Skeletonization is a term used to describe how an insect chews away a layer of the soft leaf tissue leaving behind the leaf veins. The thin layer remaining turns clear or brown and the uneaten veins appear like a skeleton. This results in an unsightly plant. Unfortunately, in the case of roses, there are many pests that skeletonize its leaves.
The pest currently eating on roses are slugs. Not the typical garden slug that’s enjoying your hostas; rose slugs are the larvae (immature forms) of sawflies, non-stinging members of the wasp family. Three species of sawflies, the roseslug (Endelomyia aethiops), bristly roseslug (Cladius difformis), and curled rose sawfly (Allantus cinctus) are pests of roses. The larvae of some sawfly species are hairy and often mistaken for caterpillars. Others appear wet and shiny, superficially resembling slugs. The larvae reach about ½ to ¾ inch in length.
Generally, rose slugs feed at night. Depending on the species, young rose slugs will skeletonize the leaves but as some species of rose slugs get larger, they chew large holes or the entire leaf with only the midrib remaining. Damage typically first appears in mid-April and can continue through mid-June. Regular inspection of roses is important because feeding typically progresses quickly, and extensive leaf skeletonizing can occur if infestations are not noticed. In addition, with their coloring, they can be very difficult to spot on leaves.
Rose slugs can be controlled by handpicking. They can also be removed by spraying with water. Once dislodged, they cannot climb back onto the plant. Insecticidal soap and horticultural oil are also effective against rose slugs. Other insecticidal sprays that are labeled for homeowner use include acephate, bifenthrin, carbaryl, cyfluthrin, esfenvalerate, lambda cyhalothrin, permethrin or spinosad. Sprays should thoroughly cover both upper and lower leaf surfaces. Soil drenches or granular applications of dinotefuran or imidacloprid will control sawfly larvae. Damage caused by rose slugs is cosmetic and will not have serious long-term effects on rose plants. However, the damage can severely reduce the aesthetic value of the shrubs.
Another challenging skeletonizer is the Japanese Beetle. This pest usually appears in mid-June and in addition to leaves, it will also feed on the flowers.
For smaller plants, it may be practical simply to remove the beetles by hand. Volatile odors released from beetle-damaged leaves attract more beetles. By not allowing Japanese beetles to accumulate, plants will be less attractive to other beetles. One of the easiest ways to remove beetles from small plants is to shake them off early in the morning when the insects are sluggish. The beetles may be killed by shaking them into a bucket of soapy water. Highly valued plants such as roses can be protected by covering them with cheesecloth or other fine netting during peak beetle activity (usually late June to mid-July).
Foliar sprays of carbaryl and several pyrethorid products such as bifenthrin, cyfluthrin, deltamethrin, lambda cyhalothrin, imidacloprid plus a pyrethroid, and permethrin are labeled for control of adult Japanese beetles. Pyrethroids generally give 2-3 weeks protection of plant foliage while carbaryl gives 1-2 weeks protection. Foliage and flowers should be thoroughly treated. The application may need to be repeated to prevent re-infestation during the adult flight period. Imidacloprid can also be applied as a soil drench in late winter or spring to susceptible plants to greatly reduce foliar feeding by this pest next summer. Botanical alternatives including Neem or Pyola provide about 3-4 days deterrence of Japanese beetle feeding. Insecticidal soap, extracts of garlic, hot pepper, or orange peels, and companion planting, however, were found to be non-effective.
Regardless of which pest you are trying to control, avoid spraying the rose flowers as many conventional insecticides are highly toxic to bees. Always follow label directions and avoid spraying under windy conditions.