Early opening for gypsy moth spray window

Gypsy moth should begin to hatch, which signals the opening of the “spray window” for nurseries and Christmas tree growers.

Gypsy moth feeding
Gypsy moth feeding on leader of spruce tree.

With the unusual warm spring, gypsy moth hatch should be starting in parts of the southern Lower Peninsula this week. If you grow spruce, fir and Douglas-fir for Christmas trees or nursery stock and ship these into non-quarantined areas, you must apply an approved pesticide during the spray window. Prior to this year, the earliest date we opened the first window was April 23, 2007.

A little background about gypsy moth and why we have a spray window

Gypsy moth spends the winter in buff-colored egg masses in sheltered areas. These eggs hatch in the spring and the young caterpillars spread to new locations by crawling to the tops of trees, where they spin a silken thread and are caught on wind currents. Once they disperse and settle they will begin to feed and develop for about seven weeks until they are approximately 1.5 to 2 inches long. Typically this occurs in early summer (June to early July). They then enter a pupal or transitional stage. The spray window targets the time between when they disperse, develop, and just before they begin to pupate.

The first window to open is for the use of Dimilin. Dimilin is an insect growth, disrupting the normal molting process of the insect larvae. This window opens at 150 degree days (DD) calculated from base 50°F. The second window used in Michigan is for insecticides with a more conventional mode of action, referred to as the “all other compounds” window, which opens at 200 DD. Because of the different modes of action, the Dimilin spray window is closed slightly earlier (at 700 DD) than all other compounds (800 DD).

As a reminder, once the spray window opens, the trees cannot be shipped to non-quarantined areas until a preventive treatment is applied to control the larvae which have hatched.

Proper application and good coverage of any product is essential for control to be effective. Since gypsy moth development will vary by location, you will need to monitor development around your farm. Weather conditions will affect caterpillar development and the end date of the spray window.

Remember, we have the gypsy moth treatment window tool on Enviro-weather. This program allows you to track the opening and closing dates of the gypsy moth spray window by county.

Pupal case and egg mass
Pupal case and gypsy egg mass.

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