End of season round up – Pests
7 Aug 2018
As year 2 of our Real Results OSR site monitoring and in-season updates comes to a close, we look back at the season with our ADAS experts. Here, Steve Ellis reviews the incidence of different pests this past season at the monitoring sites.
Cabbage stem flea beetle (CSFB)
By the third week in September, variable damage levels due to CSFB adults were being reported. Most damage was recorded in the east of England but there was also significant damage on the Yorkshire wolds.
In early October, most crops had at least four leaves. There were no reports of the 50% leaf area lost threshold being exceeded. Most crops were beyond the stage at which they are susceptible to adult CSFB by early November, when the first larvae were reported.
Water trapping (96 adults caught early September to end October), plant dissection (five or more larvae/plant) and counts of leaf scars (>50% plants with leaf scars) were recommended to determine the need for larval sprays. Sprays were only advised where thresholds were exceeded to limit the further development of pyrethroid resistance.
“ADAS data suggests that pyrethroids are effective against larvae if correctly timed”
ADAS data suggests that pyrethroids are effective against larvae if correctly timed. It is unclear whether the presence of adults resistant to pyrethroids, means that the larvae will also be resistant, although logic suggests that this is most likely scenario. Resistance is particularly prevalent in the east of England.
By mid-January temperatures had dropped, and this suggested that there would be limited further egg hatch, as egg development is believed to stop below 3oC. Surprisingly, despite the cold spell, further plant dissections at the sites in North Yorkshire and Cambridgeshire, showed larval numbers increased from 0.8 to 2.6/plant and 5.6 to 13.6/plant respectively, so conditions had still been suitable for egg hatch.
Numbers in Cambridgeshire were now 2.4 times the autumn threshold, but plants were bigger and better able to tolerate damage. Most larvae were also still within the leaf petioles (at least 91%) so had not moved into the stem despite the cold weather. By the end of April, there were reports of some csfb larval infestations affecting the ability of the crop to grow away.
Wet weather in the third week of September increased slug activity. Slug damage was recorded in Herefordshire and North Yorkshire by early October, but levels of leaf loss were relatively low. Plants also had four or more leaves and so were no longer susceptible to slug damage.
Later that month all plants at the North Yorkshire monitoring site and 75% of those at the Herefordshire site were showing damage but the level of leaf loss was too low to justify treatment.
There were reports of turnip sawfly adults in crops in the third week of September but subsequently no larvae were recorded.
Leaf miner damage was reported in early October in Herefordshire, but not elsewhere. It is likely that this was the brassica leaf miner (Scaptomyza flava). Despite unsightly mines in oilseed rape, this pest rarely justifies insecticide treatment. Generally, it is only the first developing true leaves that are affected and these are usually lost over winter.
Peach potato aphids and TuYV
In the third week of September, numbers of peach potato aphids, the vectors of turnip yellows virus, in the Rothamsted Insect Survey suction traps were low. The first reports of aphids in crops were towards the end of October.
As only a single insecticide can be applied to control TuYV, general advice is to delay spray decisions until aphid migration is virtually over, which is usually by mid-November. By early November, aphid numbers were increasing in the south of England and some sprays were applied. Temperatures were starting to dip below the flight threshold but were still high enough for reproduction and spread within crops.
By the end of November, aphid flight activity had all but stopped. The subsequent cold weather would also not have been ideal for aphid kind, with the literature suggesting that the LT 50, the temperature required to kill 50% of aphids is about -8oC for nymphs and -7oC for adults. Temperatures in some areas were below these levels.
“There were few, if any, reports of TuYV in the spring”
Another way that aphids can be killed by low temperatures is by accumulating a cold dose. Temperatures may not need to be as low as the LT50, as repeated exposure to slightly higher temperatures will affect aphid survival. Low temperatures will also stop aphid reproduction and severely limit their ability to move between plants.
Where sprays were due to be applied, the advice was to monitor crops when the weather improved, to determine if pest numbers justified treatment. Unless aphids could be easily found it was considered unlikely that any further sprays would be required.
Temperatures increased in late January, allowing aphid reproduction and spread between plants, but unless they could be easily found, no further sprays were advised. In February the cold weather returned with a vengeance, and it was difficult to believe that any aphids that had managed to survive the winter that far had not turned their toes up.
By the end of February 2017, pollen beetle migration had started across much of England and Wales when temperatures reached 15oC. This was never likely to be repeated in 2018, when cold temperatures meant that migration was delayed.
By mid-March, there had still been few sightings of beetles. It was difficult to speculate on how the cold weather and the ‘beast from the east’ would affect pollen beetle risk.
On one hand, crop growth had been slowed so it was possible that pest migration would coincide with the susceptible green/yellow bud stage. However, it was also possible that the weather had delayed migration to such an extent, that crops were beyond the susceptible stage before the pest arrived. Ultimately it appeared that the latter scenario came to pass with beetle numbers still low in early April.
“Once rape flowers are open, they preferentially attract beetles away from the buds where they do no harm”
Temperatures increased in the third week of April and beetles were more easily found, but by this stage many crops had started to flower, so the risk of significant damage was over. Once rape flowers are open, they preferentially attract beetles away from the buds where they do no harm. By early May, the pollen beetle risk to winter rape was over for another season. Pollen beetle larvae were seen in crops later in the month, and some very high beetle numbers were reported, but crops were well beyond the susceptible stage.
A potential threat from seed weevil was first reported in early May. A spray during flowering and ideally before petal fall, is advised if the threshold of more than one weevil/plant in southern Britain, and more than 0.5 weevils/plant in northern Britain is exceeded.
With continuing warm temperatures during May, reports of increasing seed weevil numbers were expected. These did not materialise, and no reports were received of crops over threshold.