Biological control for two-spotted spider mite in raspberry

Scott Raffle examines whether biological control methods for two-spotted spider mite can be compatible with control options for spotted wing drosophila in raspberry crops.

Some spray products used by raspberry growers to control the invasive fruit pest spotted wing drosophila (SWD) are harmful to biological control programmes used for two-spotted spider mite (TSSM) in raspberry. With SWD control essential to avoid serious crop losses, we funded research work to investigate if the biological control methods used for TSSM could work in tandem with SWD control measures.

Which is the most effective natural predator for two-spotted spider mite?

In the early stages of the research project, we showed that sprays being applied for SWD control were having an adverse effect on the biological control systems being deployed for two-spotted spider mite control, increasing spider mite populations.

When Phytoseiulus persimilis were introduced early in the season to protected primocane raspberry, spider mite populations were under control by early August.

Although predator numbers were reduced after Decis was applied for blackberry leaf midge control, effective control of two-spotted spider mite had already been achieved.

P. persimilis will only thrive early in the season if used in conjunction with spray products that are compatible with this predator.

Sprays of pollen (Nutrimite^TM) were applied over the crop canopy early in the season before flowering to feed natural enemies. This increased the numbers of the predatory mite Amblyseius andersoni in tunnel grown primocane raspberries, but there was no impact on the level of two-spotted spider mite control.

A. andersoni offers advantages over P. persimilis for controlling two-spotted spider mite. It occurs naturally on raspberry crops and is also commercially available for release on both protected and outdoor crops. A. andersoni is also active at a wider temperature range than P. persimilis. It can establish earlier in the season and does not need TSSM to survive as it can feed on other food sources such as pollen, fungal spores and certain other pests. It is also considered to be more tolerant of conventional spray products than P. persimilis.

We compared both predators in a crop of raspberry in propagation during the growth period, during storage over winter, and again the following spring after planting out in their final fruiting position.

Once TSSM was present in the crop, A. andersoni (with Nutrimite^TM) achieved greater control than P. persimilis applied alone. When applied together, there was evidence that A. andersoni may have predated P. persimilis or its eggs.

Nutrimite^TM helped to boost the population of A. andersoni early in the season, following a release soon after transplanting in the field. During both cold storage and overwintering in ambient conditions, A. andersoni survived until the following spring, while P. persimilis did not.

How can I improve two-spotted spider mite control in raspberry crops?

Our research has highlighted some steps you could consider to improve your integrated pest management strategies for the control of TSSM in raspberry.

• Consider the impact of plant protection products on natural predators

Sprays commonly used for SWD control result in an increase in two-spotted spider mite populations in raspberry.

• Use larger droplets on overhead spray nozzles

Using overhead sprays with a larger droplet size provides refuges for spider mite’s natural enemies, enhancing biological control.

•  Get your timing right

Timely and repeated introductions of P. persimili,s when used with compatible spray programmes, provide good control of two-spotted spider mite before the onset of SWD control sprays. However, the pest needs to be present for this predator to establish.

•  Use both A. andersoni and P. persimilis as natural predators

The predator A. andersoni can overwinter both outdoors and in cold storage. It often occurs naturally on raspberry. It can also be released early in the season for preventive control as it can survive on other food sources and is active at both low and high temperatures.

Releases of P. persimilis should still be used together with A. andersoni. Release P. persimilis when temperatures are suitable and close monitoring confirms that the first two-spotted spider mite is present.

Scott has worked for AHDB for 11 years, having spent three years at HDC and 30 years in the fruit industry in total. Prior to his time at AHDB, Scott was a fruit advisor/agronomist for 16 years with ADAS, specialising in soft fruit and apple and pear storage.