Biological Control — of interest to Growers

The use of predatory mites is not only environmentally friendly, it also has production advantages. Mites can travel into areas that sprayed pest control products cannot reach. In addition, many growers prefer working with predatory mites rather than spraying. The cost should not be a deterrent. A demonstration project by two European growers clearly showed that the cost of an integrated pest management approach to spider mite control varies very little from a chemical approach.

A pilot project was developed to determine the optimization of integrated pest management of spider mites with predatory mites. Various dispersion techniques were compared with each other, and a demonstration using predatory mites in cuttings and various crops was evaluated.

Various methods

A grower of Skimmia has compared various dispersal techniques. For this purpose a 1,000 m2 (.25 acres) greenhouse section with one year old Skimmia reevesiana on rolling benches was divided into four sections. The plants were potted outside and were moved into the greenhouse in mid-June. Because mites were present, prior to entry into the greenhouse the plants were sprayed with Floramite. Subsequently the first predatory mites were dispersed on June 19. Four different techniques were tested.

The predatory mite Amblyseius californicus was dispersed loosely with approximately 25 mites per m2 (10.75 sq.ft.). In a different section the predatory mite Amblyseius andersonii was spread at the same rate. In a third section Amblyseius andersonii  was suspended in the crop in small sachets (one sachet per m2). And a grower’s ribbon with Amblyseius andersonii was also tested in the last section at a rate of one ribbon every 2.5-3m (8’ to 10’).

It was found that the grower ribbons were the easiest method of dispersal of the predatory mites.  Marketed as Bugline, the ribbon consists of a number of connected sachets which each contain a growing system for predatory mites. This consists of gluten, feeder mites and predatory mites. The predatory mites multiply themselves in the sachets. During the 4 to 6 weeks that they are active the sachets can generate 600 to 1,000 predatory mites each.

The ribbon can simply be draped over the plants. Application is very quick. The pre-determined distance between the sachets on the ribbon improves the evenness of distribution in the crop. The number of predatory mites that emerge from the ribbon is much higher than the numbers that are usually broadcast in practice. Combined with the long duration (4 to 6 weeks) over which the predatory mites are introduced, this in turn leads to a more even and better build up of predatory mites in the crop.

To compare the various treatments, weekly scouting was done in the crop. In addition approximately 20 plants in each section were regularly counted for spider mites and predatory mites. The counts indicated that predatory mites were controlling spider mites effectively in all methods. During the last counts in September very few spider mites were observed in the crop. More than half of all plants had a population of predatory mites varying from two to eight per plant. No large differences between treatments were observed.

Observations have led to the conclusion that next year the first predatory mites need to be introduced earlier. “In 2008 we did not have to spray at all”, said the Skimmia grower. “We had several pockets of spider mites developing on a few benches after the plants were moved into the greenhouse. Likely the treatment with Floramite was insufficient and as a result any surviving spider mites were able to develop colonies in the warm greenhouse. During the first few weeks very few predatory mites were observed in the crop. As a result a second treatment with Floramite was needed to knock out the pockets of spider mites. Extra quantities of the predatory mite Phytoseiulus persimilis were introduced. After that, the spider mites were under control for the rest of the season.

In another test, conducted in early April, Amblyseius californicus was spread into a crop of Buddleia cuttings, which were transplanted two weeks later. During the finishing, a few predatory mites were observed, although It was unclear whether the predatory mites made the transition from the cuttings to the finished crop. By the same token, very few spider mites were observed. As a result the Buddleia grower was quite satisfied with the outcome and will be requesting that the predatory mites be introduced in the cutting crop prior to transplanting.

However, this is not always practical. The second batch of Buddleia did not receive the predatory mites as the shipment was part of a large crop of cuttings destined for multiple producers. The choice then was to either introduce predatory mites in all the cuttings, or in none. The other option was to ship to the nursery and then introduce the predatory mites. This was deemed as too much additional labor. Spreading the mites on the carts wasn’t an option either as the carts were too full and densely packed. As a result it was decided to introduce the mites in the middle of July by means of a backpack blower at the rate of 25 pieces per square metre (sq. yd.). This gave good results. The spider mites that were present in the crop were completely controlled by the predatory mites.

Integrated control is not only possible in container production, but also in field production. For example, as a result of blowing predatory mites into the crop, another grower found it unnecessary to spray for mites in his Magnolia crop.

Tips

• Predatory mites need to be introduced into the crop early. Amblyseius californicus and Amblyseius andersoni can survive for several weeks without spider mites. As a result they can be used on a preventative basis. The mite production systems (sachets and ribbons) can be introduced even earlier as the predatory mites are introduced with their own food supply. Phytoseiulus persimilis, however, should not be introduced until the first spider mites have been observed.
• Scout the crop weekly for both predatory mites and spider mites. If too many spider mites are observed, a corrective application of Floramite can be used. This product is safe for natural predators. However Floramite cannot be used outdoors in parcels of  land along water courses. The next best alternative is a product called Cantack.