Caterpillars are the larvae of butterflies and moths. They mainly eat leaves, young shoots, and sometimes buds. With a small infestation, you'll see some bites out of the leaf, but with a severe pest problem, a plant can quickly become bare or weakened.

What makes caterpillars extra tricky: they often eat at times when you see them less, such as early in the morning or in the evening, and they like to hide deep within the crop. Because of this, you sometimes see damage first and only later the caterpillar itself.

What attracts caterpillars?

Caterpillars don't just appear out of nowhere. Adult butterflies or moths lay eggs on plants that are suitable as food. Soft, young growth is extra attractive, as is dense planting where caterpillars can easily hide.

Beware of a blind spot many people have: you see one caterpillar and think it's not so bad, but if eggs have been laid, several can hatch at the same time. Then it seems as if the damage suddenly explodes.

Outdoors, you mainly see caterpillars from spring until late summer, depending on the species and temperature. In sheltered gardens and greenhouses, the season can last longer because the conditions are more stable.

How do I know if I have a caterpillar problem?

If you notice these kinds of signs, caterpillar damage is a serious contender:

• Irregular feeding damage on leaves, from bites along the edge to holes and sometimes almost completely defoliated parts.
• Black specks on leaves or on the ground under the plant; these are caterpillar droppings.
• Webbing or spun-together leaves where caterpillars hide.
• You see the caterpillar itself, often on the underside of the leaf or deep within the plant, especially in the morning or evening.

If you want to be sure, check the places where they hide, such as young heart leaves, rolled-up leaves, and dense parts of the plant.

Caterpillars are particularly damaging because they literally chew on your plants. This doesn't result in silver spots like with thrips, but rather actual feeding damage with bites and holes. These are the most common signs of a caterpillar infestation:

• Presence of caterpillars: You will see the caterpillars themselves on or under the leaves, often well hidden deep within the crop. Many species primarily feed at dusk or at night, so during the day you need to search specifically.
• Irregular holes and chewed edges: Leaves develop ragged bites or larger holes. In case of severe infestation, young plants, shoots, or buds can even be completely eaten.
• Skeletonization or "windowpane" damage: Sometimes, mainly the leaf tissue is removed, and veins partially remain, making the leaf appear translucent or skeletonized.
• Excrement under or on the plant: You often find dark green to black pellets on leaves, in the heart of the plant, or on the ground beneath it. This is caterpillar frass and is one of the clearest indications that a frass producer is active somewhere above you.
• Webbing and spun-together leaves: Some caterpillars create webbing, roll up leaves, or pull leaves together to create a hiding place.

Indirect damage and additional risks

Caterpillars weaken your plant by leaving less foliage for photosynthesis. This can lead to stunted growth or even collapse, especially in young plants. In some crops, caterpillars can also damage buds, flowers, or fruits, causing additional stress and reducing quality.

Coaching note, because this often goes wrong: people quickly think "caterpillar" when they see holes, but slugs, beetles, and even mechanical damage can also cause holes. The difference often lies in the combination of the feeding pattern plus frass, webbing, and actually finding the caterpillar.

Caterpillar control works best if you catch them early and interrupt their damage cycle. Nematodes, such as Steinernema carpocapsae, can help with this because they can infect insect larvae, including various types of caterpillars. You apply them with water, after which they penetrate their host and eliminate it via symbiotic bacteria.

For an optimal approach, combine this with close monitoring and immediate intervention. Think of regularly checking leaves and growing points, picking off caterpillars where you see them, and treating at the right time. Don't expect perfect silence within a few days, but with good timing and sufficient moisture, you usually see significantly less feeding pressure within a few weeks. For foliar application, humidity and avoiding bright sunlight are really crucial.

The advantages of this biological control method are:

• Environmentally friendly: you tackle caterpillars without having to use chemical agents.
• Effective and sustainable: under the right conditions, Steinernema carpocapsae can contribute to reducing caterpillar populations.
• Safe for humans and animals: with normal use, beneficial nematodes are widely considered safe in the garden and around the house.
• Easy to combine: you can combine it with manual removal and targeted control of new feeding damage.

If you see irregular holes, chewed leaf edges, or even half-bare plants, caterpillar damage is a logical suspect. Caterpillars are often hidden on the underside of leaves, in spun-together leaves, or deep within the plant. So you sometimes see damage first and only later the caterpillar itself.

The problem often worsens because caterpillars can eat a lot in a short time, especially if several hatch from eggs simultaneously. Just waiting until you accidentally see them is usually too late; you want to actively check and intervene quickly.

Biological control is often the safest route if you don't want to use sprays or harsh chemicals. The trick is not just to look at what you see, but also where the caterpillars hide and when they are most active.

Caterpillar Nematodes

Caterpillar nematodes are nematodes that you apply with water; Steinernema carpocapsae is often mentioned for this purpose. This species can be used against insects that live on the surface of the soil or on leaves, such as certain caterpillars.

Important to keep in mind: with caterpillars, the result is extremely dependent on the application. For foliar application, nematodes work best with sufficient humidity, low UV, and when the leaf remains moist long enough. Otherwise, they dry out or are damaged by sunlight, and the effect is disappointing.

Caterpillars in Potting Soil

Some caterpillars hide low in the plant or in litter during the day, and some species pupate in or on the soil. In these situations, treating the soil or the hiding place can be logical, as you target them where they already are. However, there are also species that pupate directly on the plant, in which case a soil treatment has less impact. The type of caterpillar therefore partly determines where you should treat.

Where do Caterpillars Come From?

Caterpillars don't come out of nowhere. Adult butterflies or moths lay eggs on plants that are suitable as food. In a garden or on a balcony, this can happen quickly, especially with soft new growth and dense planting where caterpillars can easily hide.

Nematodes Against Caterpillars

You use nematodes against caterpillars as a spray or drench, depending on where the caterpillar is located. For foliar application, it works best if you hit the leaf well and the conditions are right: sufficient moisture and low UV. For sheltering caterpillars around the plant base, a treatment around the base or in the litter may be more logical.

If you want faster results, a mature approach is needed, literally. You combine treatment with daily checks, removing caterpillars wherever you find them, and repeating if you still see new damage. Otherwise, you'll always be behind the curve.

What are Leaf-Eaters?

Leaf-eaters is a collective term people often use for anything that makes holes in leaves. You can distinguish them from caterpillars by three things: feeding pattern, frass, and finding the caterpillar itself. Caterpillars often leave behind black or greenish pellets and like to hide in fixed spots.

Steinernema carpocapsae Against Caterpillars

Steinernema carpocapsae is often mentioned in biological control as a species that can be effective against various caterpillars, including webworms, cutworms, and armyworms. It can be applied to the soil and, in some situations, to the foliage, but then strict conditions regarding UV and humidity apply.