the pest could ride along.
But when apple maggot turned up in a trap near one last year, they halted the practice until more research could be done to determine the risks to the state’s $2 billion apple industry.
Apple maggot has never been found in commercial fruit in Washington, yet it is a key pest of concern to some trading partners.
Heating the pupae
Last fall, Dr. Wee Yee, research entomologist at the USDA’s Agricultural Research Service laboratory near Wapato, Washington, began researching cost-effective ways to kill pupae in yard waste piles, with the idea being to kill pupae at the source before yard waste is transferred to a pest-free zone.
Little research has been done on effective methods to kill apple maggot pupae, Yee said during an evening presentation at January’s Apple Crop Protection Research Review in Wenatchee, Washington.
A 2008 study showed that apple maggot larvae do not survive rearing at 104°F or hotter, so Yee elected to apply heat in his own laboratory tests on pupae. He found that 100 percent of the pupae died after three days when heated to 122.9°F.
He moved on to field tests last fall at the Terrace Heights Landfill east of Yakima, Washington. Yee built yard waste piles composed of 30 cubic yards of waste, with 60 to 65 percent moisture content, which is ideal for composting.
Organdy bags each holding between 18 and 31 pupae were inserted into the piles — on the surface of the pile, at 5 centimeters deep and at 46 centimeters deep — with data loggers next to the bags registering the temperature over the course of the study.
Yee conducted several tests under varying temperatures and number of days and with piles uncovered or covered with tarps.
Early results were promising on pupae deep in the pile, but he found that killing pupae near the surface of the pile was more challenging.
So, he applied reflective insulation to keep the heat in the pile and came up with his best results:
After nine days, 100 percent of the pupae died in a pile covered with reflective insulation, with temperatures reaching 102°F at the surface, 111°F at 5 centimeters deep and 137°F at 46 centimeters deep, despite the test being done in December when mean ambient temperature was 36.5°F.
Yee also tracked oxygen levels in the pile. Ambient air is 21 percent oxygen by volume; as expected, he found that oxygen levels declined deeper in the pile.
“Oxygen levels may have had something to do with it too, but it’s mostly heat, I suspect, so pupae in and on the surface of waste piles can be killed by biogenic heat,” he said. “Insulating ground yard waste piles with a tarp or a tarp and additional insulation may be a simple, inexpensive way of killing apple maggot pupae during cold months at waste transfer stations.”
Washington officials hired three experts to gather information — including the results of Yee’s research — and study the threats composting facilities could pose to the state’s commercial apple industry.
Those results of that overall study are due this spring.
These preliminary research results are promising, said Jon DeVaney, president of the Washington State Tree Fruit Association, but there remain many unanswered questions about risks to the industry:
Will this be tested on a commercial composting scale? Will the composting industry and regulators accept and follow scientific guidance? Will yard waste be treated before being transferred to pest-free zones?
Who will regulate and ensure intensive monitoring for apple maggot in pest-free zones, and who will pay for it?
Dr. Mike Willett, manager of the Washington Tree Fruit Research Commission, also stressed that Yee’s research results are preliminary.
“More work and thought will have to be given as to how best to replicate conditions encountered in feed stock piles,” he said. “Also, in my experience, the number of insects tested may have to be substantially higher.”
In previous work to validate quarantine treatments, such as for the fumigation of spotted wing drosophila in cherries to Australia, USDA researchers treated 30,000 fruit infested with pests that were at their most resistant phase in the life cycle.
“As Wee noted, his work used fewer than 100 pupae per treatment,” Willett said. “The promising news was that 100 percent mortality appears to be possible.” •
– by Shannon Dininny
Apple maggot life cycle
Apple maggot overwinter as pupae in the soil. Adults emerge in early summer, peaking in July, and once mature, females lay eggs under the skin of apples.
The eggs hatch and the larvae feed within the apple, passing through three instars before the infested fruit falls to the ground and the larvae leave the fruit to burrow into the soil, molt into a fourth instar and overwinter as pupae.
In Washington, apple maggot was first detected in 1980 in Clark County in the southwest part of the state. Today, apple maggot is present in at least 22 of Washington’s 39 counties, though much of the commercial apple production region remains pest-free.