What’s Putting Holes in My Plants’ Leaves?

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Question: I need your help. The leaves of my maple seedlings, cranberrybush viburnums, runner beans and even basil have holes in them. Some are eaten right down to the ribs. It gives a unique lacey look, but that’s not what I want. What can I do?

J. Bouchard

Answer: This is the kind of question I most hate answering, because there are so many different possibilities. Almost certainly these different attacks are caused by different enemies, because the plants you mention are in no way related and most pest problems are specific to a certain plant family or even certain species within that family. The guilty party can be insects, mollusks, mammals and even plant diseases, weather or herbicides. 

Do note that, unless the damage is extensive, leaf damage might not be seriously harming the plant. Most plants produce far more foliage than strictly necessary … largely so they have extra photosynthetic material to survive on in case something does attack them.

Of course, that bit of information can be of little comfort if the leaves serve an ornamental purpose … or if they’re from vegetables or herbs whose leaves we eat. I mean, who wants to consume leaves that something else has already chomped on?

So, here are a few of the enemies that might be eating your plants’ leaves.

Rogue’s Gallery of Leaf-Eating Pests

Slugs and Snails

Probably the most common problem. They tend to attack towards the middle of the leaf, not so much the edge, leaving irregular open holes with smooth green edges. They often leave calling card: a trail of slime on the leaf. Slugs and snails are very much generalists and attack a wide range of plants, although their reputation as hosta mutilators is well deserved (not that they attack just any hosta. Here is a list of hostas slugs and snails don’t usually bother: Slug Resistant Hostas: Take Your Pick!).

Slugs and snails seek moist conditions and spend the day hiding under rocks, pieces of wood, flower pots, etc., although they may be active on cloudy or rainy days. They thrive under mulch … at first, but if you keep the mulch in place, their enemies start to build up under the same mulch and eventually come to keep their population in check.

To confirm the presence of slugs and snails, come out at night with a flashlight.

Here are some possible treatments: Slug Treatments that Really Work!

Earwigs (Forficula auriculata)

They also come out at night, have a wide host range and hide during the day … mostly in the same places are slugs and snails, but also in flowers and rolled up leaves. They make irregular, ragged holes along leaf edges and also burrow into fruits. Young leaves are a favorite (they can devastate young seedlings) as are basil leaves and they might be what’s attacking your basil. Check by coming out at night with a flashlight. 

There are all sorts of traps and controls that will kill them, but earwigs are territorial, so if you kill 10 earwigs, 10 more will move in. Since their population tends to rise to critical levels for 4 or 5 years, then drop to a level where they cause no serious damage for decades, it may be easiest to avoid growing their favorite plants for a few years. 

Caterpillars

They come in all sizes and shapes and a wide range of colors as well (although many are the same color as their host plants, so are hard to find). And they have a wide range of hosts, from trees and shrubs to herbs, vegetables and, well, just about anything that grows. That said, though, most species are highly specific. Cabbageworms (Pieris app.) and cabbage loopers (Trichoplusia ni) only attack plants in the cabbage family, for example; tomato hornworms  (Manduca quinquemaculata) only plants in the tomato family, etc. If you see butterflies or moths hanging around plants that are not in bloom, look for eggs under the leaves and destroy them. If you miss that stage, look for abundant droppings and leaves both young and old with irregular holes starting at the edges. Sometimes entire leaves are gobbled up.

Some caterpillars don’t just eat holes in leaves, they move inside them where they’re harder to control. This is the case, for example, with the leek moth.

Btk is a biological insecticide specific to caterpillars. Or try hand picking, dropping the culprits into a pail of soapy water.

Sawfly Larvae

The adults are small wasp relatives you rarely see. Their voracious larvae usually look like small caterpillars, although some look like tiny slugs. They are highly specific: each plant seems to have its own sawfly. Some species, like the rose sawfly, scrape off the top surface leaving pale irregular “windows” that sometimes dry out and open into true holes. Others start at leaf edges and work their way in, eating all the tissues but the veins. The latter are often gregarious, with many working on one leaf. Unlike true caterpillars, they are not susceptible to Btk, but insecticidal soap will kill them. However, they often skeletonize an entire plant before you have time to react and have moved on by the time you notice them and are ready to spray.

Leaf-Cutter Bees

These bees are valued pollinators, but also cut half-moon shaped, very regular disks along leaf edges, then carry the leaf bit off to line the holes that house their larvae. They especially choose rose leaves. Most gardeners learn to tolerate them as the price to pay for good pollination. Read more about them here: Round Leaves in Rose Leaves?

Japanese Beetles (Popillia japonica)

At least these pests make no effort to hide, as they’re out and active during the day. They’re big, rounded beetles about 1/2 inch (1 cm long) with shimmering metallic green heads and copper-colored wing cases. Gregarious, they feed in groups, usually starting at the top of a plant and working their way down, eating flowers as well as leaves. They start in the middle of leaves and eat the tissue between the veins, skeletonizing them. Their larvae are among the species of white grubs that attack lawn grasses. You can learn more about treating Japanese beetles here: Controlling Those #$@&% Japanese Beetles. 

