Honeybees pollinate the guava plant. Their fuzzy bodies make the pollen stick to them and then when they fly around to different flowers getting food, they leave some pollen from other flowers on stamens and pick up different pollen, effectively cross-pollinating.

There are many pests that affect and infect  P. guajava. Fruit flies all over the world will infest ripened fruits and lay their eggs. Many guava farmers try to prevent this by picking fruit before it fully ripens. This doesn't always happen so when the fruit fly infests the fruit, it must be burned to avoid it infecting humans or other animals.

India has cockchafer beetles that feed on the leaves and over 80 species of insects that feed on the tree. Three species of caterpillar, in particular, eat the bark of the tree for food. The cockchafer beetle, after eating the leave, lays its eggs in the roots of the guava for them to hatch. The larva eat at the new twigs effectively killing them.

Some pests feed on the fruit. Argyresthia eugeniella, (guava fruit worm), Theognis gonagia (citron plant bug),  Costalimaita ferruginea (yellow beetle), and Helopeltis antonii (fruit sucking bug) all feed in some way on ripe fruits. A false spider mite, Brevipalpus phoenicis (flat mite), feeds on half-grown fruit. Selenothrips rubrocinctus (red-banded thrip) causes fruit russeting and defoliation.

Human Impact

Because if the popularity of guava, it is popping up in gardens around the world, diversifying where is can be found. Genetic engineering and adaptations have allowed it to withstand harsher winters than the tropics have, much like the mandarin orange. Humans have been introducing guava to new habitats for a while. In many places, it has grown like a weed outcompeting the native plants and killing them. Some measures have been taken to control the guava population in its non-native habitat such as burning. Cutting the stems was tried at first, but that only lead to more stems sprouting, which lead to more fruit, which lead to more trees.

More random facts about guava here.