Controlling Rose Weevils

Rose weevils prefer yellow and white roses.

Insidious snout-nosed creatures wreak havoc on roses. The Marin Rose Society calls weevils "...the real bad guys of the rose garden." Their attacks leave tattered evidence of their feeding frenzies by turning flowers into bullet-riddled targets. Monitoring roses for early-season damage is the first step in controlling rose weevils. At the first sign of these pests, an integrated pest management routine eradicates them.

Rose Weevils

Rose weevils, Merhynchites bicolor, are tiny beetles with prominent black snouts. Also called rose curculios, these weevils damage flower buds and prevent many from opening. The buds that do open are drilled full of holes by the weevils’ snouts. Other parts of the plant are also drilled with holes, including the calyx, peduncle and hips. Understanding different stages in the life cycle of rose weevils and their stage-specific plant targets help gardeners learn how to control their populations.

Life Cycle

Adult rose weevils emerge from the soil in spring. They climb onto nearby rose bushes and begin drilling holes in flowers and hips to feed and to excavate tunnels where they lay eggs. Within a few days, small white larvae hatch from the eggs and begin eating tender flower buds and hips. Mature larvae drop to the ground and burrow into the soil where they pupate and hibernate for the winter before emerging the following spring.

Damage

A serious rose weevil infestation can prevent almost all blooms on a rosebush, according to the Marin Rose Society. In the absence of flower buds, weevils eat tender new-growth shoots, which prevents bud formation. They also eat bud-bearing stems, causing new buds to wilt prematurely and die. After buds form, weevils puncture them, disabling many from opening. Gardeners are unable to enjoy flowers that bloom from affected buds because they are pockmarked and ragged.

Control

An integrated pest management approach to controlling rose weevils suppresses populations and minimizes damage. The Sacramento Rose Society offers an easy way to remove adult weevils without chemical interference. Filling a wide-mouth container halfway with soapy water, tap plants where weevils are present. They will drop easily into the water because they feign death when disturbed. Larvae can be controlled by spraying them with neem oil.

Synthetic chemical controls are used when damage progresses beyond an acceptable threshold that cannot be managed by other means. The Connecticut Agricultural Experiment Station recommends applying imidacloprid early in the season as a soil drench around infected rosebushes. This kills overwintering pupae and developing larvae and provides season-long systemic control.

References

• Marin Rose Society: Garden Bad Guys – Rose Curculios
• Sacramento Rose Society: Rose Weevils (Curculios)
• The Connecticut Agricultural Experimental Station: Rose Plant Health Problems
• University of California Cooperative Extension: Merhynchites bicolor
• Plants Diagnostic Database – Missoula County Extension Office: Rose Curculio
• Colorado State University Extension: Rose Insect Pests