Flea Beetles

Contrary to Japanese beetles, flea beetles are tiny and rarely seen, as they as they jump away like fleas when disturbed. If you do, they are metallic, often black. There are thousands of species of flea beetles worldwide and just about anywhere, they will be several dozen local species, most with their favorite host plants.

 The plant’s leaves—especially the youngest ones—are riddled with dozens both small holes and shallow pits, as if they’d been hit with a shotgun. In fact, this type of injury is called shot-holing. They love vegetables and might well be what’s attacking your runner beans. More on them in the article Flea Beetles: the Shotgun Insects.

Cucumber Beetles

Well, guess which plants they are specific to? They can be striped or spotted and chew irregular holes into leaves. More on them here: Learning to Deal with Cucumber Beetles.

Other Beetles

The list of leaf-eating beetles just goes on and on. Viburnum leaf beetles are undoubtedly what attacked your cranberrybush viburnums, scarlet lily beetles attack lily leaves and flowers … and there are many, many others.

Weevils

Again, there are thousands of species of weevils, usually dull-colored beetlelike insects with a long snout. Most feed on plants, but they do their damage in various ways. Many are most harmful as larvae when they feed on roots. Others attack seeds, some live off flowers.

The best-known leaf-eating weevil is the black vine weevil (Otiorhynchus sulcatus), which chews on leaves of a wide range of plants, including roses, strawberries, rhododendrons, yews, viburnums, lilies, camellias, many perennials and trees. It damages leaves along the edges, leaving jagged crescent shaped notches with browning margins. This weevil hides in debris and soil during the day. Its larvae do major damage to roots as well. Thwart adults by applying diatomaceous earth (an organic insecticide) to plant leaves. 

Leaf Rollers

They do put holes in leaves, but are more noticeable by the way they roll them up first. This creates a shelter they then live in. Most leaf rollers are caterpillars (the larvae of moths) and can be controlled by Btk … if you catch them in the open. Otherwise, either hand pick and destroy the rolled-up leaves … or ignore them (they tend to be minor pests).

Mammals

Deer, rabbits, groundhogs, etc. They don’t so much eat holes in leaves as cut them back or eat them off entirely. Obviously, they’re big animals, so do big damage. 

Diseases

All sorts of diseases damage plant leaves. Unlike insects, that tend to eat clearly defined holes from the start, typically, holes caused by plant diseases usually begin as brown or discolored spots whose tissue eventually falls away, leaving a hole, often with a ring of yellow around it. 

Shot hole disease, seen mostly on fruit trees, is caused by a fungus (Stigmina carpophila) and leaves pellet-sized holes in leaves. Frogeye spots (common on Swiss chard, beets and soybeans, among other plants) are caused by different species of Cercospora fungus. Anthracnose, of which there are hundreds of strains affecting a wide range of plants, often leads to brown spots that fall through, leaving leaf holes. Bird’s-eye rot (Elsinoe ampelina) of grapes is one such form of anthracnose.

To help prevent plant diseases, increase air circulation and water carefully, moistening only the soil, not the leaves. Fungicides may be needed in serious cases.

Hail

It tears holes into leaves, especially larger ones, and can totally shred leaves if it falls heavily. If the leaves aren’t shredded, they tend to survive and continue photosynthesizing, so, although the holes may bother you, don’t remove too many leaves or that will weaken the plant. 

Mineral Deficiencies

When plants lack certain minerals, whether of major elements like nitrogen, phosphorus and potassium, or minor ones, like iron or chlorine, this can have all sorts of effects, including deformed, discolored leaves, stunted growth and much more. Also, how any plant will react to deficiencies varies widely. However, one sign of a lack of potassium is yellow or dead patches that can become pin holes, while a deficiency in manganese causes similar inter veinal chlorosis sometimes leading to elongated holes in the older leaves of some plants.

Herbicides

Herbicide drift, possibly from a neighbor’s lawn, can wreak havoc on gardens. Symptoms include twisted, cupped leaves, leaf discoloration and sometimes holes. It can make for strained relationships with neighbors who feel their lawn is of greater value than your garden.

Natural Slits and Holes

Some leaves naturally have slits and holes in them, notably plants in the aroid family, the Swiss cheese plant (Monstera deliciosa) being the best known. Such holes are usually symmetrical and attractive, with smooth edges, and aren’t usually confused with pest damage. 

Maple Seedlings?

I’ve given you suggestions for your other damaged plants (cranberrybush viburnums, runner beans and basil) in the text above, but I have no idea which of the above is punching holes in the leaves of your maple seedlings. It could be almost anything! Take a close look at the damage and go over the list above. Hopefully the answer will be there! 

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Leaf holes: those are the most common causes, but there are others. Finding the real cause and correcting it can be quite a struggle!