Lettuce aphid and virus control moving into unknown territory

In the third and final article in our series looking into the implications of the loss of neonicotinoid seed treatments, The Vegetable Farmer asks Fresh Produce Consultancy’s David Norman what it could mean for lettuce growers.

There is some irony in the situation lettuce growers face this year. For the last 25 years the neonicotinoid seed treatments, Gaucho (imidacloprid) and Cruiser (thiamethoxam), have largely taken care of aphid control and their loss is going to make the job much more difficult. At the same time we are better placed than ever to control caterpillars thanks to the arrival of the new diamide chemistry. As is increasingly the case in crop protection, what is given with one hand is taken away with the other.

Four aphids and three viruses of concern

In lettuce, aphids as both contaminants and virus vectors concern us. The species of concern are the peach–potato (Myzus persicae), currant–lettuce (Nasonovia ribisnigri), lettuce root (Pemphigus bursarius) aphids and to a lesser extent potato aphids (Macrosiphum euphorbiae). The contamination threshold is one or two aphids per plant which for all intents and purposes means we need to aim for nothing short of 100% control.

Peach–potato aphids are the main virus vector spreading cucumber mosaic virus (CMV), lettuce mosaic virus (LMV) and turnip yellows virus (TuYV). CMV and LMV are non-persistent viruses passed on by aphids to host plants in a matter of seconds. Plants with combined LMV and CMV show severe stunting and chlorosis making them unmarketable.

Previously, practically all summer crops (planted from mid-April onwards) had a neonicotinoid seed treatment. It saw them safely through their fortnight in the nursery and protected transplants for a further three to four weeks i.e. around half the life of the crop. As a result, just two to three subsequent foliar insecticide applications were required. With three treatments of Plenum (pymetrozine) and two of Movento (spirotetramat) permitted per crop we didn’t have much to worry about. Importantly the seed treatments gave complete control of lettuce–root aphid to the extent that few growers or agronomists under the age of 50 will have ever seen them.

Two to three insecticides short

To compound matters this year we have just passed the final sale date for Plenum and 2019 is the ‘use up’ year. In summer crops we will now need to substitute three or four foliar insecticide sprays for the lost seed treatment taking the total programme requirement to seven or eight weekly sprays. Quite simply we are two or three good systemic insecticides short of what we need for this.

Minecto One (cyantraniliprole), recently authorised for caterpillar control in lettuce may help, but we don’t yet have enough experience of it to know what it will do. We expect a subduing effect on aphids so it will fit in the June to August period when caterpillars are also an issue.

At this stage everything depends on what the year ahead brings in weather and aphids. I am hoping we will just squeak through, but if it turns into a bad aphid year we could easily see big issues with crop rejections. We are moving into unknown territory so we need to do everything we can to set the odds in our favour by working through the IPM checklist.

Varietal resistance should be considered, particularly to lettuce root aphid, as it has proved more solid than for currant–lettuce aphid. We have had such good control of lettuce–root aphid for so long that it should be some years before the species builds up again.

Physically divide plantings

Crop hygiene will be more important than ever and debris should be ploughed in as soon as possible to break the green bridge from one crop to the next. In a similar vein the sequence of plantings will also need to be planned so that they are physically divided by some distance rather than working across a field planting new strips next to the last planted strips.

Organic growers rely on barrier mesh to keep aphids off their crops and conventional growers will no doubt be considering this option too. It’s important to note that crops have to be covered on the day of planting as you can’t afford to let any aphids get trapped underneath mesh.

I would also be cautious of using mesh in the height of summer as its insulative effect can cause temperatures underneath to rise to the point that plants suffer.

Understanding our crop protection chemicals will help to ensure we get the most out of them. Spirotetramat – the active substance in Movento – moves towards the centre of the lettuce plant making it very good at controlling hidden currant–lettuce aphid. It breaks the aphids’ life cycle to achieve longer-term population control so applications should be timed to make use of this attribute.

Manufacturer Bayer says that through its two-way systemicity, Movento is translocated to the roots and will therefore help to control lettuce–root aphid. However,good coverage of older, outer leaves is particularly important, as there is little movement of the active substance to them from new leaves. When peach–potato aphid colonise the older, outer leaves, Plenum is the best bet. Its anti-feeding action may also help prevent the spread of virus.

With all products, attention to spray timing and quality will be more important than ever to get the best out of them and actives should be alternated, rather than blocked, for resistance management.

New chemistry for caterpillar control

The caterpillars of two species of moth, turnip moth (Agrotis segetum) – also known as cutworms – and silver Y moth (Autographa gamma) are particular pests of lettuce crops. Although both are sporadic and infestations do not occur every year, silver Y has been more of a problem of late. Large numbers are carried from the continent on southerly winds and under high temperatures they go through the life cycle very fast with eggs hatching in just three or four days.

The risk, particularly with iceberg, is that larvae chew through the outer wrapper leaf. LastJuly there were serious problems with silver Y moth control as the pyrethroid sprays that had been applied were breaking down fast under the high temperature and light intensity. Unfortunately, the EAMU for Coragen (chlorantraniliprole) did not come through in time but at least we have it ready and waiting for this year.

2019 will be the first time we’ve got something systemic to control caterpillars thanks to the new diamide chemistry in Coragen and Minecto One. However, growers should note the resistance management guidelines on the product labels of these two IRAC group 28 insecticides which, amongst other things, limit the total numbers of sprays from this group to two per crop, not targeting consecutive generations.

The loss of the seed treatments will bring a big change to our agronomy. In crops planted from mid to late April onwards we will have to be alert to aphid risk from day one and make good use of pest intelligence services. Monitoring with yellow water traps and pheromone traps will have to be an integral part of crop management.

AHDB aphid news and pest bulletin

AHDB aphid news provides a useful early warning of potato and peach– potato aphid catches from the Rothamsted Research/SASA suction-trap network. Lettuce-root aphids cannot be easily distinguished from other species of Pemphigus and so are not recorded. Currant-lettuce aphids are only ever caught in low numbers in both suction traps and water traps and appear to be ‘trap-shy’; the best way to monitor infestations of currant–lettuce aphids is to look at lettuce plants.

While all three species of aphid could be present during the midsummer period, currant-lettuce aphid tend to dominate during the autumn. Crops are exposed to colonisation by lettuce-root aphid over a relatively short period of time through June and July when they migrate from their winter hosts (poplar) to lettuce. The precise timing of all these events varies between years.

The AHDB pest bulletin provides useful information on lettuce aphids and lettuce caterpillars including day degree forecasts for currant–lettuce and lettuce–root aphids, summaries of sightings of silver Y moth in several European countries and results of testing for pyrethroid resistance.

With the arrival of the new diamide chemistry we can be less reliant on pyrethroids and that in turn should mean our crop protection has less impact on beneficials. More than ever before Aphidius wasps, ladybirds and hoverflies will be crucial allies in our fight against aphids so we need to treat them as such and avoid any collateral damage.