Ryegrass mealybugs are an infrequent and little known pest, with most records from South Australia. They seem to like barley, but what made this particular crop so special?
Where have they been reported?
Mealybugs, probably the ryegrass mealybug (Phenacoccus graminicola)has been found in high numbers on a barley crop (variety: Scope) east of Barham on the border of Victoria (Mallee district) and New South Wales (Lower Western district).
Approximately 20% (or 150 acres) of three barley paddocks (totaling 650 acres) were infested with mealybugs, at a density of approximately 50-100 per plant. Of the affected plants, approximately 20% had died as a result of mealybug feeding, and the rest were showing severe signs of stunting. The mealybugs were most abundant in land depressions where ryegrass had been present for a short time before being sprayed out.
Interestingly, there was no summer/early autumn ‘green bridge’ upon which these pests could maintain or build up a population; the previous barley crop had been grazed until virtually no vegetation remained. Outside the crop the vegetation was mostly native with little grass.
About ryegrass mealybugs
Mealybugs are small, sap-sucking insects, within the order Hemiptera, family Pseudococcidae.
They have an oval-shaped body (approximately 1-3 mm long) covered by a white waxy substance, and functional legs that allow them to be mobile in both the adult and immature stages.
Only the adult males develop wings and are able to fly.
Mealybugs feed on plant sap by inserting their mouthpart into the plant tissue. While feeding, they excrete a sticky waste substance that adheres to leaves and provides a medium for dark moulds to colonise and grow. This mould can reduce the photosynthetic ability of the plant.
Taxonomists in the Victorian DEDJTR are still confirming the identity of this mealybug, although they believe that it is likely to be Phenacoccus graminicola.
We don’t know much about ryegrass mealybugs.
They are not native to Australia, and have been reported previously in South Australia, Queensland and Victoria (but not New South Wales). They have been infrequently reported in South Australia since the 1950s, mostly from barley, but also from wheat, triticale, ryegrass, lucerne, clover, crassula and prickly pear.
Past attempts at controlling mealybugs with contact insecticides such as alpha-cypermethrin and chlorpyrifos have been largely ineffective, partly because of the waxy exudation that coats their body, but also due to difficulties achieving adequate spray exposure with the insects harbouring in leaf folds.
Carefully monitor barley crops that appear to be discoloured. In particular, look carefully at the leaves of plants that have turned yellow. They may also have small black marks or flecks from feeding spots.
Look in the leaf folds for mealybugs, which is where they commonly aggregate.
Insecticides with some systemic activity may be more effective than contact insecticides.
Sources of field reports of ryegrass mealybugs
Anthony McInerney – Agronomist, Elders (Lower Western New South Wales)