Factors Important to the Decline in Biological Control of Spider Mites in Florida Strawberry Production

Fresh market strawberries valued at $313.6 million were produced on 8,800 acres in Florida during the 2008-09 season.  Main arthropod pests included the twospotted spider mite (Tetranychus urticae), cyclamen mite (Phytonemus pallidus), southern and fall armyworms (Spodoptera eridania and S. frugiperda), strawberry root and melon aphids (Aphis forbesi and A. gossypii), flower thrips (Frankliniella bispinosa and F. occidentalis), and sap beetles (various Nitidulidae spp.).  Among those, the twospotted spider mite represents the greatest concern and is problematic in most fields and years.  During the 2008-09 season, this pest was managed by the predatory mite, Phytoseiulus persimilis, on about 10% of the production area, down from about 50% 10 years ago.  The technique involves scouting the crop until about 5% - 8% of the leaflets possess one or more spider mites then releasing predators once per season at about one per plant.  Miticides were used to control spider mites on the remaining area.  Season-long costs of the two methods were similar at about $200 per acre.  Farmers who once were committed to biological control methods and those never committed were asked why they were not presently using the method.  The following main factors were revealed:  (1) New, reliable miticides have been permitted thus reducing the need for an alternative method of control, (2) Financial investments for biological control must be made at one time, early in the season, before significant revenue has been generated from fruit sales, and (3) Difficulties in preserving a living control agent increased with the need for insecticides to control newly important western flower thrips (F. occidentalis), chilli thrips (Scirtothrips dorsalis), and spotted wing drosophila (Drosophila suzukii).   In addition to factors offered by growers, there were others that likely contributed to the decline:  (1) New miticides also have been available to the transplant producers resulting in cleaner shipments and reduced early to mid-season spider mite episodes, and (2)  Strawberry growers, confident in the effectiveness of   new miticides,  have concluded that problems initiated even in the early season can be managed season-long with minimal miticide use and costs significantly under $200 per acre.  Decisions by growers to elect biological control are largely economic and without new market incentives, governmental regulations, etc. favoring the technique, its use will remain low provided sufficient effective miticides remain available.