Bug Off! A Guide to Dahlia Pests & Prevention
Keeping Your Dahlias Looking Fabulous
There is no denying it: bugs LOVE dahlias. One of the most frequent frustrations I hear from home gardeners is:
“My dahlias are getting attacked by bugs!”
or
“I don’t understand why my dahlia flowers always look so bad!”
And I get it. You spend months nurturing these plants, waiting for those first gorgeous blooms…and then a bug shows up and decides your dahlia is an all-you-can-eat buffet.
Dahlia pests can be especially frustrating because there are several different insects that can damage your plants—and the damage can look very different depending on the pest.
Dahlias are also divas. They can suffer from overwatering, underwatering, nutritional deficiencies, fungal problems and viruses, so not every ugly flower is the result of a bug.
So how do you know when bugs are actually the problem?
How to Identify Dahlia Pest Damage
Here are some of the most common signs of dahlia pest damage I look for:
🐞 Petals being eaten
Chunks missing from your petals? You’re probably dealing with a chewing insect. Japanese beetles, earwigs, caterpillars and other beetles can all turn your beautiful blooms into a snack.
🪴 Distorted flowers
Twisted, deformed or just plain weird-looking flowers can sometimes be the result of sucking insects feeding on the developing bud. Leafhoppers and other small insects can damage a flower before it even opens.
Of course, distorted blooms can have other causes, too, so don’t immediately blame the bugs!
🍃 Holes or damage on leaves
Ragged edges and holes usually point toward chewing insects like slugs, caterpillars, beetles or earwigs.
Tiny speckles or a stippled appearance—especially during hot weather—can be a sign of spider mites on dahlias. Flip the leaf over. If you see tiny bugs or fine webbing, you’ve probably found your culprit.
🌱 Stem damage
Sucking insects can cause weakened or distorted growth, while stem-boring insects can cause holes, wilting or even sudden stem collapse. See below on how to deal with stem borers which can destroy a whole plant very quickly!
🌸 Brown or damaged petals
This one can be tricky. Thrips on dahlias can cause browning, scarring and distorted petals, but environmental stress and other problems can cause similar symptoms.
How to Prevent Dahlia Pests: Start With a Healthy Plant!
This is my biggest piece of advice when it comes to dahlia pest control: Don’t start with the bugs. Start with the plant.
You can’t make your garden completely bug-proof. And honestly, you don’t need to! The goal is to grow strong, healthy plants that can withstand some pest pressure without falling apart.
1. Give them healthy soil
Healthy soil supports healthy roots, and healthy roots support healthy plants. When your soil is providing the nutrients, water and biological activity your plants need, your dahlias are better equipped to handle stress and recover from minor pest damage.
Edit your soil with proper amendments if necessary including potting soil, peat moss or coco coir (for drainage), fertilizer and/or worm castings (for nutrients- see more below) and lime, ash or sulfer (to adjust pH).
Healthy plants aren’t pest-proof, but they are much better prepared for battle.
2. Don’t drown your dahlias
Dahlias like consistent moisture, but they do not like wet feet. Poorly draining soil can stress the roots, reduce oxygen around the root zone and create an environment where root problems thrive. And a stressed plant is much more vulnerable to everything else.
SIDE NOTE: Overly wet soil can actually cause your tubers to rot and die. I had to learn this lesson the hard way in 2023 when while (trying to save time) I dug 2 long ditches, placed my tubers accordingly and backfilled. Then it rained and rained and rained all the following week. The ground sank resulting in trenches that allowed water to pool and sit. At the time, I did not realize how susceptible un-rooted tubers were to rot so I let it be.
I was incredibly surprised (and disappointed) when only about 40 of the 120 or so tubers I had planted via that method sprouted. It was devastating. BUT I LEARNED MY LESSON!
NOW, I always plant my pre-rooted dahlias (more on that in another post) in a raised bed AND amend my dahlia planting hole with a large amount of potting soil. This helps to prevent and pooling water and make up for the naturally occurring heavy clay soil in my garden plot.
3. Give them room!
I highly encourage you to plant your dahlias at least 18 inches apart. Good airflow is incredibly important.
Dahlias get BIG, and cramming them together creates a dense canopy where moisture hangs around and air doesn’t move. This makes them more susceptible to fungal diseases and allows pests to more easily hide and/or jump from one plant to another.
Give those divas some personal space!
4. Feed the soil when you plant
When I plant my dahlias, I add a handful of worm castings and dry molasses to each planting hole. I don’t measure because, well…I’m planting a LOT of dahlias. 😂 For the purposes of an actual recipe, let’s call it about ½ cup of each per plant.
Worm castings provide organic matter and nutrients, while molasses provides carbohydrates that support microbial activity in the soil. Healthy soil biology can help make nutrients more available to plants, which is important for overall plant health and BRIX (a measure of soluble solids in plant sap).
Dahlia Pest Control: What to Do Once They're Growing…
Once your dahlias are established and actively growing, there are a few additional things you can do to keep them healthy—and to deal with dahlia pests when they actually show up.
1. Clean Up Your Plants
As the season progresses, I strip the lower leaves from my dahlia plants and remove debris from around the base. This helps improve airflow, removes some hiding places for pests and can help reduce spider mite populations if you remove heavily infested leaves.
And don’t forget the cleanup!
Dead leaves and debris around the base of your plants can provide a cozy little hiding place for pests, especially slugs. We don’t need to make their lives TOO comfortable.
