What’s on my plant?
For the last two years I have this bush in front of my house that produces new leaves and flowers in the spring, but as soon as the yellow flowers have bloomed it becomes infested and all the leaves get these black gritty dots, and then all that’s left of the leaf is the veins. There also appears to be little bugs on the flowers. The first time I tried spraying everything off with the hose, but that didn’t work, but it didn’t kill the plant. The next spring everything began to bloom beautifully again, but then I started to notice the black dots. I tried to spray it every couple days with a mixture of water, vinegar and dawn dish soap, and that seemed to help a little but by the middle of summer over 80% of the leaves were still destroyed. It hasn’t spread to any of the near by plants, but now that it has happened two years in a row I’m wondering if there is anything I can do early to treat it and keep the leaves healthy all summer. See a pictures of what’s on the flowers, and what the leaves first look like with the gritty dots. Sorry I don’t have a picture of the dead leaves with just their veins. Please let me know if I can provide additional information. Thanks in advance.
Lane County Oregon
Your description and photos could indicate sawfly damage. The black gritty dots on the leaf surface could be frass (larvae poop) falling from larvae on leaves above. Skeletonized leaves are typical of sawfly larvae feeding. The adult sawflies look like tiny wasps but do not feed on the leaves. They emerge from their cocoons in the soil in the spring when new leaves emerge and lay their eggs on the leaves. These hatch into larvae that eat the leaf material for 4-6 weeks and then drop to the soil to form pupae and start the cycle over again. There may be several generations.
To try to minimize leaf damage:
- Cultivate around the shrub in early spring and again in the fall to reduce the overwintering population;
- Use yellow sticky tape to detect adult sawflies in the spring and be prepared to start larvae control early;
- Look for larvae on leaves after adults spotted and/or black gritty spots appear on upper surface of leaves (this may happen shortly after shrubs start blooming);
- Physically pick off larvae;
- Use strong spray of water daily to wash larvae off upper & lower leaf surfaces (might consider using something like the bug blaster hose attachment);
- Try a soapy water (dish soap) spray daily (must contact the larvae directly).
If your shrub or adjacent plants are blooming, do not use any insecticides because they may be toxic to pollinators like bees and bumblebees.
AFTER blooming is complete you could also try Spinosad, an organic insecticide in Monterey Garden Insect Spray.
Do not consider use of any systemic insecticide except dinotefuran because other systemics may be active over several seasons and thereby pose a long term toxicity hazard to pollinators. Dinotefuran is only active for the season in which it is applied - it still may only be applied in accordance with label instructions AFTER all blooming is complete.
Thank you for this very thorough response!