Where did all the pests go?

Ellie Dearlove considers why some major crop pests were slow to get going this autumn.

Some pests were less active in early autumn than usual. For example, we received reports of lower numbers of cabbage stem flea beetle (CSFB) and the aphids that spread Barley yellow dwarf virus (BYDV) in cereals (grain aphid and bird cherry-oat aphid).

Coordinated pest monitoring activity, based on yellow water traps (for CSFB) and suction trap data (for aphids), backed up these observations.

Unfortunately, the pests did not vanish; they were just held up to various degrees. For example, recent data from the Rothamsted Insect Survey shows large increases in bird cherry-oat aphid at several sites.

Our yellow water traps also detected a relatively late uptick in CSFB at several monitoring sites. So, what’s going on?

Unsurprisingly, the weather is very influential. Insects are small, and this makes them particularly sensitive to weather extremes, including fat raindrops and gusty winds.

They also rely on external temperatures (they are ectothermic), so cool spells slow them down. In a record-breaking year for weather, unusual pest patterns are likely to follow.

This summer was the warmest on record for the UK, with sustained periods of heat and drought. As autumn arrived, conditions turned cooler and wetter than average, suppressing pests – at least initially.

In addition to temperature (and daylength), beetle populations may also be influenced by summer drought. For example, we also saw some CSFB suppression in 2022, after another hot and dry summer.

Migration was late at most of our monitoring sites, and numbers have been generally muted over the last few autumns, including this one.

Early autumn rain also encouraged crops to get up and away before beetle activity peaked.

This autumn, the migration ‘trigger’ was relatively synchronised across most sites. We will add the data to our long-term data set and continue to crunch the numbers to improve understand of migration.

• Late and low migration is associated with reduced adult feeding damage
• It is still important to monitor for larval levels during autumn, as each adult can lay up to 1,000 eggs
• Our research suggests that 10 larvae per plant may reduce yields by between 0.5 to 0.7 t/ha
• Consider grazing or topping to remove larvae, but do this before the end of the year to give crops time to recover

Autumn aphid flights are usually well under way by mid-September. The cooler and wetter conditions during early autumn encouraged pest populations to wait it out until things improved.

The suction trap network is particularly adept at catching bird cherry-oat aphids, with large spikes in numbers recorded in the most recent aphid bulletins.

Interestingly, the proportion of cereal colonisers (winged aphids moving into cereal crops) appear to be slightly lower than usual (e.g. 10.2% compared with the 10-year average of 21.5% in Aphid Bulletin 28), with most returning to overwinter on bird cherry trees.

This autumn, about a third of aphids carry BYDV, according to our latest monitoring data.

Once again, temperature and daylength are key migration drivers, with drought also being potentially influential, which can speed up aphid development and reproduction. It can also reduce natural enemy populations.

This potentially primed aphids for the current rebound, which may have been further supported by a spurt in fresh plant growth (which increased feeding opportunities).

• Monitor cereals for aphid activity during the autumn (consider using water or sticky traps)
• Late-drilled cereals are better placed to avoid migration peaks
• Aphids remain active for longer in mild conditions
• Use the BYDV management tool to time sprays to limit virus spread
• Consider resistant/tolerant winter wheat and winter barley varieties (see table footers in the RL)
• Encourage natural enemies

But what about organisms that don’t fly? Well, slugs provide another case of climate delay, not disappearance. The hot, dry soil this summer was bad news for slugs, especially eggs and juveniles (adults often retreat deep underground). Once again, when moisture finally arrived, slugs took advantage, quickly feeding and breeding.

• Monitor for slugs (especially after rain)
• Crops following brassicas (e.g. oilseed rape or some cover crop mixes) or second wheats are at higher risk
• High moisture (e.g. in heavy or cloddy soils, cover-cropped areas and areas with high levels of organic matter) are at higher risk
• Roll seedbeds to minimise places for slugs to hide (refuges)
• Increase wheat sowing depth to 4 to 5 cm (sow deeper in high-risk situations)
• Treat problematic patches with a molluscicide
• Consider mixing slug pellets with seed (admixtures), especially when direct drilling or sowing into an open, cloddy seedbed
• Repeat treatment when new feeding damage is observed, when traps indicate an increase in activity or if pellets disintegrate or go mouldy

Like last autumn, some OSR crops have been impacted more by turnip sawfly than by CSFB.

Our increasingly warmer summers may be encouraging a third generation of flies, which coincide with crop emergence (especially when drilled early).