Oilseed rape growers are facing further pest problems this autumn with increased levels of cabbage root fly larvae, as a possible consequence of early drilling.
Although there have been ‘odd bits’ of root fly in crops over the last few years, this season Velcourt agronomist, Ryan Hudson says he is seeing more of the pest.
“By taking the approach of planting oilseed rape earlier whilst there was moisture and to get the crop established before the flea beetle arrives, we have opened ourselves up to higher risk of root fly, which tends to effect crops earlier in the season.
“I started picking it up three-four weeks ago, where some areas of fields looked a bit off-colour with some purpling of the leaves. On pulling them up, we could easily find root fly larvae in the root.”
However, it is not known whether the stress and discolouration of plants is down to larvae grazing the root, or extreme rainfall, says Mr Hudson.
“The high rainfall could have reduced nutrient availability, and plants may not have enough N to feed the large canopies. Or it could be that plants are showing signs of stress from roots being sat in very wet soil,” he adds.
With little means of control, Mr Hudson says monitoring root fly and flea beetle larvae numbers over winter and into the early spring will be important to determine the impact of the larvae, and early nutrition to help get crops growing will be particularly important.
He says: “I’m not overly concerned currently because the plants with root fly are well established with large roots. The worry is if we get another hot and dry spring, because we have damage to the roots which could reduce nutrient uptake. Due to the high rainfall we are likely to need to get nitrogen into the plant as soon as practically possible in the spring to ensure the plant has sufficient nutrition for rapid development despite the damage to the roots.”
Monitoring of fly and egg numbers at Warwick University’s Crop Centre at Wellesbourne showed a significant third generation of cabbage root fly this year, according Prof Rosemary Collier, and it is this generation that will be hitting oilseed rape crops.
She says: “The highest numbers of eggs were laid between late August and mid-September at Wellesbourne.
"It’s hard to say whether cabbage root fly numbers were particularly high in 2019 as adult root flies are able to move between crops and so large infestations may arise simply because a new crop of oilseed rape has been sown close to a source of flies.”
The relatively ‘normal’ weather this summer with no real extremes and the loss of chemical control options could also be contributing, she adds.
“In the past, neonicotinoid seed treatments were probably providing some control of root fly larvae, so that may be one reason why infestations appear to be causing more damage.
Controlling cabbage root fly with insecticides applied as sprays is very difficult.”
Symptoms of cabbage root fly infestation include wilting and in extreme cases, plant die-off.
The fly lays eggs around the base of the plant and once the larvae hatch, they move down to feed on the roots, completely destroying the root system in extreme cases.
Prof Collier adds: “Small plants are more susceptible because they haven’t got much of a root system. Once they start growing away they can tolerate a certain level of infestation.”
For forecasts and up-to-date reports on most key field crop pests, visit: syngenta.co.uk/ahdb-pest-bulletin