WSDA Blog

Date: Wednesday, September 25, 2024

Author: Karla Salp

As the weather turns colder and the leaves start to turn, apple season is starting in earnest. Washington’s iconic apples are loved not only in Washington and across the country but around the world. But sending apples worldwide comes with a special requirement: no pests.
 

One of the pests of greatest concern for many international markets is apple maggot. Entomologists call this pest Rhagoletis pomonella and recognize it as part of the deciduous fruit flies insect family Tephritidae.  WSDA’s Pest Program has been monitoring for signs of apple maggots and preventing apple maggots from entering the commercial market since the pest was first detected in Washington in the 1980s.

To date, apple maggot has never been detected in commercially available apples – a resounding success considering more than half of the state is quarantined for the pest.

Keeping Washington’s commercial apples pest-free is a team effort between various WSDA programs, county pest boards, researchers, and the apple industry. But it all starts with a sticky yellow trap.

Trapping for apple maggot

_{Apple maggot trapper, Jasime James, checks an apple maggot trap near Yakima}

Each summer, WSDA’s Pest Program’s apple maggot team adds a fleet of temporary apple maggot trappers to its number. Their mission: to learn set traps, identify potential apple maggots, and submit traps with potential apple maggots for microscopic inspection.

The trappers drive routes and hang apple maggot traps in pest-free areas and near at-risk orchards. The traps are simple – yellow cards, with fly-attractive lures attached, slathered in a sticky substance to capture adult flies. The traps are checked multiple times throughout the summer, being replaced when a potential apple maggot is found on the trap – or when it is just too “disgusting” – as one trapper put it.

There are a host of less-than-pleasant challenges that trappers deal with. While most landowners are fine with having traps placed on their property, some are less receptive, to put it nicely. Trappers also deal with extreme heat, which can be particularly challenging if a vehicle breaks down. And this year – like most years – they have wildfires and smoke to work around. Sometimes the fires even burn up their traps.


_{Apple maggot trap hanging in tree in foreground next to burned wildfire area}

To be or not to be…apple maggot flies

The yellow sticky traps attract several types of flies that the trappers are trained to identify and record into an online data and mapping interface. They identify and tally the different types of flies that look similar to apple maggot – such as the walnut husk fly – and also record the total number of potential apple maggot flies.
These “potential” apple maggot flies are one of two kinds: either the apple maggot fly the program is looking for or the snowberry fly, Rhagoletis zephyria. Both are virtually impossible to distinguish with the naked eye. In fact, they normally must be dissected, and the ovipositors (the structure used to leg eggs in the fruit’s skin) must be examined and measured under a microscope to determine whether the fly is an apple maggot fly or snowberry fly.

Before the flies can be examined under a microscope, staff have the tedious job of removing the flies from the sticky goo that caught them. This involves confirming the number of flies on the trap, adding a solvent to each one, and carefully removing them and putting them in a vial to further remove the sticky residue. This is done for the hundreds of traps that come into the lab each week.


_{Applying solvent to release fruit fly from trap}

One in a million – or at least it feels like it

So far this season alone, WSDA trappers have identified over 48,000 fruit flies on their traps. Many of these flies are non-target fruit flies or snowberry flies. But the critical question must be answered – is there an apple maggot hiding among the snowberry flies? It’s a bit like looking for a needle in a stack of needles – but this needle (the ovipositor) is slightly longer than the rest.

Of the 48,000 flies so far this year, over 5300 flies were identified as potential apple maggot. The percentage of apple maggots in the total fly catch is low, thankfully. Looking through those 5300 flies, the program has identified just 123 apple maggots. But each of those detections is critical and sets off a chain reaction of events.


_{Processing apple maggot traps, each with at least one (usually many) suspect flies}

We found one! Now what?

Once an apple maggot fly is detected, the response race is one. WSDA staff return to the site and collect potentially infested fruit. That fruit is brought back to the lab and placed in containers, allowing any maggots to fall into the bins when they exit the infested fruit.* The program then attempts to rear the maggots to adult flies and confirm whether there are reproducing apple maggot populations at the site.

In addition to WSDA gathering fruit, county pest boards are alerted when apple maggots are found. The county pest boards will visit the site and attempt to mitigate the detection. Mitigation might include spraying or, when possible, removing the host material.


_{Infested hawthorn berries placed in a tub to allow larvae to exit the fruit. Hawthorn is a well-known alternate host plant for apple maggot.} 

Researching solutions

The Pest Program in collaboration with Dr. Wee Yee at USDA-ARS is also learning more about apple maggot, alternative host plants, and potential natural controls. They have discovered several parasitic wasps that has been found to lay eggs in apple maggots, killing the maggot. These wasps may help with apple maggot control outside of orchards in the future.

The program has also identified different species of native hawthorns some of which apple maggot appears to prefer over others. For example, apple maggot prefers the more widely distributed large-thorned hawthorn (Crataegus macracantha) over the more locally abundant black hawthorn (Crateagus douglasii).


<sub>Seasonal lab technician Korina Medina identifies fruit flies with the aid of a microscope. 


The lab technicians must find and measure the fruit fly's oviposter (use to lay eggs) to determine if it is an apple maggot or not.</sub> 

Multifaceted effort

From trapping to lab identification to cutting edge research, WSDA and many partners are keeping pests out of Washington’s commercial apples. Thanks to their work, people around the world can enjoy Washington-grown without worrying about introducing new pests to their country. Visit agr.wa.gov/applemaggot to learn more about WSDA’s apple maggot program.

*Apple maggot eggs are laid in fruit where they hatch and feed for a while, exiting the fruit to pupate in the soil over winter.