Morphological and chemical alterations in Psidium cattleianum (Myrtaceae) fruits infested by insects

Background/Question/Methods

Ripe fruits are attractive both to mutualists, as seed dispersers, and non-mutualists, as seed predators and pulp-eating insects. Frugivorous vertebrates can prey on non-mutualist insects developing inside the fruits. This selective pressure may promote a counter-response by insects: to make the fruit less attractive for vertebrates. Changes in insect infested fruits can be morphological (shape, coloration, consistency), and/or chemical (nutrient content, flavor, odor). We tested the prediction above in fruits of a native guava, Psidium cattleianum (Myrtaceae), in the Atlantic forest of southeast Brazil. We exposed unripe fruits of five individuals to two treatments: excluding or allowing infestation by non-mutualist insects. We used size (estimated by measures of area and mass) and shape (estimated by the variance of four diameter measures) to assess the morphological changes. To assess chemical changes we investigated the fruit odor (estimated by concentration of volatile sesquiterpenes), using gas chromatography–mass spectrometry (GC-MS). We compared fruit morphological and chemical traits between treatments by a one-way ANOVA. 

Results/Conclusions

Infested and non-infested fruits differed in all morphological and chemical traits. Area and mass were smaller in infested than in non-infested fruits. Infested fruits also had greater variation in diameter measures than non-infested fruits. Taken together, these differences indicate deformation of the fruit shape. Infested fruits showed a higher concentration of sesquiterpenes than that of non-infested fruits. These results indicate that infested fruits are smaller and misshaped, but with more odor than non-infested fruits. Morphological traits can be used as visual cues by foraging vertebrates, which could learn how to avoid or choose fruits with such characteristics. An increase in volatile concentration could be deterrent for foraging vertebrates, but also could facilitate the attraction of nocturnal and smell-oriented frugivores or even parasitoids, which could result in the larvae death. We will test these two predictions in coming experiments. If they are confirmed, we can assume that infestation of guava fruits by pulp-eating insects may affect bird and mammal foraging behavior and consequently the process of seed dispersal.