Monitor flea beetle risk in OSR

ADAS has published its latest Crop Action issue with advice to oilseed rape growers regarding the potential threat of cabbage stem flea beetle (csfb) this autumn:

A discussed in a previous issue of Crop Action, Defra has rejected a second request from the NFU to allow growers to use neonicotinoids on oilseed rape crops this autumn to help control cabbage stem flea beetle (csfb). A statement to this effect was released by Defra on 5 July. As a result there will be no neonicotinoid seed treatments seed treatments available for the 2016/17 season.

We are also aware that there is widespread resistance to pyrethroids. In 2015, almost 60% of beetles tested were confirmed as showing kdr-RR resistance. Pyrethroids are the only alternative to seed treatments, so increased usage could exert significant selection pressure. The situation is far from ideal but there are still things that can be done to minimise the impact of csfb. Last year, csfb pressure was variable across the country with the south east being particularly badly hit. It remains to be seen whether this year is going to be another high risk year.

Why is csfb important?

Csfb can cause severe damage at establishment.

Large numbers of adults feeding in the autumn can kill OSR seedlings, occasionally causing total crop failure. Adult feeding causes shot-holing which effectively reduces green leaf area. Eggs are laid in the soil and on hatching the larvae bore into the leaf petioles and continue feeding close to the surface. Later, the larvae move into the main stem to feed under the growing point. Whilst there is no doubt that adult csfb can cause severe damage at establishment, the crop is also capable of growing away from damage and anything that can be done to encourage rapid growth will be beneficial.

Establishment is key

A rapidly growing crop is the best way of combatting csfb, and the focus this autumn should be on taking all possible steps to achieve rapid establishment. There are a number of factors to take into account:

Seed source – All hybrid varieties must be grown from certified seed and cannot be home-saved. Seed samples with a low thousand seed weight should be avoided as establishment can be reduced if small seeds are sown when conditions for establishment are poor.

Cultivations – A good seedbed is important in maximising seed-soil contact. A good crumb structure will aid establishment. Seed should be sown at 2-3 cm and rolling is recommended to retain moisture. Not only will this help the crop to grow away from csfb but it will also reduce the risk of slug damage.

Variety choice – Varieties that are known to establish rapidly and have early vigour are likely to be less susceptible to loss of leaf area. Note that some open pollinated varieties can be as vigorous as hybrids.

Sowing date – Winter oilseed rape is typically drilled between mid-August and mid-September in England and Wales or from mid-August to early-September in Scotland. High yields can be achieved from drilling anytime between mid-August and mid-September. The chance of significant yield reduction becomes more likely for crops drilled after mid-September. Crops sown mid-August will take about 10 days to emerge compared with 14 days for those sown in the second half of September. Soil moisture will be crucial, and in some cases it may be better to drill seed beds when they are likely to be moist rather than sticking to more traditional sowing dates.

Seed rate – Optimal canopies and high yields can be achieved from a wide range of plant populations. However, dense crops are more likely to develop over-large canopies and have increased risk of lodging in the spring. In ideal situations the optimal plant population is 25–35 plants/m2 for both conventional and hybrid varieties. The target seed rate to achieve this plant population will depend on percentage establishment and the number of volunteer rape plants. Establishment rates usually range from 50–80% but with typical volunteer numbers of 5–20 plants/m2, a target plant population of 25–35 plants/m2 can generally be achieved from 30-40 seeds/m2. We believe it is best to resist the temptation to increase seed rates in response to the withdrawal of neonicotinoids for the following reasons: 1) oilseed rape crops compensate well for the loss of a few plants; 2) if the csfb infestation is so severe that it causes crop failure then an increase in seed rate is unlikely to save the crop; 3) leaf pruning experiments have shown that low plant population crops tolerate the loss of cotyledons and early leaves just as well as high plant population crops; 4) establishing too many plants results in an over-large canopy and more lodging, both of which can significantly reduce yields.

Monitoring csfb risk

As yet we do not know the likely pest pressure from csfb this autumn. This can be very variable. There are a number of ways to monitor pest levels to help with risk assessment:

1. Monitor the number of beetles in the grain trailers – Although not the most scientifically precise method, a quick look in the trailers of harvested OSR seed will give an indication of the number of beetles available to invade crops. There is no simple way of calibrating these observations but over a number of years, experience will tell you what level of trailer infestation poses a risk. In the first instance, if beetles are easily seen hopping around in the trailers then there is clearly a risk to an emerging crop.
2. Set yellow water traps – Yellow water traps in proposed OSR fields will catch csfb. Traps should also contain a drop of washing up liquid so that any insects caught will sink and drown. Traps should be examined regularly as they will catch large numbers of insects which can make it difficult to pick out csfb. There are no thresholds but the presence of beetles will indicate that it is important to monitor crops as they emerge. Data collected over a number of years will help to determine how trap catches relate to crop damage.
3. Set yellow sticky traps – As with water traps these can be used to monitor beetle numbers. Regular inspection is again recommended as large numbers of different insects will be caught. The presence of beetles indicates a need to monitor crop emergence.
4. Assess the need to control larvae – Even if control measures are applied against adult csfb this does not mean that there is no risk from the larvae. AHDB Information sheet 24 provides a good summary of monitoring methods for csfb at the following link: www.cereals.ahdb.org.uk/media/1079572/lr-cabbage-stem-is55-summer-2016.pdf

Monitoring pest damage

Csfb is now a relatively widespread pest so there is a good chance that you will see damage on an emerging crop of oilseed rape. However, the presence of damage does not necessarily mean that there will be an impact on yield. For example, it would be expected that the damaged plant to the left would be able to compensate for the loss of leaf area.

Currently pyrethroid insecticides are the only alternative to neonicotinoid seed treatments for control of csfb adults and larvae. As mentioned above, csfb resistance has been recorded in the UK so it is important to limit further spread. An AHDB funded project is currently investigating the extent of pyrethroid resistance in csfb in the UK. It is therefore vital that pyrethroids are used in a rational manner to minimise the development and/or further spread of resistance. Treatment thresholds currently advise that a spray is only necessary if:

•  25% of leaf area is lost at the 1-2 leaf stage increasing to
•  50% of leaf area lost at the 3-4 leaf stage

Routine sprays at the first sign of damage must be avoided.