Rose plant bugs - Ask Extension
Hi, For three years my knockout rose bush grew great, no issues. Then three years ago the leaves started getting decimated by SOMETHING. The plant sti...
Knowledgebase
Rose plant bugs #864515
Asked April 14, 2024, 5:54 PM EDT
Hi, For three years my knockout rose bush grew great, no issues. Then three years ago the leaves started getting decimated by SOMETHING. The plant still blooms, but it eats all the leaves completely. I have studied it and obsessed over it and I see no slug, bugs, tiny winged creatures, nothing. I have tried Neem oil, and all manner of Rose and Flower insect killer, plus slug pellets just to be safe. No other plants are affected.
It starts out like it is now, where some of the leaves start to get spots, then tiny holes, and then all of sudden POOF. Leaves gone. Please help!
Howard County Maryland
Expert Response
While Knock Out series rose varieties do have above-average resistance to a few of the common rose diseases, they are not immune to them all, nor are they resistant to typical rose insect pests. We suspect the main culprit chewing leaves is Roseslug Sawfly, a very common caterpillar-like insect named for its resemblance to a small (green) slug which feeds on the undersides of the foliage. While outbreaks can be an eyesore when damage is heavy, often the roses recover just fine without intervention. You can learn more about this insect and other rose issues in the linked pages.
The orange spots beginning to form on the pictured foliage look like rust, a fungal disease. You can look on the undersides of the leaves with such spots for confirmation of spores, though the rust might not be that far along in development yet (maybe next time it rains for a day or two). Unlike other rust types, this kind only infects roses, so it shouldn't pose a threat to nearby shrubs. Some fungicide sprays suppress rust (refer to their label details), but might need application several times a year to be effective. They can't cure existing infection, but since this tends to be a minor issue on roses, treatment isn't always necessary.
Neem oil is a contact insecticide (and has less ability to kill certain fungal spores on the leaf surface). It is one type of horticultural oil. This means it must directly come into contact with the body of the insect, so if an insect pest is not found (or the spray doesn't reach them, such as on the leaf undersides if the spray is only on the upper surface), the spray will not be effective. Dried neem residues on foliage does little, if anything, to deter or harm insects. (One reason it's a good choice to avoid harming pollinators and other beneficial insects.) It can be a useful low-toxicity spray when pests like Roseslug Sawfly are present, though it can admittedly be hard to use upside-down (or you'd need to flip foliage over with one gloved hand and spray with the other).
Products like a "rose and flower insect killer" might be using chemicals in the insecticide category called neonicotinoids, which are systemic (plant absorbed). Imidacloprid and dinotefuran are two commonly-used active ingredients in that group. If that is the case with the product you tried, be advised that they cannot be applied legally by the general public (anyone without a pesticide applicator certification) to outdoor landscapes/plants in Maryland, though someone hired to do so who has that certification can. Systemic insecticides risk contaminating the flower nectar/pollen, though, so their potential harm to pollinators is higher. In general, they should not be necessary to manage rose insect pests.
Miri
The orange spots beginning to form on the pictured foliage look like rust, a fungal disease. You can look on the undersides of the leaves with such spots for confirmation of spores, though the rust might not be that far along in development yet (maybe next time it rains for a day or two). Unlike other rust types, this kind only infects roses, so it shouldn't pose a threat to nearby shrubs. Some fungicide sprays suppress rust (refer to their label details), but might need application several times a year to be effective. They can't cure existing infection, but since this tends to be a minor issue on roses, treatment isn't always necessary.
Neem oil is a contact insecticide (and has less ability to kill certain fungal spores on the leaf surface). It is one type of horticultural oil. This means it must directly come into contact with the body of the insect, so if an insect pest is not found (or the spray doesn't reach them, such as on the leaf undersides if the spray is only on the upper surface), the spray will not be effective. Dried neem residues on foliage does little, if anything, to deter or harm insects. (One reason it's a good choice to avoid harming pollinators and other beneficial insects.) It can be a useful low-toxicity spray when pests like Roseslug Sawfly are present, though it can admittedly be hard to use upside-down (or you'd need to flip foliage over with one gloved hand and spray with the other).
Products like a "rose and flower insect killer" might be using chemicals in the insecticide category called neonicotinoids, which are systemic (plant absorbed). Imidacloprid and dinotefuran are two commonly-used active ingredients in that group. If that is the case with the product you tried, be advised that they cannot be applied legally by the general public (anyone without a pesticide applicator certification) to outdoor landscapes/plants in Maryland, though someone hired to do so who has that certification can. Systemic insecticides risk contaminating the flower nectar/pollen, though, so their potential harm to pollinators is higher. In general, they should not be necessary to manage rose insect pests.
Miri
