With the mild winter giving way to anunseasonably early aphid flight potato growers are being urged to apply thefirst insecticide as soon as crops reach 50-75% emerged to guard againstpotyviruses transmission. With plantingcomplete crops are expected to be fully emerged by the end of May.
The latest Aphid News from AHDB confirmsthat numbers are building rapidly. Peachpotato aphid (Myzus persicae), the principal vector for the main potyviruses, PVAand PVY, has been detected up to two weeks earlier in the south and more than amonth earlier than normal in the north of England. The Rothamsted Insect Survey (RIS) forecastpredicts the first peachpotato aphids will be caught in Scotland on 27thMay in Edinburgh and just a few days later in Dundee.
The grain aphid (Sitobion avenae), which is less influenced by winter temperaturesbecause it over-winters in the cold-hardy egg stage and is typically regardedas the greater threat to potato crops in Scotland, has also been caught atsites across England up to three weeks earlier than the long-term average. Theforecast predicts the first grain aphid will be detected in Scotland from themiddle of May.
The mild winter and the good progressgrowers have made with planting this spring mean we are likely to see largeareas of crop emerging at a time when aphid activity will be intense, saysScottish Agronomy senior agronomist Eric Anderson.
Growers should be prepared to protect cropsearly, either with a pyrethroid, a suitable oil which trials have indicated canreduce transmission by 30-60%, or a neonicotinoid, such as Biscaya(thiacloprid), he adds.
Trials also indicate that Biscaya(thiacloprid) works better when applied earlier in the season. This is perhaps due to its unique oildispersion formulation. It is also one of only two neonicotinoid products thatcan be applied to flowering crops, says Eric Anderson.
The decision as to which means of control toadopt is made complicated by farming location and the species of aphid most inneed of control. For potato growers theaphids representing the greatest source of potyvirus transmission are thepeachpotato aphid, grain aphid, rosegrain aphid (Metopolophium dirhodum) and blackbean aphid (Aphis fabae).
In England the peachpotato aphid isregarded as the most important vector because it is the most numerous while inScotland rosegrain aphids and grain aphids are considered the principal vectors.In England the grain aphids (Sitobionavenae) has been found to express some level of resistance to pyrethroidsmaking this class of products virtually ineffective as a means of control.
It is estimated that 40-50% of the cerealgrain aphid in England expresses a shift in sensitivity to pyrethroids. This isa shift in sensitivity rather than outright resistance as is the case with 95%of peachpotato aphids. In such cases, pyrethroids offer very little benefit,says Eric Anderson.
With few options available for controlhowever, growers are advised to construct a programme that utilises all theavailable products.
If growers want to pursue other modes ofprotection due to reduced sensitivity to pyrethroids they might choose to adopta tactic used by growers in France where they have taken to applying an oilevery two to five days once the crop is 50% emerged, says Eric Anderson.
The mineral oil is believed to work bycoating the inside of the stylet which prevents the virus from gathering on theinside of its tip and being passed to the next plant during the feedingprocess.
Such regular applications may not bepractical however, so consideration needs to be given as to how we use theavailable products for greatest effect. Fortunately, there is no evidence ofaphid resistance to neonicotinoids in the UK which includes thiacloprid(Biscaya), thiamethoxam (Actara) and acetamiprid (InSyst). Nor any knownresistance to Flonicamid (Teppeki) which is a pyridine-carboxamide andpymetrozine (Plenum) which is a pyridine, he says.
Growers can still rely on pymetrozine andflonicamid as well as the neonicotinoid treatments thiacloprid andthiamethoxam, but there are restrictions on permitted use which influenceproduct choice and timing.
Regulatory limits allow for a maximum oftwo applications in total from any neonicotinoid group of insecticide to a seedcrop and one application to a ware crop during the season. Products containingpymetrozine and thiamethoxam cannot be applied to flowering crops or cropscontaining flowering weeds due to their possible high risk to bee colonies, hesays.