Oilseeds and vegetable brassicas
Identification and symptoms
Adults are 3–5 mm long, metallic blue-black or light brown and are often seen crawling over trailer loads of rapeseed at harvest. They have long antennae, large hind legs and jump when disturbed. The larvae are white with numerous, very small, dark spots on the back, a black head and tail and three pairs of dark legs. When fully grown, they can reach 6 mm in length.
Adult feeding can be seen as characteristic shot-holing of the leaves. Plants infested with larvae lose vigour, becoming stunted, and die if the infestation is severe.
- Adults emerge and feed on foliage.
- Adults ‘rest’ in moist, sheltered places.
- Adults migrate into crops, feed on leaves and mate.
- Adults lay eggs and feed on leaves until temperatures drop.
- Eggs hatch and larvae feed if temperatures are 3°C or warmer.
- Larvae feed on main stem behind the growing point.
- Larvae drop to the soil and pupate.
Cabbage stem flea beetle is a major pest of oilseed rape. Originally a problem in East Anglia, it now covers England and Wales and is spreading in Scotland.
Since 2013, when neonicotinoid-treated seed was withdrawn from use in oilseed rape, management of this pest has become more challenging.
Crops are most vulnerable at emergence, when the crop can grow more slowly than it is being eaten. The growing point of the crop can also be destroyed. Large numbers of adults feeding in the autumn can kill plants, occasionally resulting in total crop failure. Larval feeding in the stems and petioles reduces vigour and can cause severe damage, which may lead to stunting or plant death.
Larvae may feed within the stems of vegetable brassicas, such as spring cabbage and kale, during autumn and winter but it is an incidental pest.
Resistance to pyrethroid insecticides has been confirmed in the UK CSFB population. Both target-site resistance and metabolic-based resistance, the latter conferring strong resistance to pyrethroids, have been confirmed.
Resistance pressures mean it is essential to use integrated pest management (IPM) approaches to minimise the use of insecticide sprays. In particular, it is important that treatment thresholds are followed.
Where a spray is deemed necessary, full recommended field rates should be applied. If control is poor, pyrethroid sprays should be avoided.
Weather conditions and drill date have the strongest influence on cabbage stem flea beetle risk, according to an analysis of data from more than 1600 sites across 14 years
Where seedbed conditions are good, crops drilled:
- Early (in August) are more likely to establish well before adult migration begins
- Late (in September) are associated with lower larval pressures in the crop
- Very late can establish after the bulk of the adult migration has occurred
- Air temperatures above 16°C are more favourable for adult cabbage stem flea beetle migration
- A warm autumn will favour egg laying and early hatch of larvae, coinciding with smaller, more vulnerable plants
- Crops drilled into dry and cloddy seedbeds can be slower to emerge, with reduced vigour
- No obvious differences in susceptibility across Recommended Lists varieties have been observed. However, varieties associated with good autumn vigour will achieve rapid establishment and growth, and move through vulnerable growth stages faster.
The target plant population for oilseed rape is 25 to 40 plants/m2. Seed rates should be adjusted to reflect expected losses. However, excess plant populations should always be avoided.
Monitoring (oilseed rape)
For early warning signs, check for cabbage stem flea beetle in previously harvested seed and shot-holing on volunteer oilseed rape. Monitor for pest damage as soon as crops begin to emerge. The amount of leaf area eaten can determine the need for treatment.
To predict larval populations, set two yellow water traps on the headland and two in the field along a wheeling in early September. Fill them with water and a drop of detergent. Empty and reset the traps weekly, recording the number of cabbage stem flea beetles and adding it to the previous total for that trap. Remove the traps at the end of October. Use the total numbers of beetles caught in each trap over the whole monitoring period to calculate an average number of beetles/trap.
Plant dissection involves taking a random sample of 25 plants from the field in late October/early November. Samples are best dissected by an accredited laboratory
Thresholds (oilseed rape)
Resistance to pyrethroid insecticides has been confirmed in the UK cabbage stem flea beetle population. Both target-site resistance and metabolic-based resistance, the latter conferring strong resistance to pyrethroids, have been confirmed.
Resistance pressures mean it is essential to use integrated pest management (IPM) approaches to minimise the use of insecticide sprays. In particular, it is important that treatment thresholds are followed. The thresholds are displayed in the table.
Sufficiently large areas of oilseed rape volunteers can attract cabbage stem flea beetle and divert them away from nearby drilled crops. Trials show that this approach can significantly reduce adult numbers and damage, and result in higher plant populations. When volunteers are destroyed in mid-September, eggs and larvae are also killed.
Carabid beetles (Trechus quadristriatus) feed on cabbage stem flea beetle eggs and young larvae before they enter oilseed rape plants and the larval parasitoid (Tersilochus microgaster) parasitises larvae in the spring. All parasitoids may be vulnerable to pyrethroid insecticides. Minimum tillage has potential for conserving carabids and parasitoids. Two entomopathogenic fungi (Beauveria bassiana and Metarhizium anisopliae) are known to infect cabbage stem flea beetles but their impact on the field populations is not known.
Winter defoliation of oilseed rape can reduce larval populations significantly – on average, 39% fewer larvae were recorded in on-farm trials that used a topper or sheep to graze the crop. Defoliation should take place prior to stem extension.
Latest information on insecticide resistance can be accessed via ahdb.org.uk/knowledge-library/IRAG
Cabbage stem flea beetle adult
Cabbage stem flea beetle larva