From our perspective, the most frequent reason for these perforations is the activity of other organisms seeking sustenance. We are a primary food source for a variety of insect larvae and adults. The most likely culprits are caterpillars, such as those of the sunflower moth or the sunflower beetle. These creatures use their mandibles to chew large, irregular holes between our leaf veins. Additionally, smaller pests like beetles, including the notorious cucumber beetle, will also munch on our foliage, creating smaller, more rounded holes. While this consumption damages our photosynthetic factories, we have evolved to tolerate a certain level of herbivory. Our robust growth often allows us to outpace the damage, and we can compensate for lost leaf area by increasing the photosynthetic rate of our remaining tissue.
Another common cause for holes, particularly those that appear overnight and are accompanied by silvery slime trails, is the work of gastropods. Slugs and snails are nocturnal feeders that use their radula—a tongue-like organ with thousands of microscopic teeth—to rasp away at our leaf surfaces. They tend to create larger, more ragged holes than many insects and often start feeding from the leaf's edge inward. These creatures thrive in moist, damp conditions. From our rooted viewpoint, we are highly susceptible to their attacks after rainfall or in areas with dense ground cover that retains humidity, as we cannot move to a drier location.
While it may seem counterintuitive, not all damage is purely external. Sometimes, what appears as a hole can begin as a localized response to stress or disease. For instance, a fungal or bacterial leaf spot infection can cause small lesions. As our immune response works to wall off the infected area, the diseased tissue often dies and may eventually fall out, leaving a hole behind in a process called "shot holing." Furthermore, we must shed older leaves to redirect energy towards new growth, flowers, and seed production. As a lower leaf begins to senesce (age and die naturally), it becomes weaker and more susceptible to minor physical damage from wind or rain, which can tear the weakened tissue and create holes.
Although less common than biotic factors, our physical environment can also be the cause. Hailstorms can brutally shred our leaves, creating numerous irregular holes and tears in a very short period. Strong, whipping winds can cause our large leaves to thrash against nearby structures, stems, or even each other, leading to rips and perforations from physical abrasion. Additionally, while rare, misapplied garden chemicals like herbicides can cause localized tissue death, manifesting as yellowing and then holes where the dead material decays and falls away.
