Marigold Bloom Beetles

Description

• The immature Japanese beetles, or grubs, are white with brown heads and three pairs of legs. They overwinter in the soil and emerge in spring when the weather warms. Grubs typically feed on turf grass or ornamental plant roots. The adults are a metallic green or bronze color, and feed in the daytime on foliage and flowers.

Damage

• Japanese beetles feed on marigolds in early and midsummer. They prefer sunny, dry days. If you notice skeletonized or brown leaves, especially toward the top of the marigold, and holes in the flowers, suspect the Japanese beetle. If the marigolds are growing near a lawn, check the lawn for Japanese beetle grubs. Look for brown patches, resembling drought damage. Peel back the turf. The grubs should be close to the surface of the soil.

Control

• Small Japanese beetle infestations can be controlled by handpicking the pests off the marigold plants and dropping them into a bucket of soapy water. Apply Heterorhabditis bacteriophora, beneficial nematodes, to the soil when you notice the grubs. The nematodes are typically packaged in a powder that you can easily mix with water in a watering can or sprayer. Water the lawn with 1/4 inch of water before and after the nematode application. Pre-mixed pyrethrum or pyrethrin insecticide products kill the adult Japanese beetle as long as it is sprayed directly on the pests. Since pyrethrum is made from several species of plants in the Chrysanthemum genus, it is considered natural. Honey bees, however, are sensitive to it, so use caution to avoid killing bees when spraying the marigold.

Prevention

• To prevent a Japanese beetle attack on next year’s marigolds, kill the grubs in the soil. Use beneficial nematodes, which are available online from large garden retailers and apply them in spring. Although this won’t prevent fly-ins, it will prevent existing Japanese beetle grub populations from pupating.