We have an infestation of our forsythia and black eyed susan plants and I am trying to determine the insect and figure out a treatment, before it gets too bad. I think it could be white flies. Would you please give me your opinion, if you can from the photos, and suggested treatments? We had this last year and thinking it was white flies, used neem oil, without much success. I'm wondering if they are in the soil and if we should replace the plants with something that white flies don't like. Or do that like all plants?
Centre County Pennsylvania
The best thing to do about no-see-ums is to remove the damaged foliage and wash the leaves with the sharpest spray of water from your hose. Get in every nook and cranny so you wash out the eggs as well as the adults and larvae. You may have to do this several times. Clean up all plant debris and bag it for the garbage. The trouble with insecticidal soap and horticultural oil on a big shrub is that you are bound to miss some of the insects. Soap and oil suffocate the ones it touches, but there is no residual effect.
Rudbeckia or black eyed susans are susceptible to a couple of leaf diseases. Again, hygiene is the best answer. Here are some tips from the University of Florida:
Aphids suck sap from the plants and coat the leaves with sticky honeydew.
Goldenglow sawfly may completely defoliate plants. The larvae are gray with dark stripes.
Four-lined plant bug causes round, brown, sunken spots on the leaves. The injury is often mistaken for a disease.
Downy mildew causes seedlings to wilt and die. On older plants the foliage is mottled light yellow.
Several leaf spots may be found but are not serious. Remove and destroy infected leaves.
Powdery mildew may cause a white, powdery growth on the leaves in late summer.
White smut causes light spots on the leaves. Destroy plant residues in the fall and get rid of infected plants as you notice them.