Cabbage stem flea beetle larvae
13 Dec 2018
Establishing winter oilseed rape has been challenging for many this autumn, with high numbers of adult cabbage stem flea beetles (csfb) decimating crops in many areas and attention has now turned to the larvae.
Preliminary results from larvae counts done by ADAS and others have found that these adults have produced variable numbers of larvae, averaging 0.8 per plant in North Yorkshire and approximately 20 per plant in sites sampled at ADAS Boxworth, Cambridgeshire. There has also been a report of a single plant from the east of England containing about 40 larvae. These larvae, like the adults, have the potential to destroy winter oilseed rape (WOSR) crops.
In this blog Dr Steve Ellis, entomologist at ADAS, discusses thresholds and treatment of csfb larvae.
“Csfb eggs are not laid inside the plant, but in the soil and it is from here the hatched larvae enter the plants over the winter period, from October to early April. If there are a lot of larvae, they can really hold the plants back and even destroy a crop as they tunnel first in to leaf stalks and then the pith of plant stems.
Assessing the need for treatment against csfb
There are three ways to assess the need for treatment:
- Plant dissection
- This is the most common method used and involves taking a sample of between 10-20 plants and cutting them open with a scalpel. A mean of 5 or more larvae per plant would justify an insecticide spray.
- Water trapping adults
- If you have caught more than 96 adult beetles between early September and the end of October, then this is also considered a threshold.
- Examine the plants for petiole scars
- Tunnelling by the beetles creates scars in the leaf petioles, so rather than cutting the plants open, count the number of petioles with leaf scars. If more than 50% of the leaf petioles have leaf scars, then this too is considered a threshold.
Csfb larvae are thought to be very difficult to control with insecticides. Pyrethroids are contact poisons so it would be expected that larvae which are within the leaf petioles would be protected from these products. However, experimental data from the 1980’s suggests that a pyrethroid spray could achieve between 73% and 93% control of larvae. It has been suggested that larvae pick up a dose of the chemical as they move in and out of the leaf petioles.
However, it is unclear whether this level of control would be achieved now with widespread resistance of csfb adults to pyrethroids. At present we do not know if pyrethroid resistance in adults means that the larvae are also resistant, although we suspect this to be the case. Until this is proven it would be sensible to avoid unnecessary insecticide sprays which could worsen the resistance situation. Pest damage does not necessarily mean loss of yield and treatment should only be applied where thresholds have been exceeded.
ADAS are trialling a novel method for csfb larvae control which involves defoliating the crop. Preliminary results have shown that this can reduce larvae populations and increase yield.”