Science into practice: Are sulphur-based fungicides bad for Encarsia?

Over the past five years, the previously successful prophylactic bio-control strategy for whitefly in tomato crops of weekly releases of parasitic wasp Encarsia formosa seems to be failing.

The situation has been exacerbated by the need to apply remedial insecticidal treatments, which has disrupted bio-control of other pests, jeopardising the industry's pesticide-free goal. One theory, which HDC project PC 293 explored, was that glasshouse whitefly problems coincided with an increase in the use of sulphur-based products to control powdery mildew.

A grower survey confirmed a higher incidence of whitefly in recent years. However, 15 of the 23 companies who responded did not mention sulphur. Records from the Tomato Working Party did not show a significant increase in sulphur use over the five years covered by the survey.

There was a clear varietal effect, with the greatest incidence of damaging whitefly infestations occurring in cherry tomato cultivars. Neighbouring crops of less susceptible varieties may have been placed under additional invasion pressure.

Excessive de-leafing may also have had a bearing on whitefly incidence in long-jointed cultivars, particularly in older and lower glasshouses where parasitised scales are often removed before adult Encarsia have emerged.

A practical laboratory study investigated whether sulphur-based products masked the chemical signals female Encarsia follow to find their host. There was no evidence that applying sulphur as a high-volume spray or a vapour interfered with the wasps' searching behaviour. There appear to be a number of factors involved that could be contributing to whitefly problems on individual sites.

Horticultural Development Company

For details on all HDC activities, visit www.hdc.org.uk.


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