Guava fruit fly   (Contra Costa County)
Guava fruit fly (Contra Costa County)

The first fruit fly of its kind to show up in Contra Costa County has agriculture officials taking swift measures to quash the threat it could pose to crops.

Three guava fruit flies last week were caught in traps that workers with the Contra Costa County Department of Agriculture had put out in residential areas of Bay Point and Pittsburg for a similar pest that's also known to destroy a broad array of fruits and vegetables.

Although the employees initially thought the insects were Oriental fruit flies, experts in Sacramento identified them as guava fruit flies.

Indigenous to countries in southeast Asia, India, Pakistan and southern China, the fly feeds on hundreds of fruits and vegetables, including peaches, nectarines, apples, pears, melons, tomatoes and citrus products.

Photo courtesy of University of California -- fruit fly
Photo courtesy of University of California -- fruit fly

"This is one where if you find it you start trapping right away," Assistant Agricultural Commissioner Matthew Slattengren said.

The specimens that his agency trapped were all males, but when paired with females, they propagate at a prodigious rate.

A female can lay 1,000 eggs at a time, each of those maturing into flies that start the cycle all over again in just over one month, Slattengren said.

The trouble begins when a fly pierces the covering of a fruit, laying her brood just under the skin.

The eggs develop into maggots that eat their way out and drop to the ground, where they form a cocoon and, if the weather is warm enough, emerge days later as an adult.


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To eradicate the threat, the county has set up more traps and expanded its reach; currently there are 80 scattered around an area just over 2 square miles, Slattengren said.

The triangular cardboard containers hold sticky fly paper and cotton wicks covered with a small amount of pesticide along with chemicals known as pheromones that female guava fruit flies secrete to attract mates.

"This pheromone is very, very good -- it'll pull in a fly from a long distance," Slattengren said.

His agency is hanging most of them from fruit trees in residents' front yards as well as some from trees on public property.

Others traps use yeast to lure hungry females -- although county officials haven't yet found any -- because it smells like rotting fruit.

The state will follow up by squirting a gelatinous mixture of pesticide and pheromones onto trees and telephone poles high enough above people's heads that the applications won't pose any risk, Slattengren said.

Trapping and poisoning will continue through at least three life cycles of the fruit fly to ensure that it's eradicated, which will take at least the rest of the summer, he said.

How the guava fruit fly ended up in Contra Costa County is anyone's guess, Slattengren said; one possibility is that someone brought fruit into the United States and then threw it out when they discovered maggots.

Although there isn't a quarantine on produce that growers are transporting out of East Contra Costa County, the county is asking residents with fruit trees to keep the food on their property to avoid introducing the flies elsewhere.

Contact Rowena Coetsee at 925-779-7141. Follow her at Twitter.com/RowenaCoetsee.