Mussel scale control
Mussel scale continues to pose a major problem for many apple growers. Since its reappearance following the withdrawal of winter tar oil ten years or so ago, it has spread throughout the South East often latching on to fruit to cause expensive downgrading.
The pest’s method of spread is by no means obvious as it cannot fly but Chris Lillywhite, Hutchinsons fruit agronomist in the South East, believes its tiny crawlers (larvae) get carried from tree to tree and orchard to orchard on workers’ hands and clothing.
“It’s alarming how many crawlers there are on badly infested trees,” he says. “They are tiny and
there are millions of them because there are up to 80 eggs under each scale. However, we have chemical sprays that bring down scale levels effectively but it takes a programme of sprays over a couple of years to do this. We have cleared up scale on some farms and got numbers down to the point where we no longer have to spray routinely.”
An HDC-funded project run by EMR’s Professor Jerry Cross found that crawler emergence from scales begins around early May and peaks in late May or early June. Migration of crawlers up trees continues for as long as six weeks.
Such long migration makes sprays difficult to accurately time “but it’s wrong to rush in during the
initial crawler emergence stage”, maintains Chris. He waits until around 50% emergence to apply the first spray and 90% for the second, stages that he is able to determine from experience and observation.
The spray programme he recommends is Envidor (spirodiclofen) first and Calypso (thiacloprid) a few weeks later. Envidor gives a very high standard of crawler control but is not so good after the scales have formed while Calypso is very effective against crawlers and the first instar scale. Although both products prevent crawlers forming scales on the wood Calypso stops them establishing scales on fruit, adds Chris.
These products have the additional advantage of giving incidental control of a range of other pests. Envidor is active against red spider mite, rust mite and flat scarlet mite and Calypso combats rosy apple aphid and other aphids, sawfly, Rhynchites and hoppers. Both are safe to most beneficials.
Professor Cross confirms that Calypso and Envidor work very well with Envidor being more effective against crawlers on bark than on fruit. “When you get scales on the fruit and leaves it’s a suicidal tactic because their offspring have no future,” he reckons.
He advocates the use of double-sided transparent sticky tape, wrapped round the tree trunks, for monitoring the emergence of crawlers. They get stuck under the tape where they are easily seen. Crawler emergence “has quite a rapid early rise and a long tail”, he says.
