Cranberry Girdler Found in Jefferson County Lawns

Posted by: Mary Small and Curtis Utley, Jefferson County Plant Diagnostic Clinic/CSU Extension

Yes, you read that right! Cranberry girdler larvae are infesting lawns.  Customers look at you strangely when you tell them this.  “Well, where did it come from? We don’t have any cranberries around here!”  Admittedly, it sounded so strange that we had to do more investigation on the origin of the insect’s name.  (We’ll get to the pest part momentarily!) 

Turns out, the larvae of this insect feed on cranberry runners at or below the soil surface, removing bark and/or conductive tissue. So their feeding actually girdles the stems and results in patches of dead cranberry plants.

Cranberry girdler larvae

In addition to cranberries, the insects have an affinity for grasses, true firs and Douglas fir. They are a member of the sod webworm group of insects. In fact, another name for this pest is the subterranean webworm, because of where it feeds. (At least this is beginning to make more sense!)
Now for the pest part. In less than a couple weeks’ time, Jefferson County’s horticulture staff found several cases of cranberry girdler damaging lawns. Diagnosis was really confusing, since the damage is similar to drought stress and can be exacerbated by drought stress.

But then, to add to the confusion, we’ve had many inches of rain and cool temperatures recently. Lawns turned around pretty quickly from summer’s heat and drought stresses.   In fact, most area lawns look pretty darn good.

While examining the lawns, horticulture staff noticed that the sod was hardly attached to the soil underneath.  We began searching for white grubs since the symptoms sure fit!  Imagine our surprise to discover ¾”, grey caterpillars crawling around in the thatch and soil.  Their feeding on turfgrass crowns and roots created the brown patches of dead grass that led homeowners to us.

Management is more difficult than other sod webworms that are surface-feeding. And cranberry girdlers aren’t very cooperative when using irritants for sampling.(It’s not that they don’t irritate, they’re usually too far down for the irritant to reach them.)

Birds can provide some natural control, but not in heavy infestations. Insect parasitic nematodes are one means of control, but must be applied early or late in the day to avoid sunlight.  They also must be watered in thoroughly to move them into and below the thatch layer where the food is.

Cranberry girdler infestations can be treated with Spinosad and certain formulations of Bt, but they are most effective when  larvae are young.  Scouting for larvae in July and August, when eggs are laid and young larvae are developing, will help with application timing.

Dose makes the poison, for this reason the late instar caterpillars that are large and deep in the ground are better protected from an insecticide application; because of their size the caterpillars must come in contact with a larger dose of insecticide to be killed. So instead of trying to control the caterpillars now  consider scouting for small caterpillars next year and treat with an insecticide if you find them. Insecticide applications should be followed with a water application to help move products into the feeding zone.

Damaged turf areas can be over-seeded *now or next spring to get your lawn looking good again. Over seeding operations are most successful if you follow these simple steps.

1.       Core aerate the lawn 3-4 different directions – just make Swiss cheese out of it.

2.       Apply 3 lbs. of grass seed per 1000 ft.²

3.       Apply lawn fertilizer at the recommended label rate.

4.       Water once per day for one month. You don’t have to stop mowing.

*Over-seeding after October 1 can potentially fail if the new seedlings freeze before being mowed a few times.