An Unconventional Find: Walnut Scale In Almonds

An Unconventional Find: Walnut Scale In Almonds

Wes AsaiA couple years ago, a fellow consultant who works in my same area had reported finding significant levels of walnut scale in almonds.

At first I thought, “walnut scale in almonds?”

However, after closer examination, that was exactly what it was. Not only that, but it was interesting that the same level of walnut scale populations that you see in walnuts that don’t seem to affect the tree, were causing significant limb dieback in the almonds.

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To make things even more interesting, it only affects the Monterey variety. Nonpareils and Carmels planted in the same blocks don’t have a single scale on them, while the Monterey is loaded. This is consistent with what I have since seen in many other Monterey blocks.

Apparently the walnut scale has a specific affinity for only the Monterey variety. I have seen it in Monterey blocks planted with Nonpareil, Carmel, Aldrich, Fritz, and Butte, and only the Monterey is exclusively affected.

Does Not Follow A Pattern
The distribution of walnut scale in a particular block does not follow a pattern. You can find several trees in a row severely affected, then a few consecutive trees with no scale at all, followed by more severely affected trees. In some situations you can find an isolated major infested area with dying limbs in the center of a block with no affected trees around the perimeter of the block. This makes me suspect that birds may be a factor in the transmission of scale from field to field.

Walnut scale has only two generations per year compared to the three to four for San Jose scale. Even with fewer generations, the increased sensitivity of almonds to walnut scale makes control decisions different.

Both scales have natural enemies. The twice-stabbed lady beetle (Chilocorus orbus) and another beetle (Cybocephalus californicus) are predators. There are also parasitic wasps (Aphytis and Encarsia) that help to control walnut scale. Close examination of the scale caps will reveal the presence of these biological controllers, but in many cases they are insufficient to reduce populations to below damaging thresholds.

The distribution of walnut scale in a particular block does not follow a pattern. You can find several trees in a row severely affected, then a few consecutive trees with no scale at all. (Photo credit: Wes Asai)

The distribution of walnut scale in a particular block does not follow a pattern. You can find several trees in a row severely affected, then a few consecutive trees with no scale at all. (Photo credit: Wes Asai)

When this occurs, chemical intervention is required to prevent further loss of fruiting wood. Options include oils for low populations, or organophosphates or insect growth regulators such as pyriproxyfen for higher populations.

If there is a positive side to this problem, it is that you only need to run a half side of the spray rig and only treat the Monterey rows.

In most cases this means only treating about one-quarter of the acreage since Monterey is commonly planted in a block with 50% Nonpareil and one other pollinizer