Spider Mite Destroyer

Stethorus is the scientific name (genus) of those little black dots, which are predatory beetles. There are three species to choose from in NZ. Most of the time when we think of “Stethorus beetle” we have a shiny black lady-bug-sized creature in mind, a generalist predator that can help us out by eating aphids, spider mites, etc.

The particular species we have pictured above is tiny (1.5mm long as an adult), and was found in high numbers on the undersides of strawberry leaves at a tabletop strawberry farm near Christchurch. Many thanks to Kiri from Fruitfed for collecting the insects and dropping a sample off to us.

The far right picture is from under the microscope. You can see a newly emerged adult beetle and its leftover pupal case, both covered with tiny hairs. The larval stage of the beetle (not pictured) looks rather like a caterpillar or a maggot.

There was a lot going on under these strawberry leaves. Spider mites, yes, but rapidly being diminished by both predatory mites and the predatory beetles.

University of California has information about their species of Stethorus beetle on a web page (https://ipm.ucanr.edu/natural-enemies/spider-mite-destroyer/#gsc.tab=0). California will have a different species to NZ, but they will act similarly. Both the adults and the larvae eat an impressive 75 spider mites every day.

The insecticides used in strawberries can be hard on predatory beetles. Have a look at the chart below, where green is safest for the beneficial insect and red is the most harmful. We can look at the Ladybird column to get an indication of the effects of various insecticides on the stethorus beetles. Carbaryl (Sevin Flo) is a broad spectrum insecticide which can be used for thrips (1 day WHP), but it will take out the beetles, as would fipronil or methomyl (Lannate). Pyrethrins are somewhat harmful–broad spectrum but not as long lasting. Spinosad (Entrust, Success) or spinetoram (Sparta) are two that are reasonably safe to the beetles.

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