Weevil on an avocado. Credit: Mark Lewis/UCR
Turns out, human COVID-19 patients aren’t the only ones subjected to quarantine. University of California Riverside entomologists Mark Hoddle and Sean Halloran recently locked up a batch of weevils at the university’s Insectary and Quarantine facility for atrocities committed to avocados.
Weevils are small beetles with long snouts that drill through avocados to lay eggs. The weevil grubs or larvae bore into avocado seeds to feed, rendering the beloved fruit and toast topping inedible. Weevils are native to Mexico, which posses a problem for the U.S.—specifically California—as avocado imports from the south are increasing at a steady rate. Right now, the risk of importing weevils along with avocados is dangerously high.
To avoid more crop-killing, invasive species, Hoddle and his team are working on ways to not only combat the weevil population in Mexico but also ensure they do not make their way to California.
One strategy for controlling pests is to introduce other insects that feed on them. However, the researchers say natural enemies of weevils are rare in areas where the pest is native. Additionally, weevils are reclusive, as they spend most of their time deep inside avocados, where they’re protected from both enemies and insecticides.
Given that set of circumstances, Hoddle has turned to pheromones as a control strategy. Working with Jocelyn Millar, a UCR insect pheromone expert, the scientists are now leading an effort to find the weevil’s pheromone, with the goal of using it to monitor and prevent them from mating in avocado orchards. Pheromones are chemicals produced and released into the environment by an insect that can be “smelled” by others of its species, and affect their behavior.
“We could flood avocado orchards with so much pheromone that males and females can’t find each other, and therefore can’t reproduce,” said Hoddle.
Additional control strategies could include mass trapping, using the pheromone as a lure, or an “attract-and-kill” approach, where the pheromone attracts the weevils to small sources of insecticide.
But first, the researchers need to identify the weevils’ pheromones—a task that is harder than it sounds.
“Weevil pheromones have complicated structures. When they’re made in a lab, they can have left- or right-handed forms,” explained Hoddle.
The entomologist recently traveled to a town three hours south of Mexico City that has a large weevil population. Using a special permit issued by the USDA, Hoddle brought weevils back to UCR’s Insectary and Quarantine facility.
At the facility, Hoddle and Halloran captured the chemicals that avocado weevils release into the air. Possible pheromone compound formulas were identified from these crude extracts and synthesized in Millar’s laboratory.
For the first batch, Millar’s group made a mixture of both left- and right-handed forms to see if the blend would work as an attractant since it is much less expensive to make the blend than the individual forms. However, subsequent field work back in Mexico with the pheromone cocktail was unsuccessful. Hoddle said they did not get a big response from the weevils, suggesting that one of the forms in the blend could be antagonizing the response to the other.
Now, the researchers are working with Millar to synthesize the individual forms of the chemicals and again test the insects’ response to each in Mexican avocado orchards.
“The tools we develop now can be used to make sure crops from any exporting country are much safer to import,” said Hoddle.