How to tailor flystrike controls to flock setup in warming climate

The season for flystrike will lengthen as the climate gets warmer. This will affect the timing of preventative chemical controls for ewes and lambs.

It was once rare for flocks to succumb to blowfly strike in March and October.

Now, the risk period can extend into both of those months as warmer conditions support the breeding season of the green bottle fly.

This longer season is an important consideration for planning when to administer controls.

See also: Q&A: How to prevent and treat blowfly strike in sheep

So, too, is understanding which groups of animals to target, and when, says veterinary entomology expert Richard Wall.

Ewes and lambs are at risk of blowfly strike at different times in the season, and control programmes should therefore be arranged accordingly.

The green bottle fly – Lucilia sericata – can start breeding in March or April, and ewes are under threat from that point until shearing.

“The risk reduces once the fleece is removed, but increases again as the wool regrows,” explains Prof Wall, of the University of Bristol.

In contrast, the risk to lambs is initially low, but increases rapidly over the course of the summer, as they encounter pasture nematodes.

“There is a range of product efficacy lengths and withdrawal periods, so choose the right product to fit in with the group of animals and time of year,” Prof Wall advises.

He recommends that farmers seek guidance from their vet or a Suitably Qualified Person (SQP) on which product to use for their situation.

Control for ewes

For ewes, Prof Wall says using a short-acting product at the start of the season will provide protection until the fleece is removed. “It is important to always follow the label guidance when treating to avoid contamination of the wool,” he warns.

As wool regrows, applying a long-acting product will protect the ewe through to the end of the blowfly season, he adds.

Control for lambs

The farm’s lambing period will dictate when a chemical control is needed for lambs.

January-lambing flocks may not need any intervention, as it is likely that most lambs will be sold before the start of the higher-risk period, Prof Wall advises.

“Lambs are not very susceptible when they are first born.

“So, applying chemical treatments really depends on when the flock lambs and, importantly, when the farmer plans to sell the lambs, to comply with the withdrawal period for meat.

“In general, for all but the very early lambing flocks, treating in May with a longer-acting product will provide protection in the key risk period.

“But always cross-check the withdrawal period against the predicted sales window.”

Early-season treatment, however, will reduce the fly population at its lowest point. The benefits of this persist beyond the residual activity of the treatment, particularly if the entire at-risk group is treated.