Silver Y Moth Caterpillars detected in crops – advice on identification, monitoring and control

Over the past 7–10 days, there have been increasing reports of caterpillar feeding in crops of spring beans, sugar beet and maize, Lousie McNamara, Michael Gaffney and Ciaran Collins share key advice on identification, monitoring and control.

Over the past 7–10 days, there have been increasing reports of caterpillar feeding in crops of spring beans, sugar beet and maize. While low levels of leaf feeding are not unusual at this time of year, a few crops are showing more significant damage.

Figure 1. Feeding damage from Silver Y moth caterpillar in spring beans (left) and fodder beet (right).

Samples of caterpillars from crops of spring beans and beet examined by Teagasc are consistent with the features of Silver Y moth caterpillars. Despite some initial concerns based on reported findings in the UK, there are no reports of Beet Armyworm (Spodoptera exigua) being present in crops to date. In relation to Fall Armyworm (Spodoptera frugiperda), while there have been recent findings in Greece and Romania, it has never been detected in northern Europe.

The adult Silver Y moth is greyish brown with a distinctive silver coloured ‘Y’ marking on each of their forewing. Silver Y moths are native to southern Europe, North Africa and parts of Asia. Each spring, they migrate northwards into Europe, although they do not usually reach Ireland in large numbers. However, the warmer weather experienced this season appears to have favoured migration and survival, resulting in higher numbers than are normally seen, similar to the situation experienced in 2018.

While the moth itself causes no damage, its caterpillar is capable of feeding on a wide range of broadleaf crops including beans, peas, sugar beet and maize. As the caterpillars grow, they consume leaf tissue, often leaving the veins intact and giving affected leaves a characteristic ‘stripped’ appearance (Figure 1). In severe infestations, they can also feed on pods.

The caterpillar feeds for up to four weeks before pupating in a silken cocoon (Figure 3). Adult moths emerge 10 to 14 days later and can live for up to three weeks, allowing two or sometimes three generations to develop during a favourable season.

Figure 2. Silver Y moth caterpillar feeding on spring beans, (left) and with its characteristic ‘loop’ in fodder beet (right).

Figure 3. Cocoon of Silver Y moth.

Silver Y caterpillars are green with pale side stripes and grow to around 30–35 mm long. They have three pairs of pro legs. They have a distinctive looping movement, making them easier to recognise when inspecting crops (Figure 2). Another distinguishing feature is the presence of three pairs of pro legs (Figure 4), most caterpillars found in Irish crops will have more than three pairs of pro legs.

Figure 4. Caterpillars typically have 3 pairs of true legs (Blue Arrow) just behind its head capsule. At the anterior end, you will often see proleg structures (Yellow Arrow), which look like fleshy legs. With Silver Y caterpillars you will see 2 of these prolegs and an additional proleg at the very end called the anal clamper. Caterpillars with more or less prolegs are not Silver Y caterpillars.

As it is an atypical pest in Ireland, there are currently no treatment thresholds for Silver Y moth established to date. However, applying the guidance from PGRO which uses pheromone trap catches to indicate migration into pea crops; the threshold is 50 moths by the first pod stage, while in sugar beet a threshold of five caterpillars per plant is suggested before treatment is considered. There are currently no thresholds for maize.

It is important to note that the presence of caterpillars alone does not necessarily justify an insecticide application. Most crops can tolerate a degree of leaf feeding without any measurable effect on yield, and unnecessary insecticide use will harm beneficial insects that naturally help suppress pest populations.

Growers are advised to:

• Inspect susceptible crops regularly over the coming weeks.
• Where feeding is observed, identify the cause and confirm what caterpillars are responsible before making any treatment decision.
• Revisit affected fields after a few days to determine whether caterpillar numbers or feeding damage are increasing.
• If damage has stabilised or caterpillar numbers have declined, treatment is unlikely to be required.
• Where caterpillar numbers continue to increase and significant feeding is occurring, a pyrethroid insecticide such as Karate or Decis may provide effective control.

As with all insecticide applications, products should be used according to product label and only be used where there is evidence that treatment is justified. Limiting insecticide use to fields with significant and increasing damage will help protect beneficial insects.