Orange wheat blossom midge spray thresholds have been radically altered for this season after new research.

The new recommendation is to spray only if more than 120 midges a trap a day are caught. That replaces the previous lower guideline of spraying when 20 or more male midges were caught in a trap over a two night period.

The change follows a second HGCA LINK project aiming to develop a decision support system for the pest.

While the new thresholds are much higher they are comparable to those used very successfully to control pea midge, Toby Bruce of Rothamsted Research points out.

“Although the figure seems much higher than before, part of the new advice is a decision support system,” he says. “And that tells growers to monitor crops daily once trap catches exceed 30 a day.”

He accepts that catches above 30 and below 120 could be seen as a grey area, but highlights that the new advice is based on a two-step threshold.

“Remember that the pheromone traps used in fields only catch the males. It’s female midge movement which causes concern, as they lay the eggs inside emerging wheat ears.

“That’s why we have suggested that growers keep checking crops once they’ve found 30 midges in a trap. But spraying isn’t necessary until 120 midges a trap a day is exceeded.”

Attracting male midges provides the earliest warning of midge activity, explains Dr Bruce. “Only the females fly between fields and crops are susceptible at ear emergence, between GS53-57, not at flowering. In the last couple of years, they’ve emerged too late to cause much damage.”

The weather conditions in late April and early May will determine the threat this season, he adds. “If the soil is warm enough, the larvae will move to the soil surface and pupate. We need soil temperatures of more than 13C.”

Pheromone traps should be in place before ear emergence, in fields of susceptible varieties where wheat was grown in previous years, recommends Dr Bruce.

“These fields are sources of the pest. Where thresholds are exceeded and spraying is necessary, make sure that you treat the surrounding fields too, as you will get localised movement of the female insects.”

Previous guidance to give priority to milling and seed crops remains, he adds. “Again, this is where field inspections come in. Look in the evenings, as they fly up to the ears to lay their eggs towards the end of the day.”

The latest project has also identified that humidity, not light intensity, affects midge flight, reveals Dr Bruce.

“There’s more flight activity when it’s humid. We recorded twice as much flight at 70% humidity than we did at 35%.”

Orange wheat blossom midge thresholds

• New spray threshold recommended
• Spray only when trap catches exceed 120 midges a trap a day
• Monitor fields daily when trap catches exceed 30 midges a day
• Milling wheat and seed crop priority remains