Small (1 cm),greenish/yellow caterpillars are eating the leaves on my roses. Can you please tell me what they are and what I can do to control them?
Thank you for contacting Toronto Master Gardeners.
The pests you see on your roses look like caterpillars but are indeed rose slugs. They are the larvae of primitive wasps called sawflies. There are three species: the bristly rose slug (Cladius difformis), the European rose slug (Endelomyia aethiops), and the curled rose slug (Allantus cinctus). The caterpillars on your roses are likely one or more of these species.
Adult sawflies emerge in early spring and lay their eggs on the underside of host plant leaves. Larvae appear several weeks later, feed on soft leaf tissue for about a month, and then drop into the soil to pupate, and then emerge as sawflies. Depending on the species, there can be one to many generations each year.
Heavy defoliation gives plants a brown scorched appearance. In general, light to moderate infestations are cosmetic in nature and rarely harm the host plant. Heavier attacks, however, can weaken plants when leaf loss stresses them to the point of vulnerability to other insect and disease attacks.
Remove the infested leaves and destroy the larvae. A forceful spray of water out of a garden hose can also provide control by knocking off and killing many of the soft-bodied larvae. Be sure to aim the water at both upper and undersides of leaves. Continue checking plants throughout the growing season.
While it may be tempting to use insecticides, the Toronto Master Gardeners fully support and endorse the pesticide legislation of the Ontario provincial government and do not recommend chemical control. Insects such as parasitic wasps, insectivorous birds, small mammals, predaceous beetles all assist in keeping sawfly populations lower.
If the infestation is severely hampering enjoyment of your garden, lower risk insecticidal products such as horticultural oils and insecticidal soaps can help with severe infestations. If you choose to use these products, please be sure to follow the directions on the product’s label. Be aware that the use of these products may also kill the beneficial insects that help to control the sawflies.
Good luck.