Black bug egg infestation on Echinacea and Black-Eyed Susan
I am seeing black spots that I suspect are bug eggs on Black-eyed Susans and Echinacea growing in full sun on a dry sloping yard. The leaves on these plants also have chewed edges and holes, and are looking pale and dry and are clearly not thriving. The black bugs have also spread to nearby Asters and to Hosta, to a lesser extent. This is the second year this garden is impacted by this infestation. I am located close to the Magothy River, and within 10 miles of the Chesapeake Bay. Can you tell me what the bugs are, and advise how to treat them? I have sprayed with an organic bug killer, but have not seen an improvement.
Anne Arundel County Maryland
They may not be insect eggs per se but they could be residue (like frass - insect poop) from leaf-chewing insects. Do the spots appear on the undersides of the leaves as well? We cannot identify the insect without a picture of the culprit, and there are a number of generalist plant-feeders which come and go and aren't always present on the plant when not actively feeding. Some insects also only come out at night to feed, so you can inspect them after dark to see if something appears then.
The damage visible is not severe and shouldn't require treatment if all plants have this level of spotting or less; other "pale and dry" plant symptoms are probably tied to something else like moisture availability (too much or too little) or feeding by a different insect. Pictures of this damage would be helpful in case it's more diagnostic of a particular pest. Insecticide spraying, even organic, is not recommended unless the pest can be identified. Many sprays are contact-killing, meaning the pests have to be covered in the spray in order for it to be effective; the residue left behind isn't always enough to affect them if they come back later. The spray can also negatively impact beneficial insects, so it's best to hold off on its use until there's a better understanding of what is going on - that will determine if a spray is needed at all, when to apply it, and which type to use.
Given the array of plants this is appearing on, are they all underneath a tree or other overhanging structure where this residue may be falling from? (We realize you said they're in full sun but they could still get enough light if the tree branches were high enough.)