Black-eyed Susans (Rudbeckia) are tough, drought-tolerant perennials, but they still attract a handful of insect pests that can disfigure foliage, stunt growth, or even transmit plant diseases. The most frequent culprits are aphids, spider mites, Japanese beetles, sawfly larvae, and aster leafhoppers. Most of these are manageable without reaching for heavy-duty pesticides, especially if you catch them early and understand what each pest actually does to the plant.
Aphids
Aphids are probably the single most common insect you will find on black-eyed Susans. They are tiny, soft-bodied, pear-shaped bugs that cluster on new growth, flower buds, and the undersides of leaves. Colors range from green to yellow to black depending on the species. You often notice the damage before you notice the insects themselves: curled or yellowed leaves, sticky honeydew residue on the foliage, and sometimes a sooty black mold growing on top of that honeydew.
Aphids feed by piercing the plant’s tissue and sucking out sap. A small colony rarely does serious harm to an established Rudbeckia clump, but a large infestation can weaken young plants, distort new growth, and make the whole stand look sickly. They reproduce fast in warm weather, so a handful can become hundreds in a week or two.
The simplest fix is a hard spray of water from your garden hose, directed at the undersides of the leaves where aphids congregate. This physically knocks them off, and most cannot climb back. Repeat every few days until you stop seeing new colonies. For heavier infestations, insecticidal soap sprayed directly onto the aphids works well because it disrupts their soft outer coating. You need good coverage since the soap only works on contact. Neem oil is another option, though it has a stronger smell and can leave an oily residue on petals. One thing to keep in mind: aphids are a primary food source for ladybugs, lacewings, and parasitic wasps, so if you see those predators already at work, giving them a few days before intervening often solves the problem on its own.
Spider Mites
Spider mites are not true insects but tiny arachnids, and they are the pest most likely to explode during a hot, dry summer. The twospotted spider mite is the species you will usually encounter. These mites are nearly invisible to the naked eye, so you typically notice the damage first: a fine stippling of tiny yellow or bronze dots on the upper surface of the leaves, as though someone tapped the leaf with a pin thousands of times. In heavy infestations, you may see fine webbing stretched between leaves and stems.
Twospotted spider mites generally reach damaging levels in late July or early August when conditions favor their growth, but they can start building populations as early as June during years when temperatures climb above 85°F, humidity drops below 90 percent, and soil moisture is low.1Semantic Scholar. You “Mite” Want to Scout Drought-Stressed Crops That combination of heat and drought stress is exactly the kind of summer many Rudbeckia growers experience, which is why spider mites and black-eyed Susans have such a reliable collision course.
Because mites thrive in dry conditions, keeping the area around your plants adequately watered and occasionally hosing down the foliage disrupts their preferred environment. A blast of water also physically dislodges them. Insecticidal soap and horticultural oil both work on mites when applied thoroughly to the undersides of leaves, where the mites feed and lay eggs. Avoid broad-spectrum insecticides for mites because those products kill the predatory mites and ladybugs that naturally keep spider mites in check, often making the problem worse a few weeks later.
Japanese Beetles
Japanese beetles are hard to miss. They are about half an inch long with a metallic green head and copper-brown wing covers, and they tend to feed in groups. On black-eyed Susans, they chew the petals and skeletonize the leaves, eating the tissue between the veins and leaving behind a lacy, translucent shell. A few beetles can strip a flower head in a day, so the damage looks worse than it usually is for the overall health of the plant.
Adult Japanese beetles are active for roughly four to six weeks in early to midsummer. The adults lay eggs in lawns, where the larvae (white grubs) feed on grass roots through fall and the following spring before pupating and emerging as new adults. This means your lawn and your flower bed are connected pieces of the same pest cycle.
Hand-picking is genuinely one of the most effective controls for Japanese beetles on ornamental plants. They are large, slow, and easy to spot. Drop them into a bucket of soapy water in the morning when they are sluggish. Japanese beetle traps, on the other hand, are a mixed bag. The traps use pheromone and floral lures that attract beetles from a wide area, and research has consistently shown they pull in far more beetles than they capture, often increasing the damage to nearby plants. If you use traps at all, place them well away from the garden beds you are trying to protect.
For the grub stage, milky spore disease and beneficial nematodes applied to the lawn in late summer or early fall can reduce next year’s adult population. These biological controls target the larvae in the soil without harming other organisms. They take time to build up, so expect a season or two before you notice a meaningful drop in beetle numbers.
Sawfly Larvae
Sawflies are less well known than aphids or beetles, but several species feed on Rudbeckia and can defoliate plants quickly. The larvae look like small caterpillars, usually pale green or gray-green with darker heads, and they line up along the edges of leaves, eating inward. People sometimes mistake them for butterfly or moth caterpillars, which matters because Bt (Bacillus thuringiensis), the popular organic caterpillar treatment, does not work on sawfly larvae. Sawflies are actually related to wasps, not moths, so Bt is useless against them.
Handpicking works for small infestations. For larger numbers, spinosad-based sprays are effective against sawfly larvae and are approved for organic gardening. Neem oil also has some activity against them. Since sawfly adults lay eggs on the underside of leaves, inspecting your plants in late spring and early summer and removing any leaves with rows of tiny eggs on them can head off trouble before the larvae appear.
Aster Leafhoppers and the Aster Yellows Problem
Aster leafhoppers are small, wedge-shaped, pale green insects that hop away when you brush against the foliage. They feed on sap, and their direct feeding damage is usually minor. The real threat is what they carry: a phytoplasma organism that causes aster yellows, a disease that can dramatically deform black-eyed Susans. Infected plants develop bizarre symptoms, including green, leaf-like structures where the petals should be, abnormally bushy growth, and yellowed or stunted foliage. Once a plant is infected with aster yellows, there is no cure. The plant will not recover, and leaving it in the garden allows leafhoppers to pick up the pathogen and spread it to healthy neighbors.
