Arable pests taking advantage of the late season and a lack of natural predators, could still cause an epidemic, if conditions allow.
That’s the warning from independent consultant Dr Alan Dewar, who says that insecticide choice will be important to ensure control due to the high level of grain aphid resistance.
“Despite the harsh winter and lack of migration, we can’t discount the risk,” he says. “Crops are way behind and wide open to exploitation. And history tells us that aphid epidemics can, and have followed cold winters.
“It is never what you think it is going to be,” warns Dr Dewar, noting that the grain aphid can have both an asexual and sexual lifecycle. “The pest tends to exploit mild winters by reproducing asexually; yet when adults are unlikely to survive a harsh winter, they revert to a sexual lifecycle, with eggs hatching during a period of warm weather.”
Natural predators such as ladybirds, lacewings and parasites that can normally be relied upon to curb grain aphid populations, are also at very low levels, with the majority thought to have been caught out by the poor early Spring weather.
Dr Dewar notes that further summer pest threats such as the Orange Wheat Blossom midge and Saddle Gall midge also have the potential to take advantage of late crops, although the latest Dow AgroSciences egg count data, conducted in conjunction with ADAS, reveals a mixed picture. Orange Wheat Blossom midge populations are currently very low, with even the highest recorded levels still rating below the 10/kg soil threshold at which damage is likely if conditions are favourable for midge emergence. Yet, high numbers of Saddle Gall midge eggs have been recorded in some infested sites.
“It’s the current pest of note,” says Dr Alan Dewar. “Yet it appears to be sporadic. Until the recent mini epidemics in 2010-2011, the last time it was a serious problem was in the 1970s. It lasts for a few years and then disappears again, although I am sure it is more widespread than has been reported.”
For this season, the sampling data shows high Saddle Gall midge levels locally, with, for example, 165 larvae per m2 recorded in one infested site in Buckinghamshire, which equates to 1,650,000 larvae per hectare, levels that could result in significant, yield-stripping damage in backward crops. Yet the picture is rather different at the North Yorkshire site where early reports indicate many of the larvae have died or are unviable.
In terms of control options, Dr Alan Dewar says that chlorpyrifos should be the insecticide of choice where there are mixed midge and aphid populations, warning against employing pyrethroid chemistry if resistant grain aphids were also present, due the risk of ineffectual control. “Resistance to pyrethroids is rife in grain aphids, and has increased significantly in the last three or four years, although the control implications of using pyrethroid chemistry remains unproven on wide scale, field situations.
“But, at least half the aphids tested last year in some Rothamsted Insect Survey suction trap catches were carrying the resistance gene,” he confirms.
