This study aimed to investigate the biological activities of eight essential and five vegetable oils against Queensland fruit fly Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), a major insect pest of a wide variety of fruit and some vegetable crops in Australia. Currently, synthetic insecticides are mostly used to control this pest but insecticides of botanical origin are generally safer to humans and the environment. Therefore, botanical insecticides represent a potential alternative to the synthetic insecticides.
Under laboratory conditions, repellency and oviposition deterrence effects of essential and vegetable oils were evaluated against female B. tryoni, using artificial substrates and apple fruits. The results showed that vegetable oils were more effective than essential oils in deterring oviposition. The oil with most potential was from safflower, Carthamus tinctorius (Asterales: Asteraceae). This vegetable oil at a concentration of 10 mL/L significantly reduced oviposition in apples by 56.4% in a choice test, but none of the essential and vegetable oils tested had a significant effect on oviposition in apples in a no choice test. The likely mechanism is that safflower and other vegetable oils created a slippery surface and females were unable to make punctures in the fruit for egg deposition. Essential oils especially lemon-scented tea tree Leptospermum petersonii (Myrtales: Myrtaceae), peppermint Mentha piperita (Lamiales: Lamiaceae), honey myrtle Melaleuca teretifolia (Myrtales: Myrtaceae) repelled female B. tryoni, but their persistence on apple fruits was very low, only for a few hours (2-4 hours). Safflower oil was further investigated for its mechanism and effectiveness against female B. tryoni. In a laboratory test, safflower oil treatments (2.5 mL/L and 5.0 mL/L) reduced the number of fly punctures on treated artificial fruits no matter whether prepunctures were present or absent.
Safflower oil treatments also reduced the number of fly landings and eggs laid, but only when the treated artificial fruits were without prepunctures. These results confirmed that safflower oil is active against female B. tryoni mainly by physically preventing this fruit fly from making oviposition punctures, not by discouraging them from depositing eggs or by repelling them. Further tests using fruit bearing tomato plants (a no choice test) in a glasshouse situation revealed that safflower oil application at concentrations of 10 mL/L and 15 mL/L reduced the number of oviposition punctures but failed to reduce the number of eggs laid. The fumigant toxicities of the eight essential oils against B. tryoni eggs and larvae were evaluated in the laboratory using a filter paper method with a 950 ml glass jar as the fumigation chamber. During fumigation, eggs were put on a moistened black filter paper, whereas larvae were placed in carrot-based diet. The results of a 24 hour exposure test showed that peppermint oil had strong fumigant effect on eggs. However, this oil showed low fumigant effect against the larval stages. Broad-leaved peppermint (Eucalyptus dives) (Myrtales: Myrtaceae) oil showed low fumigant effect against eggs but a strong fumigant effect on the first and second instars. None of the tested essential oils showed fumigant effects on third instars. Further investigations were conducted on the effects of fumigation with essential oil alone or when followed by cold storage on survival of the fruit fly in “Gala” variety apples. Efficacy was based on the number of pupae recovered from treated and untreated fruits and on phytotoxic effects. In a 24 hour fruit fumigation test, peppermint oil applied at 100 and 200 µL/L air was found to be active against B. tryoni eggs, whereas broad-leaved peppermint oil was active against both eggs and larvae but only at the highest dose tested (200 µL/L air). However, both peppermint and broad-leaved peppermint oils sometimes had a phytotoxic effect on the apples. In a 6 hour fruit fumigation test, an equal mixture of peppermint and broad-leaved peppermint oil (100 µL/L air) did not cause phytotoxic effect but had only a slight effect on B. tryoni eggs and no effect on the larvae. There was no synergism or additive effect when this essential oil mixture was applied in combination with subsequent cold storage. Cold storage (4 + 1oC) alone was confirmed to be a very effective treatment against B. tryoni larvae and eggs in “Gala” apple without causing fruit damage. Contact and oral toxicities of eight essential and five vegetable oils were evaluated against female B. tryoni. In the contact toxicity tests, the oils were sprayed onto females at a concentration of 10 mL/L, whereas in the oral (ingestion) toxicity test, the oils were incorporated into fruit fly baits at a concentration of 20 mL/L bait. The results of contact toxicity tests showed that honey myrtle was the most potent oil. This essential oil at the concentration tested had moderate toxicity against female B. tryoni. However in oral toxicity tests, none of the oils had a significant mortality effect on female B. tryoni.