Unless you are the culprit, the next rational choice for what is eating the leaves of your basil plant is insects. Basil (Ocimum basilicum L.), as a species, is prone to several insect problems that can cause damage to the leaves of the plant. Slugs, aphids, Japanese beetles and spider mites are just a few of the many insects that plague this plant.
Basil is an herb grown for its smell and usefulness as a spice for cooking. There are several different varieties of basil, which all stem from the parent plant Ocimum basilicum. Basil foliage can range from green to purple, depending on the variety. In addition, different varieties of basil are better suited for use as potpourri, or in vinegar because of their smell. One interesting fact about basil is that it can actually be used as an insect repellent, usually against mosquitoes. However, this property does not extend far into the insect world because basil is a favourite snack for many common, and some uncommon, garden insects.
Common Basil Pests
Some of the more common insects that feed on basil leaves are aphids, slugs, spider mites and Japanese beetles. Aphids are small insects that suck the life out of the leaf, leaving a discoloured spot that eventually decays and can resemble chewing damage from afar. Slugs on the other hand will do damage to the basil leaves that will closely resemble a bite or chewing on the leaf. However, due to the slow pace of slugs, they are usually visible on the plant.
Spider mites are a common problem on basil, but the damage they do more closely resembles the damage done by an aphid and close examination of the damage would show that it actually does not seem to resemble chewing. Japanese beetles however, are an absolute menace that mechanically feed on the leaves of basil plants, giving it that distinct chewed-upon look.
Uncommon Basil Pests
In the rare chance that your basil plant was not a snack to one of the above insects, you might be dealing with a less common insect. Grasshoppers have been known to munch on basil plant leaves as well as June bugs. Cutworms can also be a potentially threatening infestation for the plant.
Lastly, ants can pose a problem to basil leaves because they have been known to carry and transfer mealybugs, a sucking insect that could potentially cause damage to the basil leaf that looks consistent with chewing damage.
According to basilgardening.com, most of the insects that attack and eat basil leaves do so because of water stress, cold or cool weather and insufficient sunlight or drainage. The take away from this is that the better you care for your basil plants the better their chance of surviving and fending off insect invaders.
Insecticidal soaps can also be used to coat and help protect the basil leaves during particularly bad parts of the season when insects are at their worst. Placing traps near the plants is another idea however, some experts caution against this, saying that it will only attract the insects to your plants in large populations.
If the damage has got to an egregious point, the gardener may need to take immediate action to kill the insects before they kill the entire plant. There are several different varieties of insecticides available that range from pest-specific to all-purpose that can be very helpful for killing insects off your basil plants. Those that prefer the organic route, should spray liquid seaweed on their plants to deal with aphids, mealybugs and mites. Citrus oil/d-limonine sprays and canola/pyrethrum mixtures are great for eliminating larger insects like beetles and grasshoppers