Evil weevil strikes back

“Fighting fire with fire” describes the philosophy of biological control. Classically, biological control involves the introduction of an alien predator to control an alien invasive species. While there have been success stories utilizing such biological control, a recently documented case demonstrates the ability of nature to outcompete attempts of human management. As Dr. Ian Malcom says in one of my favorite movies of all time, “Life..uhhh…finds a way.”

The Argentinian Stem Weevil has been damaging New Zealand crops, and subsequently livestock, for almost a century. First attempts to manage the pest involved plant resistance but Dr. Stephen Goldson, an adventurous entomologist, personally selected a parasitic wasp from South America as a biological control agent to combat the devastating weevil. The wasp lays eggs inside the weevil’s body. When the larvae hatch, they eat weevil from the inside out. Gross. The stuff of nightmares. Yet effective…for awhile.

After a boom in the 1990s, the effectiveness of the murderous wasp declined. Dr. Goldson, a thorough scientist, documented the decline in a paper published in PNAS this April along with NZ AgResearch mathematician Federico Tomasetto. They discovered that no matter when, where or how many wasps were released, resistance in the weevils emerged after about 7 years, strongly suggesting an evolutionary source rather than one caused by an environmental or experimental variable.

Remarkably, this is the first case of resistance to a macroparasitic (parasites that develop and do not multiply in the host) biological control agent in the field. In the lab, however, scientists have given us clues as to want might be happening. Using the fruit fly model, studies show resistance to parasitoids can occur through alterations in immunity or behavior. In the former case, the fly encrusts the parasitic egg in immune cells to block further development. In the latter, the fly changes it’s behavior to decrease the risk of being targeted as a host. Certainly either explanation could be occurring in the weevils and genetic tests are being conducted to identify the causative mutation.

Interestingly, this relationship has a positive control. The equilibrium of wasp-weevil in the native environment of Argentina favors the wasp. So what’s the evolution-favoring factor of the NZ weevils? Some think it may be the simple biodiversity or lack of other natural weevil-predators in the archipeligo. I haven’t mentioned yet another key clue; the weevils are much more likely to become resistant on a certain type of plant, Lolium perenne, also known as ryegrass, as opposed to common pasture grass. Current studies are exploring whether the plant directly influences weevil behavior. As a microbiologist, however, I can’t help but wonder if the root cause (pun intended) is a specific plant-associated microbe that influences the plant or maybe even the evil weevil itself to cause increased resistance.

Competition and advantage are crucial for natural selection. Resistance is simply natural selection against something humans want to work consistently: for example antibiotics or the biological control of pests. Studying and understanding examples of competition and resistance, such as the NZ weevils, can help us mitigate resistance and more successfully administer the biotechnologies in the future.

Thanks to Dr. Rebecca Hasley for the sangria and congrats on graduating medical school today! Enjoy happy hour y’all.

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