Growers Not Suffering from Avoiding Autumn Insecticide Spraying

The encouraging number of growers deliberately avoiding the use of insecticides to support winter oilseed rape establishment do not seem to be suffering from doing so, according to the latest annual Dekalb benchmarking study from Bayer.

Of the 170 growers well spread across the country involved in this season’s study just over a third are now deliberately avoiding insecticide spraying at establishment to encourage predators (Figure 1).

Figure 1: To what extent have you used an insecticide at establishment?

Source: Dekalb National Winter OSR Management Study 2022, Bayer

Almost certainly reflecting a growing understanding of the ineffectiveness of contact insecticides against cabbage stem flea beetle (CSFB), this is a welcome increase on the 16% avoiding insecticides in the previous year’s study.

What’s more, the further reduction in autumn CSFB pressures reported in the current season meant a substantial proportion of those not deliberately avoiding insecticides didn’t need to spray. As a result, nearly 60% of growers didn’t spray at all.

The key question, though, is are they suffering by not doing so? Well, Dekalb study coordinator, Richard Williams suggests the data here is encouraging.

“The relatively few growers reporting more than moderate challenges with CSFB make it difficult to be certain here, but the data certainly indicate that those avoiding insecticides altogether as a management tool to reduce cropping risk are not necessarily suffering as a result,” reveals Richard Williams.

“Indeed, they seem to be taking a noticeably higher proportion of their crops to harvest than those continuing to be committed to autumn insecticides (Figure 2). They also appear relatively more positive about this season’s crop condition and potential.”

Figure 2: What proportion of your crops are you taking to harvest?

Source: Dekalb National Winter OSR Management Study 2022, Bayer

“This bodes well for the future – especially if insecticide spraying can be reduced across the rotation, and other strategies are employed to support predator and parasitoid populations.

“There’s a lot more we need to understand about the dynamics of the biology involved,” agrees Richard Williams. “But with what we know already researchers believe there’s plenty we can do to maintain an ecological balance that can do much to improve the consistency of OSR performance.

In addition to avoiding the most damaging autumn insecticide spraying where there are no canopies to shelter beneficials, he suggests following the latest Rothamsted IPM advice to:

• Till the ground as shallowly as possible in the rotation – to avoid burying parasitic wasp pupae too deeply and disrupting ground predator foraging.

• Provide diverse field margin habitats to support the richest insect populations – including tussocky grasses favoured by ground beetles and pollen/nectar resources for other species.

• Improve the micro-climate and diversity within the establishing OSR crop with long cereals stubbles or nurse cropping – to encourage greater predator activity and protection from birds.

• Support the most diverse and robust insect populations across the farm and rotation – through well-managed hedgerow, tree and other non-cropped habitats.