WSDA Blog

Date: Wednesday, July 23, 2025

Author: pio@agr.wa.gov

Every summer, thousands of sticky yellow traps go up throughout Eastern Washington, hung on trees in backyards, parks, orchards, and public spaces. They’re part of a carefully coordinated effort by the Washington State Department of Agriculture (WSDA) to monitor one of the state’s most persistent fruit pests: the apple maggot fly.

Cruz Gonzalez, a WSDA lab technician, is learning how to perform a delicate, difficult, and important task: remove tiny flies from those sticky traps and dissect them. The goal? To help identify whether any of the insects are Rhagoletis pomonella, the scientific name for the apple maggot fly.

What’s the process like?

The work starts in the field, where seasonal trappers place bright yellow sticky cards with lures that are irresistible to flies near apple orchards. After collecting the traps, the real detective work begins in the lab. Flies that look like possible apple maggot candidates are examined under a microscope.

But before the fly ever reaches the microscope, it must first be carefully removed from the sticky trap where it was caught. That’s the job Gonzalez is learning this week from Tanya James, WSDA pest biologist, as part of the Apple Maggot Survey Program.

“Pulling flies off sticky traps isn’t as simple as it sounds,” James said. “They’re small, delicate, and completely stuck. You must go slowly, or you’ll damage the body and make it harder to identify later.” 

First, Gonzalez and other lab techs use a citrus-based solvent to dissolve the sticky glue without damaging the insects. With a pair of tweezers, he gently lifts each fly from the card, placing them into a labeled vial.

WSDA staff are trained to dissect the flies and look for specific internal features unique to apple maggots. It takes a sharp eye and steady hands, but this process helps the department confirm whether a fly is just a lookalike, or the real deal.

It’s painstaking work and accuracy matters; only adult apple maggot flies are counted and identifying them requires that key features, like wing patterns and body markings, stay intact.

“Once you start seeing the differences in the flies, it becomes easier to identify an apple maggot fly,” Gonzales added.

From there, samples head to dissection and identification, where another layer of precision begins. The lab technician determines whether the adult fly is male or female. If it is a male, the tab tech flips the fly on its back and examines its underside at 50-fold magnification to determine the species. If it’s female, in order to determine species they must remove and measure the length of the fly’s ovipositor, or egg laying tube. If the ovipositor is longer than 1 mm, that confirms the adult fly is in fact the notorious apple maggot.  

Why though?

Washington is the #1 producer of apples in the world, and the apple maggot is considered one of the top threats to this crown. If apple maggots were to spread into commercial orchards, the consequences would be significant: crop damage, trade restrictions, and stricter pesticide requirements that could impact both growers and the environment.

That’s why WSDA runs the Apple Maggot Survey Program, to detect the pest early and keep it out of critical growing regions. The program is part surveillance, part science, and 100% essential to protecting Washington agriculture.

Every dissected fly, every trap checked, and every skilled technician trained — like Cruz Gonzalez — plays a part in defending a billion-dollar industry and the thousands of families who depend on it.

Want to learn more about WSDA’s pest detection work?

Visit agr.wa.gov or follow us on social media to see behind-the-scenes stories of the people protecting Washington’s agriculture.