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Defend your plants against destructive asparagus beetle

My husband, Bill, and I were walking in the yard to the garden when we were shocked to find tiny gray caterpillars chowing down on our asparagus ferns.

Many of us don’t even look this time of year as we harvested the sprigs of this plant very early in the spring. So, we were lucky to have noticed the caterpillars. We love asparagus, and this bed is so special. What to do? Research the issue and find a solution.

Upon researching, I found these culprits are common asparagus beetle (Crioceris asparigi). These larvae are one-third inch, gray, with humpbacks, plump with black legs and emerge from tiny black eggs laid on the spears in early spring.

The adults emerge from debris, leaf litter, hollow stalks, etc., and begin to feed on the spears. They are blue / black, one-quarter inch long, with three yellow squares with red margins on the wing covers on the back. Then they lay eggs on the plants.

The larvae are destructive to the ferns as they steal nutrients from the roots and reduce the vigor of the roots for next year’s harvest. The adults are destructive to the spears, and the spears curl over when attacked.

Another culprit may have been responsible. The spotted asparagus beetle (Crioceris duodecimpunctata) — the 12-spotted asparagus beetle — is ¼ inch with six black spots on each wing cover and is orange / red colored.

The beetles lay greenish eggs in singles on fronds, and the larvae hatch and feed on the asparagus berries. These are not as destructive to the plants themselves. They winter as adults under debris, in soil, in stems, emerging in spring to feed and lay eggs.

These orange larvae eat up to 14 days and molt four times. They make cocoons in the soil to pupate. After five to 10 days (depending on weather temperature), they emerge as adults for a new season. They usually have two generations per summer.

Scout your plants in the afternoon. Pick them off and drop them into soapy water to reduce their impact. From eggs to adults are many beneficial insects to help with the battle.

These predators include a tiny parasitic wasp (Tetrastichus aspapagii) capable of reducing the beetle population by 70%. It attacks the eggs, laying its eggs that eat the larvae. Lady beetle larvae eat the eggs and the larvae as well.

If populations remain high into the fall, remove and destroy the ferns before winter. You can use insecticides as a last resort. If doing so, read and follow all label directions.

Hughes is an Ohio State University Extension Master Gardener Volunteer in Mahoning County.

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