Aster yellows is not a fringe concern. In parts of western Canada, the prevalence of aster yellows in susceptible crops jumped from around 5 percent in 2022 to 65 percent in 2023 in Saskatchewan, and from 4 percent to 21 percent in Manitoba over the same period.2Oxford Academic. Aster yellows integrated management in Western Canada and the United States Upper Midwest Those figures come from crop surveys rather than ornamental garden counts, but aster leafhoppers do not distinguish between a canola field and your Rudbeckia patch. A year with high leafhopper migration can mean high infection rates in home gardens too.
The most important management step is removing and destroying any plant showing aster yellows symptoms. Do not compost it. Pull it out, bag it, and trash it. Controlling leafhoppers with contact insecticides is possible but difficult because the bugs are mobile and constantly arriving from surrounding areas. Row covers can protect vegetable crops, but they are impractical on ornamental flowers you want to enjoy. Keeping weeds around the garden under control helps somewhat, since many common weeds host both leafhoppers and the phytoplasma.
Thrips
Thrips are slender, barely visible insects that rasp the surface of petals and leaves, leaving behind silvery streaks or stippled patches. On black-eyed Susans, they tend to concentrate inside the flower heads, where they feed on pollen and petal tissue. The damage is mostly cosmetic, showing up as streaked or discolored petals, but a heavy thrips population can reduce the vibrancy of the blooms noticeably.
Blue or yellow sticky traps placed near the plants can help you monitor thrips numbers. Insecticidal soap and spinosad are the two most effective low-toxicity controls. Because thrips hide deep inside the flower, thorough spray coverage is important. Removing spent flowers promptly also reduces the sheltered spots where thrips breed.
Low-Toxicity Management That Works Across Multiple Pests
Several cultural practices reduce pest pressure on black-eyed Susans regardless of which specific bug is causing problems. Good air circulation, achieved by spacing plants properly and dividing overgrown clumps every few years, discourages both spider mites and fungal diseases that can weaken plants and make them more susceptible to insect attack. Adequate watering during dry spells keeps plants vigorous and makes conditions less hospitable for mites. Cleaning up dead foliage in late fall removes overwintering sites for several pests.
Insecticidal soap and horticultural oil are the two most broadly useful spray treatments. Soap works on soft-bodied insects like aphids, mites, and young sawfly larvae. Horticultural oil smothers eggs and small insects on contact. Both break down quickly in the environment and have minimal impact on beneficial insects, provided you spray when pollinators are not actively visiting the flowers (early morning or evening). Neem oil has both contact and mild systemic activity, meaning the plant absorbs some of it and pests that feed on treated tissue are affected. It works on aphids, mites, whiteflies, and some beetle larvae, but it takes longer to show results than soap or oil and can leave an unpleasant smell.
Spinosad, derived from a soil bacterium, is effective against thrips, sawfly larvae, and some caterpillars. It is approved for organic use but is toxic to bees when wet, so the same timing advice applies: spray when pollinators are not active. Once dry, the risk to bees drops substantially.
When Black-Eyed Susans Are Part of the Solution
Here is an angle most gardeners do not consider: black-eyed Susans are not only a target for pests but also a tool for managing them. Research on organic greenhouse production found that growing Rudbeckia hirta alongside tomato plants resulted in significantly higher densities of Orius pirate bugs, a tiny predatory insect that feeds on aphids, thrips, and whiteflies.3Oxford Academic. Arthropod Pest Management in Organic Vegetable Greenhouses When prey populations on the tomato plants dropped, the pirate bugs sustained themselves on the Rudbeckia and then migrated back to the crop plants when pests returned.3Oxford Academic. Arthropod Pest Management in Organic Vegetable Greenhouses In other words, the black-eyed Susans acted as a living reservoir for beneficial insects.
This principle applies in home gardens too. Black-eyed Susans attract a wide range of beneficial insects, including parasitic wasps that attack aphids and caterpillars, hoverflies whose larvae eat aphids, and various predatory beetles. Planting Rudbeckia near vegetable beds or other ornamentals vulnerable to aphids and thrips can increase the background population of these natural enemies in your garden. The pests you see on your Rudbeckia may be feeding the predators that protect the rest of your garden, which is worth considering before you spray everything in sight.
Pests That Look Alarming but Are Not Actually a Problem
Not every insect on your black-eyed Susans is there to cause trouble. Soldier beetles, sometimes called leatherwings, are long, narrow beetles often seen crawling over Rudbeckia flower heads in late summer. They are predators and pollinators, not pests. Ambush bugs, which are small, chunky, and well camouflaged against the yellow petals, sit motionless on flower heads waiting to grab visiting insects. They may occasionally catch a pollinator, but they also take out plenty of pest species. Spiders building webs among the stems are similarly helpful.
Even some of the pests themselves are less concerning than they look. A handful of aphids on a vigorous, established black-eyed Susan clump will not meaningfully affect the plant. A few Japanese beetles chewing petals on one flower head will not threaten the stand. Black-eyed Susans are resilient plants that tolerate moderate insect feeding without much fuss. The threshold for intervention is higher than many gardeners assume, and the plants often outgrow minor damage on their own, particularly when they are well established and growing in decent soil with adequate sunlight. Save the spraying for situations where the pest population is clearly escalating or the plant is showing obvious stress, and let the minor nibbling go.