1. The Squash Beetle and the Round Holes on Cucumber Leaves
The adult squash beetle is a small insect about 7 to 8 mm (0.3 inches) long. It has an orange-yellow back and a black underside. [1] These beetles eat the leaves of cucurbits, such as cucumber, pumpkin, melon, and watermelon. Their feeding leaves clean, circular marks on the leaf surface. [1]
The beetles emerge once a year and spend the winter as adults under dead grass or in gaps in stone walls. In early spring, they also eat broad bean leaves, so they do not limit themselves only to cucurbits. [1] However, their presence on cucurbit plants is prominent. There seems to be a clue specific to this insect. The clue lies in the bitterness that cucurbit plants deliberately produce.
2. Cucurbitacin: The Bitter Compound That Defends Plants Against Pests
The source of bitterness in cucurbits is a compound called cucurbitacin. It is found in cucumbers, zucchini, and pumpkins. For the plant, this is a defense substance meant to protect against pests and pathogens. [2] The amount of cucurbitacin can increase due to environmental stress, such as heat or drought. [2]
Cultivated varieties grown for food are bred to have low levels of this compound. This is why we rarely encounter strong bitterness in our daily meals. [2] In 2014, research reported that genes and regulatory factors controlling cucurbitacin C production were found in cucumber leaves and fruits. The same report noted that while fruit bitterness was lost as people cultivated them, wild cucurbits use it to defend against predators. [3]
If bitterness is a defense, insects should avoid it. Yet squash beetles appear to be drawn to it.
3. How Bitterness Tells Squash Beetles That Food Is Nearby
A 1980 study showed that cucurbitacin acts as a cue for leaf beetles in choosing their host plant. It promotes frantic feeding behavior, functioning as a kairomone. A kairomone is a chemical signal that benefits the receiver (the insect) rather than the sender (the plant). [4]
A 1983 report stated that for New World leaf beetles and Old World squash beetles, cucurbitacin causes them to stop moving and begin intense feeding. They can detect even tiny amounts. [5] Note that the specific figure of reacting to "1 nanogram" was obtained from New World Diabrotica species, not from squash beetles themselves. [4]
Japanese tests using squash beetles (Aulacophora femoralis) also exist. When filter paper was soaked with cucurbitacin B, E, I, and a glycoside of E, the beetles were strongly stimulated to eat all four types. [6] The bitterness plants use for defense becomes a signpost saying "food is here" for these insects.
4. Do Other Insects That Eat Cucurbits React to Bitterness Too?
Not all related insects react the same way. A closely related species in Japan, Aulacophora nigripennis, showed almost no reaction to cucurbitacin. The researchers concluded that not all cucurbit-eating insects are strongly stimulated by this compound. [6]
Bitter melon (goya) leaves are also eaten less by squash beetles. This is because momordicin I and II in the leaves suppress feeding. Momordicin II works on squash beetles only at high concentrations, but it affects A. nigripennis at lower concentrations. [7] However, the study does not say they are never eaten.
Wax components on the leaf surface (straight-chain alkanes) also stimulate feeding behavior in squash beetles. But this is reported as a general stimulus, not a clue for identifying the host plant. [8] The beetle’s choice seems to be a result of bitterness acting as an "eat" signal, combined with other cues or repellents. The evolutionary path that led to this specialization is not clear from the collected materials.
5. How Some Beetles Store Bitter Substances to Protect Themselves From Birds
A 1992 study reported that three species of Aulacophora beetles in East Asia, including relatives of the squash beetle, eat cucurbit roots and store a substance related to cucurbitacin (23,24-dihydrocucurbitacin D) in their bodies. This compound deters bird predators from eating them, suggesting it is used for defense. [9] The summary does not clarify if the squash beetle itself stores this, so we refer to them as "relatives of the squash beetle."
Squash beetle larvae eat cucurbit roots underground. When they reach the main root, the plant may weaken and die. Egg-laying begins in early June and peaks in mid-July. New adults emerge from late July onward. [1]
From the perspective of damaging crops, these are troublesome pests. At the same time, they use plant defense chemicals as meal signals, while some relatives store them for their own protection. They are participants in a tug-of-war between plants and insects.
6. Why Beetles Make Round Holes: Trenching and the Unanswered Question
The behavior of digging a circular trench before eating the inside is called "trenching." It has been studied in ladybird beetles (Epilachna) in America. [10] A 1980 study proposed that bitter compounds accumulate in damaged leaves within 40 minutes, so the trench might block this accumulation. In contrast, a 1995 study offered the theory that cucurbitacin stimulates eating, and the trench is to avoid sticky phloem sap. Both are based on abstracts, and the theories differ. [10]
It is not known if squash beetle holes form for the same reasons. Online explanations claiming it is "to avoid cucurbitacin" often lack cited papers, so they are not included here. The fact that we do not know clearly shows that the entry point for research into this question is open.
7. How to Observe the Round Holes on Leaves Yourself
If you have the chance to see cucurbit leaves, observe the shape of the round holes. Are the edges smooth? Is there any line or groove outside the hole? Note where the insect is located. Do not touch the insects; just observe from a distance.
Write down your observations alongside your own candidate explanations for the "round reason." Later, when you read books or papers, you can compare which theory matches your notes. For Japanese readers, prefectural agricultural technology centers provide pest information with photos summarizing squash beetle life and damage. [1] If you are interested in the link between bitterness and beetles, papers by Japanese researchers from the 1980s or articles on J-STAGE serve as entry points to original sources.