The high acidity of a cynipid wasp’s gall may be a new way to defend against predators

Oak galls. By Dazzii – Own work, CC BY-SA 4.0,

A tiny insect known as a cynipid wasp has a larva that was recently found generating plant growths known as galls that possessed acidity levels comparable to lemons.

Entomologist at Penn State and main author of an article on the discovery that was published on March 1 in Biology Letters, Antoine Guiguet, noted that it is a unique defense system that has never been observed before.

It has long been known that the majority of cynipid wasp species inject chemicals into oak trees’ leaves to cause the formation of protective galls (or growths) around their larvae, ensuring the security of their growing young. The gall functions as a protective mechanism to ward off natural enemies while housing and feeding the insects as they develop. The wasp larva ultimately eat their way out of the galls as they fall off the tree, leaving the little balls left to decay on the forest floor.


As all of this activity requires chemistry, tannins, which build up on the gall’s surface and protect it from herbivore attack, have generally been the principal defensive compounds found in galls. In fact, the tannin content of oak galls is so high that when they are crushed and soaked in water, they produce a dark brown liquid that serves as the foundation of a lasting ink that was once used to write the U.S. Declaration of Independence, Constitution, and Bill of Rights.

John Tooker, an entomology professor at Penn State and a co-author on the paper, said that it is really amazing because here is an animal utilizing chemistry to force a plant to do its bidding. The nutritional hypothesis for why galls evolved is explained by the insect’s ability to direct the plant to produce the precise food it needs. Nevertheless, this explanation must certainly be linked with a defense strategy since if you have a good food supply, other organisms will want to consume it.

The wasp decreased the pH level of the inside of their growing gall to the acidity levels adopted by pitcher plants in their study, which may be a unique manipulation of host-plant chemistry in transparent oak galls.

The researchers are aware that plants seldom have pH levels this low, according to Tooker.  “Furthermore, the pH value that the researchers obtained was comparable to the acidity of the contents of a pitcher plant, which is about equivalent to a lemon. They  believe that this serves a defensive purpose. That acidic climate would discourage anything from boring in there.

Mass spectrometry, an analytical method used for the study of chemical compounds, was employed by the researchers to determine the amount of organic acid present in the transparent oak gall and to compare it to fruits and other galls. They discovered that 66% of the organic acid identified in the galls is malic acid, an acid that is particularly abundant in apples. In comparison to other galls and apples, the concentration of malic acid was two times greater. Also, they discovered that the gall’s pH ranged from 2 to 3, which is among the lowest values discovered in plant tissues.

Malic acid, which is present in all plant and animal cells, even if in small amounts, is a crucial component of cell metabolism, according to Guiguet. What’s astonishing is that this wasp is able to induce its accumulation in the vacuole, a storage space found in plant cells.

The transparent oak gall is one of the most acidic plant tissues ever recorded, with a pH of less than 3. Only citrus fruit tissues were known to be capable of this extreme acidity prior to this finding.

The scientists speculate that the wasp may have produced acidic galls as a substitute for the tannin buildup seen in the majority of other oak galls. They claimed that because caterpillar hindguts are quite alkaline, low pH might impair the effectiveness of protein digestion in insects similarly to tannins.

Contrary to tannins, acidic conditions may also be effective at repelling parasitoid wasps—the primary predators of cynipid wasps—by degrading the tissue of the organ that some parasitoid wasp species use as a needle to lay their eggs in the gall.

Cynipid wasps produce galls by a chemical mechanism that is yet unknown as stated by Guiguet. Now that the researchers have demonstrated that the wasp have developed with the ability to modify pH, they have contributed to this enigma.


Antoine Guiguet et al, Extreme acidity in a cynipid gall: a potential new defensive strategy against natural enemies, Biology Letters (2023). DOI: 10.1098/rsbl.2022.0513

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