If you want to study ambrosia beetles in a laboratory, you’ve got to have a way of not only keeping them alive but also encouraging them to thrive and reproduce. The simplest way of raising beetles in captivity has been to cut pieces of wood and place them in a jar with the beetles, effectively reproducing their wild habitat. But there are problems with this approach. The University of Florida’s Forest Entomology Lab has recently experimented with a new method that could become common.
The idea is to replace wood with a material that can be standardized, easily sterilized, customized to suit a particular experiment, and easily collect beetles from.
“Wood’s a little messy to deal with,” says Dr. James Skelton, a scientist involved in the lab’s experiment with a new method. “Opening it up, especially these xyloborus beetles that make extensive galleries, it’s hard to open that up without smashing the beetles inside.”
The new alternative involves mixing agar and sawdust with milk protein casein, wheat germ oil and a salt mixture. It forms a soft, wood-like substance that can be poured while hot into a vessel, where it solidifies.
The Carillo Lab at the University of Florida conducted experiments with this method for years, publishing a number of papers and establishing the model now being used by the UF Forest Entomology Lab. The Carillo Lab, in turn, was inspired by early experiments by Dr. Louela Castrillo of USDA.
Sawyer Adams, the Forest Entomology Lab’s manager, helped make the lab’s first batch of Castrillo media and will be producing it for the lab going forward. She says that the substance is simple to make.
“Overall a takes few hours,” Adams says. “You mix it up, autoclave it, pour it, and then let it dry in the hood for week.”
Making a beetle media from agar and sawdust is nothing new — the lab has used that form of media for years as a sort of temporarily housing to keep beetles alive on plates during shipping. Few species thrived in it for long. The new addition of milk protein, wheat germ oil, salt and trace minerals takes a page from other areas of entomology where those substances are used to improve the nutrition of lab-reared insects. But it is something of a mystery as to how it is helping ambrosia beetles, which are thought to feed exclusively on fungus that they farm in their galleries.
“It seemed a little strange to me because the whole thing about ambrosia beetles is that they eat only their fungus,” says Dr. Skelton. “So why would you want to make a media that has a bunch of stuff in it that’s nutritious for beetles if they aren’t eating the media? It seems like you would want to make a media that’s good for fungus with malt extract or whatever. But for whatever reason this media works really well.”
It isn’t clear how or whether the added nutrition reaches the beetles. Are they eating the media directly, or are the nutrients mediated through the fungus? We don’t know yet.
The results from the Castrillo media impressed scientists at the UF Forest Entomology Lab very quickly.
“It’s easy and it’s fast,” says Skelton. “We’re producing dozens of progeny per initial beetle in a month. It’s like Drosophila, almost. The galleries looked nice and clean with tons of ambrosia growing in there. I think it could be a super powerful tool for testing hypotheses about what maintains particular relationships between certain fungi and certain beetles. Whether there is any adaptive value to beetles having multiple symbionts or one symbiont.”
Ambrosia beetles can be a powerful model for the study of evolution. With thousands of species radiating from multiple instances of convergent evolution, combined with their rapid rates of reproduction and close relationships to specific fungi and plants, ambrosia beetles allow scientists to test ideas about evolution far faster than is possible with mice and in more complex situations than using fruit flies.
“With this method, it’s easier [than with wood] to get everybody out,” says Dr. Skelton. “And presumably we can introduce all sorts of things to the media. Control the amount of nitrogen that’s available or phosphorus or carbon… whereas with wood it’s got what it’s got.”
