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How to overcome two-spotted spider mite challenge in rose growing

Dr Lisbeth Riis, CEO at Scarab Solutions, has highlighted a three-step formula to help rose growers apply the lessons learnt on Kenyan farms and succeed with biological control of spider mites

Spider mites are common crop pests in many parts of the world, but it has taken time for rose growers to learn crucial lessons on effective biological control methods and share this knowledge. Although applying phytoseiid predatory mites to a rose crop is a viable option to get spider mite infestation under control, it does not constitute the classical biological control where predators released feed on the prey, multiply and reduce the prey population until the two populations eventually oscillate into a balance. 

What Scarab Solutions experts have learnt through working closely with Kenyan rose growers for the past 15 years, is that predator reproduction in industrial greenhouse roses is low – probably due to the direct and accumulative effect of other necessary pesticides and fungicides used in the crop. Industrial rose growers should therefore think about predatory mites in much the same way as a biopesticide and keep applying them using the appropriate release methodology. To get this right, they need to follow a simple three-step formula.

Step one: Begin when the pest population is still low

The first step is to bring down existing spider mite population with compatible chemicals that have short-lived persistence in the crop – for instance Organo-silicone adjuvants, such as Silwet Gold, as long as they are not sprayed in direct sunlight, or other compatible chemicals recommended by the supplier. The most important outcome is that the product has a short-lived persistence – not that it kills predatory mites, which should not have been released at this stage.

Step two: Establish excellence in scouting operations and data analysis

Releasing predatory mites is a costly endeavor, so growers need to ensure they are confident in the quality of their scouting and mapping of the locations with predator deficiencies before they begin.

Rather than having the scout spend time on deciding which score class to record, the same amount of time is much better invested in recording the real counts. There are excellent methods to master quick counting and a good scouting company can train scouts to implement them. When counts are recorded accurately, it allows the corresponding spatial analysis to map the deficiency and sufficiency of predatory mites. 

Step three: Release the predators – apply targeted and blanket methods

Now growers can begin with the continuous, spatially targeted release of predatory mites in the locations with predator deficiency. Aiming these releases to bring the predator-to-spider mite ratio below 1:10 will help them get the outbreak under control rapidly. These targeted spot releases will be most effective when applying Phytoseiulus persimilis – effective predatory mites that specifically and exclusively feed on two-spotted spider mites.

Blanket releases should also be carried out by applying the resilient, more ‘generalist’ and cost-effective Neoseiulus californicus (formerly known as Amblyseius californicus) which can survive on alternative preys such as other mites and pollen. It is effective where or when the spider mite population is very low and the great hunter Phytoseiulus persimilis does not have many feeding options.

Common mistakes to avoid

There are many ways cutting corners can lead to painful delays in getting to grips with spider mites. If growers release too few predatory mites – instead of just the right amount – or waste time waiting for the classical biological control dynamics to kick in, they will only extend the pain. 

Another dilemma is posed by miticides and pesticides. On the one hand, when growers replace miticides with predatory mites, pests that have been previously suppressed by miticides will inevitably flare up. On the other hand, as growers turn to pesticide use against ‘new’ pest problems, these pesticides are likely to negatively affect the predatory mite population – an uncomfortable challenge in biological control.

Following this three-step method will benefit growers through improved crop quality and lower long-term pest control costs, but also the predator supplier, who will have grown its market share in terms of hectares.