Three steps to curb spider mites in greenhouse roses 

Spider mites (Tetranychus urticae) are a common crop pest 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 harmful population until the two populations eventually oscillate into a balance.

Working closely with Kenyan rose growers for the past 15 years, what we have learnt 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

It is essential to release predatory mites when and where spider mite population is low. Starting at a heavily pest-infested greenhouse will help neither the grower nor supplier as reproduction of predatory mites in industrial rose greenhouses does not follow the same logic as classical biological control – predators will feed on the spider mites, but they produce comparatively few offspring.

The first step is to bring down existing spider mite population with compatible chemicals that have short-lived persistence 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 

Releasing predatory mites is a costly endeavour, 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.

Only spatially targeted releases of predatory mites make the effort worthwhile and help growers avoid wasting predatory mites on areas where there are no spider mites.

Consider how scouts work. Most of their time is spent on walking diligently through the crop, searching thoroughly for the pests and diseases in each observation point and recording their observations.

But if the scout simply records presence, the only information the farm manager will glean is pest incidence, not the full understanding of the spatial distribution or the pest severity.

Step three: Release the predators 

Begin with 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, which are 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.

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.

Treating the problem quickly and effectively requires the right number of predatory mites, rigorous control of the process and excellent monitoring and analytical tools.

The good news is that once spider mite population is reduced to zero, growers no longer need to be so sensitive about the predatory mite population.

Dr Riis is the CEO, Scarab Solutions