Saddle gall midge rises up pest league table

Saddle gall midge is certainly worth watching out for, especially in spring cereals; but while yellow cereal fly, gout fly and cereal leaf beetle have prompted headlines over the past two seasons, growers should not be unduly concerned by them, according to ADAS.

“All these pests have been turning up in crops more often than usual, which raises the question of whether any deserve, as in football, promoting to a higher league,” says entomologist Steve Ellis.

An HGCA Research Review (No. 73) of treatment thresholds for controlling insect pests, conducted in December 2009, offers some tentative answers, he suggests.

“The review shows that thresholds are valuable for spray decision making, but there may be less confidence in some because they were developed up to 30 years ago. Varieties have changed and agronomy has moved on significantly since then.

“Our knowledge of crop physiology has also improved, which should help us develop more robust thresholds and predict whether crops are at risk from so called new pests. It should also allow us to tell whether the impact of other more established pests may have been over-estimated.”

Dr Ellis believes the prime candidate for possible promotion is saddle gall midge which affects both wheat and barley.

“It tends to be a problem where there is frequent cereal growing on heavy land. Winter crops can suffer damage, but spring sowings are at greater risk.

“The adults, appearing from May onwards, look much like blossom midges, but they’re red. They lay distinctive rafts or chains of eggs which are quite easy to spot on both leaf surfaces. The hatching larvae then crawl down the stems to hide under the leaf sheaths, usually on the top three internodes. Their feeding causes galls which appear as saddle-shaped depressions.”

The galls reduce yield by restricting nutrient flow to the ear, he explains. “As the straw ripens the damaged stems, particularly of barley, may bend over or even break.

“It’s certainly a pest we need to keep an eye on, especially in spring crops which are more susceptible because they have limited time to compensate for any damage.

“Yield losses seem worst in wet humid conditions, but there are several questions we need to answer – namely, how should we time sprays correctly, how should we monitor the pest’s numbers and how do these relate to yield?”

By contrast, yellow cereal fly – whose larvae are among several cereal stem borers – is unlikely to rise in the league table, Dr Ellis believes. Its increasing presence in wheat, with the larvae making distinctive spiral holes in tillers, may have resulted from changes in barley yellow dwarf virus (BYDV) control strategies, he speculates.

“The traditional timing of BYDV sprays tended to coincide with the presence of adult flies in the field, so they were killed. Where sprays have been replaced by seed treatment it’s possible that more adult flies are surviving.”

However, given that well established winter cereals can lose up to half their Jan/Feb tillers without losing yield and that the insect’s egg population is unlikely to exceed 500/sq m, the risk of yield loss is low, he believes.

“Yellow cereal fly is probably only damaging in backward crops whose plants have only one tiller when the eggs hatch.”

Gout fly falls into much the same slot in the league table, its more frequent appearance possibly also being linked to the changes in BYDV control practice, he suggests.

“There’s been a vague treatment threshold that if eggs can be found on more than half the tillers at GS12 then it’s worth treating – but that’s a lot.” But with some secondary spring generations cutting yield by up to 30% that treatment threshold could require re-examining.

Cereal leaf beetle’s league progression is equally unlikely, believes Dr Ellis.

The adult beetles over-winter in sheltered spots like hedge bottoms, move into cereals in April and May and lay their eggs in June and July. The hatched larvae feed on the upper surfaces of the leaves often leaving characteristic white tramlines.

“While feeding they cover themselves with their own excrement presumably as a defence against predators. The easiest way to monitor for cereal leaf beetle larvae is to walk through the crop in light coloured trousers and look for the tell-tale stains.”

Although the loss of green leaf area, particularly in spring barley with its relatively small flag leaf, could in theory cut yield, in practice that is unlikely, Dr Ellis believes.

“That’s because the beetles don’t arrive until after flowering, and barley is mainly a ‘sink’ limited crop whose yield is governed more by the storage capacity of the grains rather than assimilates in the leaves.”

Crop damage league table

• Possible promotion: saddle gall midge

• Unlikely risers: yellow cereal fly, gout fly, cereal leaf beetle

Need a contractor?

Find one now
See more