Give Soft Scale the Boot
GrapeGrower Magazine, (June 2003)
By Devin Carroll
The European fruit lecanium, Parthenolecanium corni, is a soft scale found in many grape vineyards, but usually not at damaging levels. Occasionally, severe infestations cause some table grape bunches to be discarded. Natural enemies will control this soft scale unless ants interfere. Chemical control depends on correct timing.
In the San Joaquin Valley, lecanium has two generations on grapes. UCIPM guidelines say one, but that needs to be revised. Second instar or “rubber stage” scale overwinter under bark on the vine trunk or canes. Just before budbreak they move to spurs or canes, where they typically settle in spots where the thin park has peeled away, providing access to the cambium layer. They are susceptible to the Lorsban sprays that are often applied at this time for mealybugs.
The scale develop quickly and begin turning into adults in mid April. This is a good time to monitor, because the adults, all females, are easily visible on old canes. They are almost black and about the size of a pencil eraser. The inside is filled with many hundreds of eggs.
New nymphs begin appearing in early May and settle on leaves. As they grow larger, the honeydew they produce becomes very noticeable, especially after it is covered with black sooty mold. Most have hatched and settled by mid June, when they can be controlled with Lannate or Provado. Provado, which has a long residual, probably will work earlier. In 2003 I noticed that blocks with Provado applied in mid-May, at thrips timing during bloom, had good scale control. The June timing is too early for mealybug, which is best treated in mid July.
The heaviest honeydew is produced in late June and early July by large nymphs, just before they turn into new adults. Adults do not produce honeydew. Most of the honeydew and sooty mold is deposited on lower leaves, where it looks bad but does no economic harm. Only fruit directly below heavily infested canes will have serious deposits. Sometimes individual scale will find their way to a bunch, where they are more damaging.
New crawlers appear on leaves in July and August, but they do not grow large enough before winter to deposit significant amounts of honeydew.
Several parasites attack lecanium. Coccophagus scutellans and C. lycmnia attack the small second and third instars. Parasitized scale appear translucent, with the white larvae visible inside. After the adult emerges, you can see the emergence hole with a hand lens. Metaphycus luteolus has a similar appearance in larger instars. When it attacks adult scale, it lays multiple eggs; so many emergence holes will be seen. If parasites are found in the March-April generation or early in the May-June generation, control is likely.
Predators also help control lecanium. The UCIPM website has nice pictures of several small lady beetles, a predaceous sap beetle, and lacewing larvae, all known to feed on soft scales. I have also observed the black hunter thrips, Leptothrips mali, eating the eggs or newly-hatched nymphs under the adult scale cover. In July and August, if lecanium is heavy, these thrips may clean up 80% or more of the population. If you find dead bodies instead of cast skins under old adults, thrips are the likely cause.
Ants like lecanium even more than they like mealybugs. If ants are heavy, they will deter natural enemies from controlling the scale.