Rose curculios—Merhynchites species
Holes in rose blossoms can be caused by chewing insects including Fuller rose beetle, hoplia beetle, and rose curculios. The rose curculio (Merhynchites bicolor) and western rose curculio (M. wickhami) feed on caneberries and roses.
Adult rose curculios are red and black, hard-bodied, snout beetles (weevils) about 1/4 inch long. They have an elongated head and mouthparts, distinct antennae, and long, black legs.
Eggs are oval, about 1/25 inch long, and occur hidden in the bracts and buds of flowers. They are white to yellowish, then darken before hatching.
The legless, plump larvae have distinct segments and are pale orange to whitish with a brown head. Larvae grow up to 1/4 inch long and occur hidden in flower buds and rose hips (fruit). Larvae of rose curculio resemble those of the rose midge, Dasineura rhodophaga. However, rose midge larvae grow to only 1/16 inch long and are rarely pests in California.
Adults emerge from soil beginning in the spring and seek young flower buds. The females chew small holes in the rose buds or hips to feed and lay eggs there. Adults can be present from about April through July.
Eggs hatch within a few days of being laid and the larvae feed hidden within bud tissues. Larvae develop in buds that remain on the plant and in buds that fall to the ground. Later in the season larvae also feed in rose hips.
Mature larvae emerge from buds or fruit on the plant, drop to the ground, and burrow into top soil. Overwintering is as inactive, mature larvae (prepupae) in a cell in soil. In late winter or early spring when soil warms, larvae pupate and adults soon emerge. Rose curculios have one generation per year.
Rose weevil adults chew and puncture feed on rose buds, calyxes, and peduncles (flower stems). Injured parts may drop prematurely or remain on the plant. If damaged buds remain viable enough to open, the petals show distinct rounded holes and ragged areas. Unless buds are inspected regularly, damage occurrence can be overlooked until the buds open and display the damaged petals. Adults also chew petals of opened blossoms.
If blossoms and flower buds are not plentiful, adult rose curculios may feed on the tips of new rose shoots, causing the death of terminals. Adults may also chew the stem of buds, causing the buds to wilt and die. Adult females lay their eggs in buds and the larvae feed on the petals and receptacle area. Larval infested buds commonly drop instead of maturing into flowers.
Where rose curculios have been a problem, consider avoiding and replacing cultivars that have been most susceptible to damage. Handpick adults and dispose of infested buds. Rose buds or entire rose bushes can be protected with netting; choose a material that allows sunlight and water to penetrate but excludes insects. Certain insecticides can also provide control.
Hand picking is very effective for controlling adults. Once rose buds appear, inspect them regularly for the presence of adult weevils and their damage. Because rose curculios prefer certain cultivars (commonly yellow and white blossoms), keep records of which plants the weevils are found on and focus subsequent monitoring on those. Regularly clip off and dispose of damaged buds, rose hips, and spent blossoms to reduce the abundance of weevil larvae that may infest them. If alternative hosts such as blackberries, boysenberries, and raspberries are present, also monitor their buds and handpick any weevils and clip and dispose of damaged buds and fruit.
Adult weevils commonly drop from plants when disturbed. Hold a wide-mouth container half full of soapy water beneath infested parts and shake or touch the plant where the weevils are feeding. This induces weevil adults to drop down into the soapy water and drown.
Parasitic (entomopathogenic) nematodes (e.g., Heterorhabditis or Steinernema spp.) can be drenched onto soil beneath infested plants to reduce larval abundance if soil is well drained (e.g., high in organic matter, sandy) and not compacted or high in clay content. Soil must be warm (at least 60°F) and moist (well irrigated) but not soggy before applying nematodes and for 2 weeks after application. If warranted, irrigate every 2 to 3 days after applying nematodes to soil. Late summer and fall may be the most effective application times. During hot weather, apply nematodes in the early morning or evening.
Applying a systemic insecticide (e.g., imidacloprid) to soil or trunks may control both adults and larvae. Foliar sprays are generally not recommended. If plants are especially susceptible to damage and handpicking is inadequate, a persistent, broad-spectrum insecticide can be applied, such as carbaryl mixed with horticultural oil.
See Pest Notes: Roses: Insect and Mite Pests and Beneficials from UC IPM and Rose Weevils (Curculios) from the Sacramento Rose Society for more information.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).