5
 Timing the Predator Release
 The most critical phase in the implementation of biological control on
 strawberry is the release phase. Proper timing of the predator release is
 essential to achie e adequate control. Timing of releases is primarily based
 on the spider mite density on the cultivar of concern, the plant size, the
 possible presence f other pests which have to be controlled, and the costs
 for obtaining and leasing the predators versus the anticipated benefits.

 1. Spider mite nsity. The spider mite density at which predators should
 be released d pends on cultivar susceptibility. 'Selva' and 'Seascape'
 are more sensitive to spider mite damage than 'Oso Grande' or 'Pajaro';
 'Sweet Charli seems to have an intermediate susceptibility.
 Satisfactory biological control on 'Selva' and 'Seascape' was obtained
 when the predators were released when 2 5% of the leaflets had spider
 mites present. On 'Oso Grande' and 'Pajaro', good biological control
 was obtained w en the predators were released when 5 to 10% of the
 leaves had spi er mites. From limited experience, good control was
 realized when he predators were released also when 5 to 10% of the
 leaflets of 'S eet Charlie' were inhabited by spider mites.

 If spider mite ensity exceeds these levels, a corrective spray with a
 selective mitic de is recommended. For this purpose a low rate of
 Vendex (fenbuta in-oxide, hexakis) can be used, because it does little
 or no harm to t e predators. Predators can be released within a few
 days after appl ing a low rate of Vendex. Also, when the development of
 the spider mite population after releasing the predators is faster than
 was anticipated, a "knock-down" spray with a low rate of Vendex may be
 appropriate. Re ember that predators need time to become established
 and have to disp rse and reproduce to sufficient numbers to become
 effective.
2. Plant size. When the plants have two to four new leaves, the
 environment is mo t suitable for the predators; if the number of new
 leaves is higher he predators have to cover a larger area for detecting
 the prey and it m take longer before adequate control is realized.

3. Presence of other ests. It is essential that no pesticides are used
 shortly before or after releasing the predators. Usually a period of 4
 6 days is suffic ent to accommodate the predatory mites. For
 information on com atibility of pesticides with Ph. persimilis see Table
 2.

4. Economics of bioloqgcal control of spider mites. The economics of
 biological control f spider mites depend on (1) costs of the predatory
 mites and the labor involved for releasing; (2) the costs of chemical
 control; and (3) th secondary advantages associated with biological
 control. The costs f predatory mite releases and the costs of chemical
 control are easily d rived. The secondary advantages of successful
 biological control a e: more and larger leaves due to the reduction of
 chemical pesticides; the presence of various parasites and predators of