Saving Avocados: Scientists Use Pheromones That Disrupt Mating to End Invasive Insecticide-Resistant Weevils

Pheromones are being used by scientists to interfere with the mating behaviors of weevils, an invasive and insecticide-resistant beetle. Farmers refer to the pesky insect as "evil" weevils.

Scientists are working on an innovative way to stop these long-snouted beetles because insecticides are ineffective because they bore through the fruit to lay their eggs inside.

In particular, scientists want to use pheromones to prevent weevils (Heilipus lauri) from mating in avocado seeds. Many animals and plants produce pheromones, which are chemicals that act as signals for other members of the same species. Those who "smell" them may behave differently.

The potential impact of applying the proper pheromones on weevils in avocado orchards piqued scientists' interest.

The Way Pheromones Work

Mark Hoddle, a UC Riverside entomologist explains that since pheromones are used by insects to locate potential mates, flooding an orchard with them will introduce a few factors that will reduce mating and the subsequent production of pest offspring.

Weevils will not be able to find each other easily because they are following erroneous trails that lead from a pheromone release receptacle. After all, there are too much pheromones in the orchard.

Additionally, weevils may become accustomed to the presence of high concentrations of pheromones in an area and stop responding to them.

Hoddle says that it is similar to the way aftershave or perfume smells. Initially, it is strong and noticeable, but after a while, it becomes less noteworthy.

For growers, being able to eradicate these pests could have a significant impact. On roughly 50,000 acres in California, 4,000 farmers grow avocados. The annual value of avocados is about $350 million.

Hoddle is battling avocado weevils in Mexico and working to stop their unintentional introduction to California.

Hidden in the Fruit

Little is known about these weevils among researchers. Insects pierce the fruit to lay their eggs, and the larvae then make their way into the avocado seeds to feed.

Where they are safe from predators and insecticides, these solitary insects are almost always found deep inside the fruit. They are difficult to identify and poorly researched. Although they pose a significant threat to avocados, researchers know little about them.

Hoddle continues to say that because they spend a significant portion of their life cycle concealed inside the fruit, where insecticides cannot reach these delicate stages, weevils are difficult to eradicate. Additionally, because fruit is the part of the crop that is sold and is damaged by insects, tolerance for infected fruits is very low because the fruit will be downgraded or culled, costing growers money.

As a result, people should search for alternative solutions or supporting technologies, such as pheromones, that can offer control and/or reduce grower reliance on insecticides. Control methods including insecticides are not the most effective control options. Unintended consequences include residues on food, accidental eradication of natural enemies, and the death of other advantageous insects that thrive in orchards.

The Proactive Approach

Researchers are approaching management in a proactive manner. Despite not existing in California, these weevils pose a clear invasion threat. There is a higher chance that weevils will be introduced as more fruit is imported from nations like Mexico or Colombia. In Mexico, weevils are native pest species, while in Columbia, weevils are invasive. Another accidental introduction method that is still a problem is the fruit that is being smuggled.

Hoddle says To avoid wasting years developing management programs that should be implemented after the pest invades an area, establishes its habitat, spreads, and causes damage, they are attempting to get ahead of an obvious problem that lurks just outside of California so they would be prepared to go should weevils be found there.

Other Strategies: Cocktails and Lures

Researchers have been experimenting with different pheromone formulations that weevils produce. They spread them throughout commercial avocado orchards that are severely infested with weevils. The pheromone "cocktails" were released through colored sticky traps, just in case of weevil behavior was influenced by color.

Since they have not yet received a significant response, one of the formulations may be negatively affecting the effect of another. The effectiveness of various chemical forms on weevils is currently being investigated.

Use of the pheromone as a lure, either to trap them or to direct them to an insecticide, is another strategy that may be employed.

Hoddle says that in this approach, the pheromone attracts weevils, but it is encased in a matrix that also contains a contact insecticide. Curious weevils come into contact with the dollop, but instead of mating, they are exposed to a lethal amount of insecticide, which kills them.

The orchard is not sprayed with insecticide in this highly targeted method for killing weevils; rather, several hundred point sources are laced with pheromone-insecticide. These point sources are placed on trunks, rocks, fences, and other surfaces; weevils fly to the sources, interact with them, and die, Treehugger reports.