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Talking agronomy with Maddy Vaughan: True extent of flea beetle damage as yet unknown

On the third day of advent not only did I receive my first Christmas card, but also my first phone call to tell me that the sprayer had been anti-freezed. It was in the shed and no more spraying was to be done for 2018.

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For many, the autumn onslaught of drilling, pre-ems, residual top-ups and applications of insecticides and Kerb (propyzamide) are almost complete and for the most part cereal crops and winter beans have established well.

Favourable conditions meant delayed drilling could be used as an anti-black-grass strategy and the application of pre-ems and tops-ups was not hindered by adverse weather. Temperatures did plunge briefly to chilly, minus figures in October, but they soon crept up again. The return to mild weather has meant many cereal crops have also received a barley yellow dwarf virus spray this year.

Oilseed rape crops still vary across the counties. There are some triffid-like crops waving at you from the roadside. Others still stare limply at you from below the stubble, looking wounded and dishevelled from the ravages of flea beetle.


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Unknown damage

Unfortunately, the crops that we have managed to nurse through the shot-holing are still vulnerable. Flea beetle larvae are easy to find in crops but the effect of their damage is still a big unknown. The larvae hatch out of eggs which are laid by the adult beetles on the soil at the base of the OSR plants in September and October. After hatching, the larvae migrate into the plants and burrow into the leaf petioles where they over-winter. Later in spring, the larvae bore into the main stems which can lead to loss in plant vigour and impact on yield.

Low chance of survival

When dissecting rape stems at this time of year, larvae are often to be found lurking within and will continue to feed on the plant throughout winter, causing leaves to die off. If small plants are badly infested with larvae, survival chances are low. Bigger plants with more branching may be able to withstand the invasion. There are no chemical control methods available to us to target the larvae once it has entered the plant. Cocooned within the plant, the pest is shielded from any pesticide application.

Due to our lack of chemical control options since the ban of neonicotinoid seed dressings, there has been more attention given to how we can culturally control the larvae. The use of companion cropping has been investigated as a larval and flea beetle control method. The use of mustard has been explored where mustard is broadcast into a sown crop of Clearfield rape in the hope that the beetle will prefer the mustard to the oilseed rape.

Trials results

In a tramline trial NIAB conducted it could be seen that the beetle did prefer the mustard and consequently feeding on the oilseed rape plants was diluted (but not eradicated). With flea beetle more attracted to the companion crop, they are more likely to lay their eggs at the base of the mustard and therefore the larvae should migrate into the companion crop rather than the OSR. The companion crop can then be sprayed off with a Clearfield herbicide, hopefully resulting in the death of the larvae as well as the host plant.

Defoliation of an OSR crop over winter could also help to reduce flea beetle larval numbers. Flailing crops nearly to the ground in December, January or February has been shown to reduce larval survivors with little or no impact on yield. Grazing infected rape crops could also help to control larval numbers. However, the practicalities of achieving this are yet to be explored and further work in this area is required.

For the present, we have few options for controlling the larvae already in our rape crops. Dissecting plants can help us understand the level of infestation, but it won’t be until spring that we know the true extent of the problem. For now, we look forward to a healthy and happy Christmas, a prosperous new year, and hope spring is kinder in 2019 than it was in 2018.

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