How to Kill Catalpa Worms
Catalpa tree worms swarm catalpa trees during the summer months, causing damage that can eventually lead to defoliation. These voracious pests can affect native trees, such as the northern catalpa (Catalpa speciosa; USDA zones 4 to 8) and southern catalpa (Catalpa bignonioides; zones 5 to 9), as well as the Chinese catalpa (Catalpa ovata; zones 4 to 8), all of which are common landscaping trees grown for their attractive foliage and showy flowers.
Dealing with catalpa, or catawba, worms at home takes a multi-pronged approach that focuses on good cultural practices to keep the tree healthy, which will help it survive an infestation. Killing catalpa worms is not always the best option, because it is not only very costly, but it's also difficult to do because mature catalpa trees are large and expensive to treat with insecticide.
The Northern catalpa tree is sometimes known as a "cigar tree" for its long seedpods.
All About Catalpa Worms
Catalpa worms are the offspring of catalpa sphinx moths (Ceratomia catalpae), which are a variety of hawk moth native to the Southeastern United States. They are the only species of moth to feed on catalpa trees, so it's not difficult to identify which worm is wrecking havoc on your tree.
The moths are harmless, but they lay 100s or even 1,000s of eggs on the undersides of leaves, twigs and branches, which eventually hatch into caterpillars or worms that feed on the catalpa.
In large numbers, the worms can stress a tree to the point that it drops smaller branches, or it can cause the tree to defoliate entirely. Some trees will defoliate multiple times, each time rebounding with a fresh flush of foliage and no long-lasting damage. Other catalpa trees will decline and die after multiple infestations.
Catalpa moth larvae or pupae make good fishing bait for anglers and fishermen, because they attract fish such as largemouth bass and catfish. This earned the tree the common name “fish bait tree.”
Use Good Cultural Practices
Catalpa trees that are stressed from poor growing conditions or bad cultural practices are more susceptible to serious problems and can die from a bad catalpa worm infestation. Keeping your tree healthy will not only help it look better, it will also allow it to withstand a catalpa moth infestation with minimal damage.
Catalpa trees grow quickly and adapt to a wide range of growing conditions, but they should be planted under the best possible conditions to encourage strong, healthy growth.
- Choose a growing site with full sun or some light midday shade. Both deep shade and baking hot sun in desert climates are stressful for catalpa trees.
- Catalpa trees are moderately drought tolerant once established but will suffer less stress when grown in moist, well-draining soil.
- Catalpa trees can reach a mature height of 50 feet with a 40- to 50-foot spread. Be sure to choose a growing location that provides enough space for them to spread out, since cramped conditions can stress the tree.
Catalpa trees are invasive in many areas, including Maryland and California.
Care and Maintenance
The right care and maintenance make all the difference when it comes to keeping your catalpa tree healthy and resistant to damage from catalpa moth caterpillars.
- Water newly planted catalpa trees weekly. If no rain falls, provide 1 inch of water each week during the summer months. Spread a 3-inch-thick layer of mulch around the base of the tree to hold moisture in the soil.
- Established catalpa trees don't need as much water, but they still grow best in soil that retains moisture during the summer months. Check the soil moisture around the tree during hot, dry weather and water only if the soil feels very dry a few inches below the soil.
- Catalpa trees fare well without fertilizer unless they are grown in very nutrient-poor soil. They should not be fed at all until they are established in the ground. Feed very sparingly in early spring with 10-10-10 or organic fertilizer.
Watch your tree for signs of stress, such as yellow, drooping or dropped leaves. Dry soil, too much fertilizer and catalpa worms could all be the culprit.
Manual Removal Methods
Good cultural practices will help prevent serious damage from catalpa worm infestations, but removing the pests from young or smaller catalpa trees is also a good idea because it will prevent damage in the first place.
- Removing the eggs, white cocoons and worms by hand is an option for smaller catalpa trees. Search on the undersides of the leaves on twigs and anywhere else on the tree for the catalpa worms at any stage of their life cycle. Pull them off or trim them out of the leaves using pruning shears and drop them in a bucket of soapy water.
- Tilling the ground beneath a catalpa tree in spring or fall will dislodge the pupae from the soil before they hatch. Pick up any worms or pupae that are unearthed by tilling; then drop them in a bucket of soapy water to drown them.
Using the Right Treatment
Treating a catalpa worm infestation with insecticide or pesticides should only be done as a last resort. You are more likely to have success treating younger trees due to their smaller, more manageable size, because mature catalpa trees are simply too large to effectively treat.
The only effective treatment for catalpa worms is a Bacillus thuringiensis-based insecticide, which is often shortened to the name Bt. Specifically, you must use a variety of Bt called Btk, which is Bacillus thuringiensis kurstaki. Btk-based insecticide works only on the larvae of catalpa sphinx moths, so it must be applied in spring before they pupate.
Saturate the entire tree with a Btk-based insecticide. The undersides of catalpa leaves are an especially common hiding spot for these pests, so pay special attention to those areas when applying it. Repeat applications of Btk are not necessary, but the solution takes time to work.
Btk-based insecticide is not harmful to humans, pets and beneficial insects, but it must be applied according to the manufacturer’s directions.
Sasha Degnan holds a Bachelor of Arts degree in English Literature and Anthropology. Her written work has appeared in both online and print publications. She is a certified Master Gardener and dedicated plant enthusiast with decades of experience growing and propagating native and exotic plant varieties.