Improving integrated pest management in strawberry – A summary of AHDB Project SF 156

AHDB Project SF 156 sought to develop novel and alternative approaches to the control of some of the key pests affecting commercially produced strawberry crops. Thrips, aphids and capsids were the main focus of the research. Growers and agronomists can now adopt some of the findings within their integrated pest and disease management strategies.

Western flower thrips – monitoring, sampling and control

Without effective crop protection products for western flower thrips (WFT) in strawberry, control relies upon integrated pest management (IPM) techniques and biocontrol using predators such as Neoseiulus cucumeris, Orius sp. and others. Growers and agronomists have mixed success with this approach and can sometimes be unsure as to whether a control programme is working effectively.

  • When monitoring populations of N. cucumeris predatory mites in the crop, focus your search on young button fruits (those which have just set and where the petals have just withered). Assess the populations of WFT adults on mid and old aged flowers. WFT larvae appear on both button fruits and flowers.
  • Should WFT be present in the crop, significantly more N. cucumeris are found on flowers and fruits, but if WFT is absent, introduced predators tend to be spread more evenly across the plant.
  • Scientists demonstrated that the solvent, methyl isobutyl ketone (MIK) can be used to extract N. cucumeris and thrips adults and larvae from flowers and button fruits. A prototype extraction device designed by NIAB EMR uses MIK to extract pests and predators from flowers and fruits. In field tests, it recovered 27% of predatory mites from fruit and 5% from flowers. It recovered 68% of WFT from button fruit and 81% from flowers. The MIK dispenser can be used at least 60 times, before it needs replacing.
  • The device helps in the detection of pests in the crop and improves the detection of predators, including N. cucumeris and Orius, giving confidence that controls are working. NIAB EMR will produce the device for grower or agronomist use. Rather than replacing crop monitoring, it assists in determining numbers of predatory mites in the crop and informs the need to make further predatory mite introductions. Compare the practicality of the device with existing methods of detecting thrips and predators in the crop.
  • Distribution trials showed that as temperatures increase, numbers of N. cucumeris and WFT larvae, generally, decline in the device. Conversely, the numbers of WFT adults and Orius adults increase. When sampling, if N. cucumeris or WFT larvae numbers are low, it is worth trying again when temperatures have decreased.
  • Bioassays showed that Met 52 (Metarhizium anisopliae) sprays killed up to 60% adult WFT in lab trials. Further lab assays showed that Met 52 had minimal effect on natural enemies.

Investigating other thrips species being found in strawberry crops

Typical bronzing damage in strawberry associated with the western flower thrips (WFT) is now common, even where IPM programmes have controlled the pest. Initial research identified other thrips species living in such crops, including rose thrips. The predatory mite Neoseiulus cucumeris which is relied on to control WFT larvae, does not predate adult thrips, so adults of other species may be flying into strawberry crops and causing damage. This work set out to investigate which other thrips species are present in strawberry crops, whether they are implicated in the damage being found and how best to manage and control them.

  • In addition to WFT (Frankliniella occidentalis), rose thrips (Thrips fuscipennis), rubus thrips (Thrips major), onion thrips (Thrips tabaci) and flower thrips (Frankliniella intonsa) appeared in commercial strawberry crops.
  • Although rose thrips had been suspected of causing the damage where WFT had been controlled, confirmation was difficult as different species predominated in different seasons and at different times of the year. 
  • No larvae of the rose thrips were found in strawberry flowers, but only adults, so it is possible that this species doesn’t breed in strawberry, explaining why N. cucumeris is not gaining control. 
  • Orius controls both adults and larvae of thrips but only when temperatures are regularly above 20oC for good establishment, so is unreliable every season. It is also very sensitive to a number of commonly used plant protection products. 
  • The predatory banded wing thrips (Aelothrips sp.) was found during this study and is known to feed on other thrips larvae, but it is not known if it feeds on adults. 

An effective IPM programme needs to be developed for thrips species other than WFT, but in the meantime, growers are urged to heed the following guidance for WFT control, which has been developed over a number of AHDB funded projects on thrips along with practical experience: 

  • Release the predatory mite Neoseiulus cucumeris throughout the season from first flowers.  The minimum release rate should be 25 per plant every week or fortnight, increasing to 50 per plant if numbers of thrips start to increase.  This predator feeds only on young thrips larvae so it may not control rose thrips, which might not breed in strawberry flowers. 
  • Apply the ground-dwelling predatory mites Statiolaelaps scimitus (formerly known as Hypoaspis miles) once at about 10 per plant. It is not yet known how effective these are against larvae of thrips species other than WFT that might drop to the ground to pupate, but as they are effective against WFT it is a sensible option. 
  • Release Orius laevigatus in addition to N. cucumeris once temperatures are suitable.  This predator needs a minimum of 15°C for egg laying and over 20°C for good establishment.  Commonly used release rates are a minimum of 0.25 to one Orius per plant, repeated after two weeks. Orius laevigatus is very sensitive to plant protection products so avoid using any that are harmful (consult your supplier or adviser). 
  • If fruit bronzing is seen, consider using an IPM-compatible plant protection product for control. Options include spinosad (Tracer) but growers may wish to reserve this for control of SWD. Do not use Tracer if only WFT are present as they are likely to be resistant to this product. 

Thrips species identification can only be confirmed using a high-power microscope and specialist expertise.  Consult your adviser on getting the thrips species identified and choosing the appropriate plant protection product if required.