2. Give them a nutritional boost:
I use a foliar treatment of liquid molasses + fish/seaweed fertilizer every other week. I like this as part of an overall plant-health strategy, especially during periods of rapid growth or high pest pressure.
The fish/seaweed fertilizer provides nutrients and trace minerals to support vigorous growth, while the molasses provides carbohydrates and other compounds that can interact with the plant’s surface and microbial community.
But there’s another reason I like the molasses component: BRIX.
BRIX is a measurement of the soluble solids in plant sap, and higher BRIX is often associated with healthier, better-nourished plants. There is a long-standing theory among some growers that insects are more attracted to plants with lower soluble-solids levels—and that increasing BRIX can make plants less appealing to certain pests.
The idea behind a foliar molasses spray is that you’re essentially giving the plant a little boost while also coating the foliage with a sugary solution. I think of it as making the buffet a little less appealing to the bugs.
My recipe:
- 2 tablespoons unsulfured liquid molasses
- 2 tablespoons liquid fish/seaweed fertilizer
- 1 gallon warm water
Mix the molasses into the warm water first and stir thoroughly.
When to spray: Early morning or late evening. Avoid hot, direct sunlight.
Where to spray: Both the tops and undersides of the leaves. And avoid the open blooms—you don’t want to stain those pretty petals.
3. Treat the specific pest
Once you identify what’s actually causing the damage, you can use a targeted treatment rather than giving up or raging by using a broad spectrum pesticide which will kill good bugs as well as bad.
A few examples:
🕷️ Spider mites: Spider mites on dahlias are especially problematic during hot, dry weather. If they get out of control, I use Mite Massacre, a miticide that targets spider mites. I treat this as more of a last resort and follow the product label carefully. The product I use needs to be applied during low-light conditions, so I spray just before sundown.
QUICK TIP- Strip those bottom leaves first as recommended above. The less leaves you have to spray the quicker it goes!
🐌 Slugs & earwigs: Slugs and earwigs are common dahlia pests who love to nibble on leaves. Slugs are worst when the dahlia plant is young. They can devour the leaves off an entire plant in one night as seen below.
Sluggo Plus is one option I use. It’s labeled for both aforementioned dahlia pests, which makes it particularly handy when you’re dealing with both at once.
I have heard of people using copper rings around the base of dahlias to create a chemical-free barrier that deteres pests like slugs (and snails). When a soft-bodied dahlia pest touches copper with its slimy body, it creates a mild, unpleasant sensation or reaction that makes the dahlia pest turn around.
🐛 Vine borers: If you find a dahlia stem that looks brown and mushy or an entire plant that suddenly wilts or collapses (not due to dehydration), you are most likely dealing with a vine borer. You can confirm this if you see a small hole in the stem with frass (insect waste) around it. This is the point of entry.
Don’t give up on the plant just yet! If you can find the borer inside the stem, you have a couple of options. You can make a small surgical incision in the stem and physically remove the borer.
Or, if you can locate the entry hole, you can use a syringe to inject BT (Bacillus thuringiensis) directly into the stem.
As a last ditch effort you can cut the entire plant down to the level below where the vine borer may be living, if effect demolishing the little bugger’s home. This is, for obvious reasons, easier to do on young plants than mature ones. Yes, you sacrifice a lot, but it’s better than the whole plant dying. The dahlia WILL most likely grow back.
I’ve had success with all these methods. Just be sure to seal or support the damaged stem afterward if necessary—the goal is to get the borer out without sacrificing the entire plant!
The key here is identification. Different pests require different approaches, so don’t spray something just because you saw a bug on your plant. Figure out what you’re dealing with first, then choose a treatment that’s appropriate for that particular pest.
And remember: seeing a bug doesn’t necessarily mean you have a pest problem. A few bugs here and there are part of having a garden. It’s when their numbers or damage start affecting the health or appearance of your plants that it’s time to intervene.
4. My #1 Dahlia Pest-Fighting Trick: When in doubt, bag it!
When I have a dahlia that I really, really want to look perfect, I put a 6″ x 9″ organza bag over the developing bud.
Think of it as a deer fence for dahlia loving bugs. It creates a physical barrier that keeps many of the larger chewing and sucking insects (earwigs, caterpillars, beetles, and leafhoppers) from getting to the prize. It won’t stop everything—particularly tiny pests like thrips—but it can make a BIG difference in the appearance of your blooms.
If you have a small backyard cut-flower garden, you could even take this a step further and cover the entire plant in tulle or fine mesh. That’s obviously not practical when you’re growing hundreds of dahlias for cutting, but for a small garden, it can be a great way to keep many insects from reaching the plants in the first place. Just make sure your covering still allows plenty of airflow.
The Bottom Line: Don't Let Dahlia Pests Ruin Your Blooms
If you take away one thing from this newsletter, let it be this: Start with the plant.
It’s tempting to see a bug and immediately reach for the bug killer. But you’re never going to have a completely bug-free garden. And that’s okay!
Focus first on giving your dahlias what they need:
- Healthy soil
- Good drainage
- Plenty of sun
- Adequate nutrition
- Plenty of space
- Consistent water
- Regular monitoring
Then deal with the dahlia pests when—and if—they actually become a problem.
Healthy plants aren’t bug-proof. But they are much better equipped to handle a little nibbling, recover from damage and keep producing flowers.
And let’s be honest…You’re growing flowers, not plastic. For most home gardeners, a few chewed leaves are okay. A few missing petals are okay. The goal isn’t a garden where a single bug never lands on your dahlias.
The goal is to enjoy the fruits—or in this case, flowers—of your hard work!
Have a Beautiful Day!
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