(Beyond Pesticides, October 5, 2012) Researcher Paul Kendra of the Agricultural Research Service’s Subtropical Horticulture Research Station and others are investigating natural essential oils as traps for Red Bay Ambrosia beetles, the primary vector of laurel wilt fungus, which attacks trees, including avocado trees in the southeastern U.S. Building on previous research, the researchers have identified two important oils, phoebe oil and manuka oil, as potent antifungal agents that can be applied to avocado trees. They have gone so far as to start shipping fungicide-treated avocado trees from the Miami avocado germplasm collection to disease-free sites.
The invasive beetle from Asia has spread to the Carolinas, Florida and west to the Mississippi, killing 90 to 95 percent of infected trees and significantly altering forest ecosystems. Scientists are concerned that the beetles will soon reach Mexico and California, which are major avocado production areas. If only half of California’s commercial avocado trees died, estimates indicate it would mean a total economic impact of about $27 million. In response to growing concerns, the U.S. Department of Agriculture (USDA) is researching alternative strategies to monitor and eventually control the spread of the ambrosia beetle.
Both manuka oil and phoebe oil are sourced from plant based materials, that are readily available and effective alternatives as a trap bait for monitoring distribution and population trends. Manuka oil is sourced from a shrub native to New Zealand. Research at the New Zealand Cawthron Institute indicates that its primary components, leptospermone and flavesone, are 5 to 10 times more effective at treating fungal infections than Australian tea tree oil.
But it’s the longevity of phoebe oils, sourced from the Brazilian walnut tree, that have researchers excited. Phoebe lures not only capture significantly more ambrosia beetle than manuka lures, it also is effective for up to 12 weeks while the manuka lures last only about three weeks. Knowing how long the manuka lures work will be useful for officials to set up early monitoring programs.
Essential oils are complex mixtures of different organic components, the most prominent single substance is triketone leptospermone in manuka oil. This combination gives a high level of antimicrobial activity. However, few studies have analyzed the toxicity of manuka oil outside the cosmetic and medicinal industry. Those that have indicate that it in comparison to other myrtaceous essential oils, manuka demonstrates moderate toxic behavior in cell cultures.
Phoebe oil on the other hand, is primarily composed of Î±-copaene, cadinene, and Î±-humulene which all attract beetles. Again, the chemical, physical, and toxicological properties of the active ingredient have not been thoroughly investigated.
Beyond Pesticides has long been an advocate for the use of non-toxic and least toxic pesticide alternatives. However, while essential oils are traditionally classified as a least-toxic method for pest management, products that are designed to kill living organisms should always be treated with caution. The concern with essential oils is its volatility and ability to vaporize into the air. It is important to remember that there is still a potential to cause harm to human and environmental health and consumers should read labels on all products to make sure it does not also include any toxic pesticides, synergists, or non-disclosed inert ingredients.
Previous research shows that bark beetles, like the Redbay ambrosia beetle, “sniff out” particular compounds within manuka oil and phoebe oil. Field experiments were conducted at a Florida conservation area where the beetle has infested trees since 2007. By comparing the number of Redbay ambrosia beetles attracted to manuka oil lures, phoebe oil lures, and bolts of wood cut from lychee, researchers discovered not only that beetles prefer phoebe lures, but that they also prefer lychee trees. The results indicate that there are three compounds that particularly attract the beetle, and the lychee wood has large amounts of all three. Just as roses are planted by grape vines to warn of infestations, so lychee trees could be planted to signal the need for beetle management.
These mechanisms are a way forward in organic agricultural systems. Currently, conventional avocados are grown with a wide variety of toxic chemicals. Though avocados grown on conventional farms show low pesticide residues on the finish commodity, there are 32 pesticides with established tolerances (residue limits for pesticides used in the U.S. or by countries exporting to the U.S.). There are 13 pesticides registered for use that are considered acutely toxic, 29 are linked to chronic health problems, five contaminate streams or groundwater, and 29 are poisonous to wildlife. Clearly there is a need to move beyond conventional agricultural system that poison our food and sickens agricultural workers and nearby residents.
Beyond Pesticides works extensively to promote organic practices and policy throughout the country. With proper design and preventive practices, there is little to no need to use any pesticide product.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.