Pheromones Could Protect Avocados From Invasive Weevils

They call them "evil" weevils. Just ask any farmer who has tried to protect crops from the invasive insects.

Insecticides don’t work as these long-snouted beetles drill through the fruit to lay their eggs inside, so scientists are working on a novel way to stop them.

Specifically, researchers are hoping to control avocado seed weevils (Heilipus lauri) by using pheromones to disrupt mating. Pheromones are chemicals produced by many animals and plants that send signals to others of the same species. They can affect the behavior of those that “smell” them.

Scientists were particularly interested in how spreading the right pheromones could affect weevils in avocado orchards.

“Since insects use pheromones to find each other to mate with, if you flood an orchard with pheromones a couple of things can happen to reduce mating and hence fewer pest offspring are produced,” UC Riverside entomologist Mark Hoddle explains.

“Because there is so much pheromone in the orchard, weevils can't find each other because they are following false trails that originate from a pheromone release receptacle.”

And it’s possible that when there are constant, high levels of pheromones in an area, weevils may get used to it and not respond to it any longer.

“This is like a perfume/aftershave that you smell,” Hoddle says. “First, it is strong and obvious, but when it is present for a long time, you don't notice it anymore.”

Being able to stop these pests could have a major impact for growers. Avocados are grown by about 4,000 farmers on around 50,000 acres in California. Avocados are worth about $350 million per year.

Hoddle is working to fight avocado weevils in Mexico, while preventing them from being accidentally introduced in California.

Reclusive and Understudied

Researchers know very little about these weevils. They drill through the fruit to lay eggs, then the larvae bore into the avocado seeds to feed.

These reclusive insects are almost always deep inside the fruit, where they are protected from predators and insecticides. They are hard to spot and they are not well-studied. Researchers know that they are a serious threat to avocados, but don’t know much about them.

“The weevils are hard to control, because a large part of their life cycle (eggs, larvae, and pupae) is spent hidden inside the fruit where insecticides cannot reach these vulnerable life stages. Also, because the fruit is damaged and this is the part of the crop that is sold, tolerance for insect damage is very close to zero, as fruit will be downgraded or culled, and growers lose money,” Hoddle says.

“Consequently, control methods like insecticides may not be the most effective control options, and we should look for alternatives or supporting technologies (like pheromones) that can provide control and/or lessen grower reliance on insecticides (which also have unintended side effects—residues on food, accidental killing of natural enemies and other beneficial insects that live in orchards).”

Researchers are taking a proactive management approach. Although these weevils aren’t present in California, they are an obvious invasion threat. As more fruit is imported from places like Mexico, where the pest is native, or Colombia, where weevils are invasive, there’s a greater risk that weevils will be introduced. In addition, there’s a continuing issue with smuggled fruit, which is another means of accidental introduction.

“We are trying to get ahead of an obvious problem that sits just outside of California, so we would be ready to go should weevils be found in California, and not waste years of time doing the necessary work to develop management programs after the pest invades, establishes, spreads, and causes damage,” Hoddle says. “So why not do the work now? You can think of this as an insurance plan: You hope you don't need it, but good to have it just in case the worst happens!”

Other Strategies

Scientists have been testing various formulations of the pheromone weevils produce. They spread them in commercial avocado orchards that have significant weevil infestations. The pheromone “cocktails” were deployed through sticky traps of different colors, just in case colors affected how weevils responded.

They have not had a huge response yet, which suggests that one of the formulations could be having a negative impact on the effect of another. They are now testing the weevils’ response to individual forms of the chemicals.

Other possible strategies might include using the pheromone as a lure, either for trapping or to direct them to some insecticide.

“In this approach, weevils are attracted to the pheromone, but the pheromone is embedded in a matrix that contains a contact insecticide. Curious weevils interact with the dollop, but instead of finding a mate, the contact results in exposure to a lethal dose of insecticide that kills the weevil,” Hoddle says.

“This is a highly targeted approach that uses an insecticide to kill weevils, but the entire orchard is not sprayed. Instead, several hundred point sources of pheromone-insecticide are placed on trunks, fences, rocks, etc. in the orchard and weevils fly to the source, interact with it, and get killed.”