Plants evolve their own defense to threats to its survival (Ryan, 1990). To prevent attacks to their fruits as a vehicle of sustaining the species, plants secrete substances that make them vulnerable to pests. Either they produce substances that are toxic to animals such as alkaloids, saponins, volatile oils, resins and phenolics. Or maybe a highly acidic sap that can kill intruding insects? This article presents the case of highly acidic sap that probably helped control mango pulp weevil infestation.
Mango Pulp Weevil
The latter mechanism may be at work in the recent experience I’ve had with mangoes grown just within our lot. Almost five years ago, I posted an article titled “Mango Pulp Weevil: A Pest Control Problem in Palawan Island” I saw first-hand how the mango pulp weevil (Sternochetus frigidus) escapes from the mango pulp to again re-infect other mangoes with its pulp destructive reproductive strategy. The mango weevil grows within the mango pulp! This reproductive strategy is damaging to the mango industry where many farmers depend as a source of livelihood.
Hence, out of curiosity, after years of not benefiting much from mangoes grown in our yard, I harvested fifty mangoes from our naturally grown mangoes. No pesticide was sprayed on the two 29-year old trees through the years. I wrapped the harvested green but mature mangoes with paper and cut a diamond hole to monitor its ripeness as the days go by.
Categories of Mangoes
Anticipating the presence of mango weevil, I listed three categories on whiteboard: 1) uninfected, normal mangoes, 2) infected mangoes, 3) mangoes infected with other mango diseases. I added the last category noting that there is a tendency of the mangoes to rot (Figure 1) even before they ripen.
After consuming and giving some mangoes to family and friends while asking for their feedback on the state of the mangoes, the following are the results:
Uninfected, normal mangoes: 48
Infected with mango weevil: none
Infected with other diseases: 2 (mango rot)
More Acidic Sap in Mango Controls Mango Pulp Weevil?
While this may be just anecdotal evidence, it is possible that the mango has developed a natural immunity against the mango weevil. Apparently, the mango sap has become more acidic than usual as when I harvested the mangoes, trickles of acid from the harvested portion hit my face. I felt a little burning sensation but regarded it lightly. However, after taking a bath and looking at the mirror, I noticed ugly streaks of burnt skin run through my face (Fig. 2). I never thought it could happen although when I searched online, indeed mango sap can cause phytoallergy among mango harvesters. Thankfully, the burnt skin healed after several days.
While detaching the mango from the petiole, spurts of white sap burst forth from the fruit. There is strong pressure of escaping sap from the mango. So detaching a mango from its attachment should be done carefully to avoid skin contact with the highly corrosive sap.
Mango Sap Hypothesis
I recognize that a rigorous study need to be done to affirm this observation. The hypothesis for that full-scale study would be: The highly acidic sap of mangoes can kill or at least prevent mango weevil eggs from being inoculated in the mango pulp. The mango may have developed natural resistance to the mango weevil as pest. This may be a demonstration of Barry Commoner’s third law of ecology: Nature knows best.
Ryan, C. A. (1990). Protease inhibitors in plants: genes for improving defenses against insects and pathogens. Annual review of phytopathology, 28(1), 425-449.