Master Gardener: Minnesota’s newest fruit pest

By Jerry Vitalis, Chisago County Master Gardener

Whenever I write or give classes on harvesting fruit, I always preach to remove overripe berries to prevent picnic beetles, grasshoppers, ants, etc. from laying eggs or eating soft fruit.

Well now we have a new pest that will eat the ripening fruit.  The following article from the University of Minnesota Extension Yard & Garden Brief of August 15, explains more about this new pest.  The article was written by Bob Koch of the MN Dept. of Agriculture and also by Jeff Hahn and Eric Burkness of the University of Minnesota.

A new fruit pest, the spotted wing drosophila (SWD) (Drosophila suzukii), has arrived in Minnesota. This pest feeds on small fruits and stone fruits.

The SWD is an invasive pest of Asian origin that was first detected in the continental United States in California in 2008 and has since spread to several western and eastern states. It was found in Minnesota in August, 2012.

The first two detections of this pest were made by members of the public who reported the flies to the Minnesota Department of Agriculture (MDA).

First, a homeowner from Hennepin County contacted the MDA about some flies she found in a yeast-baited trap she placed near a raspberry patch. Days later, the MDA was contacted by a citizen who found an abundance of maggots in some wild raspberries picked in Ramsey County.

The MDA quickly followed up on both of these reports to visit the sites, collect specimens and confirm the identity of this new pest.

It is impressive that people noticed such a small fly (or maggot), realized that it could be a new invader, and knew to contact agricultural authorities regarding the finds. It goes to show how much people care about protecting our resources.

The SWD looks very similar to the small fruit flies you might occasionally see flying around overripe fruit on your kitchen counter. However, unlike these other flies, which typically feed on overripe or deteriorating fruits, the SWD feeds on healthy, intact, ripening fruits.

In particular, the SWD will feed on thin-skinned, soft fruits such as raspberries, blackberries, blueberries, strawberries, grapes, plums and cherries.

With this pest being so new to Minnesota and the United States, little is known about how big of an impact it will have and what management tactics will be most effective.

The MDA will be working with the University of Minnesota (Extension and the Department of Entomology) to determine how widespread this pest is in Minnesota and to alert farmers and gardeners of its presence and potential impacts.

The University of Minnesota will also be developing recommendations for management of this pest on Minnesota fruit crops. SWD could be particularly devastating to blueberry, raspberry and grape growers, but we will need more information on when the pest is active in Minnesota and how well it can survive our winter weather.

Extension programs from other states have suggested several items for consideration in management of this pest. Sanitation is an important consideration to lessen local buildup of SWD populations.

Sanitation practices include frequent harvest of crop to ensure ripe fruits are not in field for extended period of time and removal and destruction of old fruit remaining on stems and fallen fruit. Furthermore, crops can be monitored with traps baited with yeast or vinegar; however, yeast-baited traps appear more effective.

Traps should be checked frequently (at least weekly) to determine the presence and abundance of SWD males and females. Monitoring for activity of SWD adults is also important, because once eggs are laid in the fruits it will be too late for other management tactics (for example, insecticides) to be effective.

If SWD are found in the traps, an insecticide that is registered for use in the specific crop and effective against the pest should be applied. University of Minnesota Extension is evaluating what insecticide options will be effective in Minnesota. After treatment, monitoring of SWD should continue, with additional timely treatments applied as needed.

The adult flies are difficult to distinguish from other small flies; however, if you find an abundance of small, white maggots in what were apparently healthy fruits at the time of harvest, contact the MDA’s “Arrest the Pest” hotline at 1-888-545-6684 or at Arrest.the.Pest@state.mn.us.

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