How Bacteria Tell the Oriental Fruit Fly “Do Not Lay Eggs Here”

For many insects, selecting a plant on which to lay eggs requires first determining whether the plant already harbours potential competitors. If other larvae are already present, newly hatched offspring may face severe food shortages and fail to survive. When competitors are visible on the plant surface, the situation is easy to recognize. However, if larvae develop hidden inside fruits, assessing whether the resource has already been occupied becomes far more difficult.

The oriental fruit fly, Bactrocera dorsalis, is among the most destructive agricultural pests worldwide. Females are capable of laying eggs on more than 400 species of plants. They typically deposit their eggs between the peel and the flesh of fruits, and once the larvae hatch they burrow into the pulp, eventually causing the fruit to rot. Research has shown that female flies possess the ability to avoid laying eggs in fruits that already contain eggs deposited by other females of the same species. This avoidance behaviour becomes stronger as the eggs inside the fruit age. Such a pattern indicates that females can detect and integrate signals emitted from the fruit in order to identify the most suitable oviposition sites and provide their offspring with the best chance of survival, a pattern consistent with the “mother-knows-best hypothesis”.

Female oriental fruit fly laying eggs（Image source：Unknown，CC0 1.0 ）

The mechanism behind this avoidance behaviour has been found to involve bacteria. When the first female deposits eggs into a fruit, bacteria attached to the egg surface are introduced into the fruit at the same time. These bacteria multiply within the fruit and continuously produce molecules such as β-caryophyllene. As the concentration of these compounds increases, other females approaching the fruit can detect that the resource is already occupied and subsequently leave in search of a new fruit for egg laying. Bacterial sampling and sequencing revealed the presence of genera including Klebsiella, Providencia, and Enterobacter. The abundance of these bacteria increases markedly about 48 hours after egg deposition, corresponding with a stronger avoidance response in gravid females.

Oviposition avoidance is not unique to this species. Similar behaviours occur in other insects. For example, the tobacco hawkmoth, Manduca sexta, detects the odour of faeces produced by conspecific larvae and avoids laying eggs on the same plant. Conversely, the fruit fly Drosophila melanogaster displays the opposite strategy: individuals detect pheromones released by conspecifics and prefer to lay eggs in the same location, allowing larvae to aggregate and thereby increase their chances of survival. In some cases, the signals involved originate from the plants themselves. When plants experience damage, such as herbivore feeding, they release defensive chemicals known as herbivore-induced plant volatiles (HIPVs). These compounds can attract natural enemies of herbivorous insects or drive insects away from the plant. Such interactions illustrate the complex relationships and diverse survival strategies operating among organisms.

The discovery that Bactrocera dorsalis females avoid certain fruits because bacteria associated with eggs generate compounds such as β-caryophyllene may have important implications for pest management in agriculture. If β-caryophyllene were applied to fruit trees or crops, could it effectively reduce infestation by oriental fruit flies? How many other insect species rely on similar mechanisms when selecting sites for egg deposition? These questions remain open and await further investigation.

Author: Shui-Ye You

Reference:

Khallaf MA and Knaden M. (2020). Insect Host Choice: Don't Put All the Eggs in One Basket. Curr Biol.