Mites Overwintering on Berry Crops

I was hoping the cold weather this year would cut down on the mites in my berries.  Instead, my plants are already really loaded up with mites- what is going on?

As many growers and gardeners know, the twospotted spider mite, Tetranychus urticae, is a very common pest in many crops, including strawberries, raspberries and blackberries.  This small, pale green mite is easily distinguished from others by the two large black blotches on the back of the abdomen, as well as the webbed colonies it forms on plants when numerous.

Although many people believe that a cold spell in the winter featuring freezing weather reduces the numbers of twospotted spider mite surviving until the following spring, this is not borne out by experience.  Twospotted spider mites may cease to reproduce during the coldest months of the year, but do not have difficulty surviving.   Mites will spend the winter in the crowns of fresh strawberry transplants, and at the base of brambles and weeds surrounding the field.

It is important to know that that the appearance of female twospotted spider mites changes from the usual yellowish green to orange in the winter when they are dormant.  These orange colored females slightly resemble the predatory mite, Phytoseiulus persimilis, or the carmine spider mite, Tetranychus cinnarbarus, so berry growers should take care that they are identifying the mite correctly.

In conclusion, growers and gardeners should recognize that they cannot depend on cold winter weather to reduce mites.  Mite reduction will only be achieved by selection of plant stock not infested with twospotted spider mites, selection of varieties less susceptible to mite infestation, correct nitrogen fertilizer use and efficient mite management through predators or pesticides.

The above has been a brief guide to the ability of twospotted spider mites to overwinter in berry crops.  Please contact Mark Bolda at UCCE Santa Cruz if you have more questions on this topic or any other topics concerning blackberry, raspberry or strawberry production.