"OYSTERSHELL SCALE ON KNOCKOUT ROSES. Joe Boggs noted that the widespread winter dieback of roses, including Knockout Roses, may have an up-side: cutting and removing winter damaged stems can also help to remove pests and pathogens. Joe reported that as he was pruning dead stems from his Knockout Roses, he came across a heavy OYSTERSHELL SCALE (Lepidosaphes ulmi) infestation on one of his rose plants. On a bright note, Joe observed numerous scale bodies with round parasitoid emergence holes in their coverings (tests); evidence that bio-allies were providing some help with killing the scales!

Oystershell scale is tiny with females measuring no more than 1/16" in length. Their elongate and slightly convex shape causes the scale to resemble miniature clam or oyster shells; thus their common name. Their size, dark gray to brown color, and slight banding cause the mature females to blend-in with the bark making low populations difficult to detect. As with all armored scales, the oystershell scale feeds by inserting their piercing-sucking mouthparts into plant stems to withdraw nutrients from non-vascular stem cells. Since they do not extract juices from vascular tissue, oystershell scales do not exude the sugary, sticky "honeydew" that is associated with "soft" scales. Thus, black sooty molds do not develop to help disclose an oystershell scale infestation.

The scale has a wide host range and may be found on over 130 host plants including trees as well as shrubs. Although this scale has only one generation per year in Ohio, undetected infestations can rapidly build within 1 - 2 years to levels that cause significant plant injury including branch dieback and even plant death. Unfortunately, this is one of the most difficult scale insects to control. Removing and destroying heavily infested stems is a recommended "first step" in an oystershell scale management program.

The scale spends the winter as eggs protected by the bodies of dead females with the eggs hatching in mid-to-late spring. While the resulting mobile first instar nymphs are susceptible to standard contact insecticides including soaps and oils, if the infestation is heavy, the bodies of dead females may protect some crawlers from contact with the insecticide. Systemic neonicotinoid insecticides are effective against soft scales, but some, including the imidacloprid (e.g. Merit), will not kill oystershell scale or other armored scales. Dinotefuran (e.g. Safari) is a highly soluble neonicotinoid insecticide that moves rapidly into plants and has been particularly effective against armored scales as well as soft scales. However, insecticide applications should be coupled with selective pruning to reduce the overall oystershell scale infestation."

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