Scarab Larvae in Gardening Containers

Scarab Larvae in Gardening Containers

I recently noticed that four of my container garden beds had surface soil being disrupted almost daily. Two of the beds contained onions, the third had leaf lettuce, and the last had rutabaga. Part of the regular garden (not in containers) also had evidence of soil disruption and 3/8 inch to over 1/2 inch holes at the surface.  All of the disruption noted was on the east side of the garden area.

One of the onion beds was really doing poorly, with the onions wilting and dying. I suspected scarab beetle larvae. I transplanted (10 Feb 12) the onions to another container; they all lacked roots or had roots badly damaged.

I removed (15 Feb 12) the dirt from the onion container (an inner drum from a washing machine) and recovered 55 large 1 1/2 to 2 inch long scarab beetle larvae (grubs). All the larvae were at, or near, the bottom of the container, about a foot below the surface of the dirt. None where found touching each other. The larvae were not identified but I suspect they were larvae of green june beetles, which I had previously seen in the garden area.

These larvae were highly disruptive in containers:
---Turning over surface soil almost nightly.
---Uprooting sprouting, or young plants.
---Eating on plant roots and either weakening plants or killing plants.

The 55 grubs were in a container 21 inches in diameter; the surface area was pi r squared or 3.1416 x 10.5 squared = 346.36 square inches = 2.40 square feet. So there were 22.92 grubs per square foot. This would appear to be a high concentration, probably because a batch, or more, of eggs were laid in the container and the larvae were thus trapped inside the container during their larval life.

An estimate of the number of grubs in the regular garden soil from a count of the holes at the surface gave 0-2 grubs per square foot of garden. In prior years the grubs were 0-1 per square foot in the garden area.

It is not known why grub production appears to have increased in 2011. I have raised part of my garden in containers for over 50 years, and never previously had such a problem with scarab larvae.

Adrian R. Lawler,  (C)  2012 --