Tricounty News

Where did my radishes go?

Entire plantings of radishes in some home gardens are being severely damaged or, in some cases, completely disappearing. If the radishes are in the seedling stage, not only the above ground part, but even the root system is consumed.

The culprit is the red turnip beetle. The adult beetles are about 1/2 inch long and are brick red with three black stripes that run lengthwise down the back. They look very much like a red version of the Colorado Potato Beetle. The adult beetles emerge from the soil or fly into an area and feed for about two weeks. They feed primarily on plants from the mustard family such as turnips, radishes, cabbage, hoary alyssum, sweet alyssum and wild mustard. They have also been found feeding on plants from other plant families.

They are more prevalent in areas with sandy soils. The beetles may migrate into home gardens from nearby fields where plants from the mustard family are growing - from an alfalfa field infested with hoary alyssum, for example.

From mid June to mid-July, the adults burrow into the soil to rest for about a month. They re-emerge in late July or early August and feed, mate and lay eggs. The density of adult beetles at this time is generally not high enough to cause any significant damage. High populations of red turnip beetles are sporadic and 2009 appears to be a year of high populations.

As the eggs are laid in the soil or on debris on the soil, tilling the soil in fall or spring will bury the eggs so that the larvae that hatch from the eggs cannot climb out and reach host plants to feed. Plants from the mustard family are a primary food source for larvae as well as adults so eliminating weeds of the mustard family such as shepherd's purse, wild mustard, yellow rocket and flixweed may help reduce the number of adult beetles.

If a large population moves into a garden, chemical control may be necessary. Use a residual insecticide labeled for use on vegetables like carbaryl (Sevin) or permethrin (Eight). When using an insecticide on any vegetable, you may have to wait a specified number of days before you can harvest and eat the crop. Read the label to determine the number of days between insecticide applications and harvest for the specific vegetable you are treating. Use the insecticide only on plants listed on the label.