Mealbugs – gettting to the root of a hidden problem in New Zealand pastures

By Chris Pennell, AgResearch Lincoln

Farmers - are your pastures looking stressed, even though you have done everything you should?

Insect pests are often the cause, and the bad news is it may not be grass grub munching on the roots or Argentine stem weevil burrowing into tillers that you have to worry about.

Scientists at AgResearch have now confirmed that yet another insect is active in the paddocks, causing its own form of destruction.

The pasture mealybug (Balanococcus poae) is the culprit. Mealy bug is a small insect found around and in the crown and on the upper roots of a range of pasture grasses where it sucks the sap from the plant’s tissues.

The pasture mealybug  was originally identified on common tussock grass and is endemic to New Zealand, where it is known to occur in the Canterbury, Nelson and Manawatu regions but is likely to be even more wide spread.

Adult mealybugs are pink, round and up to five millimetres in size. The crawlers (the smallest immature stage) are less than a millimetre in length, and have a bit of a pink tinge to them. Nymphs (the next immature stage) become progressively larger as they feed. Crawlers are dispersed by wind, while older nymphs and adults become sedentary; surrounding themselves with white wax secretions once they become established on host grasses.

A visiting US expert, Dr Fred Baxendale, found on a recent visit to Lincoln that there are at least two generations per year.

The pasture mealybug can be confused with another common insect, a root aphid, which also surrounds itself with white waxy secretions and lives on grasses. The aphid is smaller than the adult mealybug, yellow/white in colour and oval-shaped. The adult pasture mealybug on the other hand looks more like a tiny pink shrivelled pea. Root aphids also tend to occur deeper in the soil than mealybugs.

Mealybug feeding causes a general browning off of pasture, much like drought does.  Plants become progressively weaker as the mealybugs continue to feed and severely infested plants are often unable to recover.

Damage often is misidentified as drought initially, but sometimes there will be evidence of pulling in paddocks after grazing. With grass grub, the pulling and pasture damage tends to be patchy, whereas with mealybugs damage symptoms are very widespread.

Damage may first become apparent in January, if their presence coincides with a dry spell, but is usually most severe in autumn and early winter.

If any of this sounds familiar, AgResearch scientist Chris Pennell suggests you use a spade or a pocket knife to poke into the soil and look for the telltale white waxy secretions. A keen eye, or better still a small magnifying glass, will help identify the presence of these small pink pests.

The good news is that endophyte in ryegrass protects the plant against mealybug attack. The recently released AR1 endophyte that does not cause animal toxicity is every bit as effective at reducing mealybug attack as the wild endophyte that causes ryegrass staggers.

AgResearch scientists hope to investigate other management options for controlling this insect in the future.