The failure of B. tryoni to puncture artificial fruit treated
with vegetable oils was likely due to the slippery nature of the
oils, though the presence of non-volatile deterrent chemicals
in vegetable oils cannot be eliminated. This result may also be
attributed to the process of oviposition by B. tryoni, where the
ovipositor is positioned to form an angle of 60–70° with the
fruit surface, and only after the fruit skin is penetrated is
the ovipositor brought to an angle of 90° (Pritchard 1969). An
angle of 60–70° would have made it more difficult to obtain
the purchase necessary to pierce the smooth slippery surface
created by the vegetable oils.
This suggestion is supported by the recovery of a small
number of eggs deposited on the surface of vegetable-oiltreated artificial fruits. If a fruit fly had been able to puncture
the surface, the puncture site would have been used by other
fruit flies to lay eggs. In comparison, eggs were well distributed in the untreated artificial fruits. Pritchard (1969) has previously reported that hard and shiny fruits such as Granny
Smith apple received more B. tryoni eggs per puncture than
softer fruits with protuberances over the skin surface, such as
pears. The application of vegetable oils appears to have modified the surface of the artificial fruit so that they were less
susceptible to fruit fly punctures.