When Bt — or Bacillus thuringiensis — was introduced more than 30 years ago, there were good reasons to be optimistic about this new material.
These bacteria were very effective in controlling specific groups of insects and were considered more benign to the environment and beneficial insects compared to traditional pesticides.
Bt also expanded the insecticide arsenal — an important factor in trying to curb the inherent ability of insect populations to develop resistance over time to specific chemicals and other biological controls.
Bt's strong attributes still hold true today.
However, the rapid adoption of using genes from Bt to successfully develop insect-resistant crops does present a challenge to researchers and plant breeders.
For example, relying too heavily on Bt could allow any insects with a natural resistance to Bt to survive and reproduce more generations of resistant pests.
This is a key reason why planting a non-Bt refuge has become an important practice in insect resistance management, according to rules established by the Environmental Protection Agency (EPA).
These refuges reduce the chance of Bt-resistant pests mating with other Bt-resistant target pests and help preserve genetic diversity. Avoiding such refuge practices will only hasten the spread of insects with Bt resistance, according to researchers.
Besides planting refuge areas, searching for additional bacteria that can provide the same or better insect control than Bt has also been an ongoing effort — one driven by the need to offer producers more options and help protect the useful lifespan of Bt and other insecticides.
One promising alternative to Bt that has captured the attention of researchers has been Photorhabdus luminescens — a bacterium that produces a family of highly potent insecticidal proteins that can kill a wide range of insects from cockroaches to boll weevils, including white grubs, corn rootworm and other subterranean insects.
“It has also been effective against the Colorado potato beetle — a tenacious insect with a big appetite and a reputation for its ability to quickly develop resistance to insecticides,” says Phyllis Martin, microbiologist for the ARS Insect Biocontrol Laboratory in Beltsville, MD.
The insecticidal proteins from this bacterium were first announced in 1998 when a team of scientists from two laboratories at the University of Wisconsin, Madison, along with scientists from Indianapolis-based DowElanco (now Dow AgroSciences) published their findings in the journal Science.
The Photorhabdus bacterium was described as a potent pathogen. It was also characterized as being a widely dispersed, multiple-strain bacteria that lived symbiotically inside of soil-dwelling roundworms or nematodes.
The bacteria live inside the gut of nematodes that invade insects. Once inside an insect host, the bacteria are released from the nematode and kill the insect. The bacteria also begin reproducing — a process that turns the insect into a “protein soup” or food for large numbers of nematodes.
The corpses left behind by the bacteria glow in the dark since the microbe produces luminescent proteins in addition to potent insecticides. Hence, the species name luminescens.
Since its initial discovery, efforts continue today to bring this new bacterial insecticide into the marketplace.
Dow AgroSciences is actively exploring the utility of insecticidal proteins from Photorhabdus luminescens and related bacteria for the control of insect pests, according to Tom Meade, research scientist and group leader, Dow AgroSciences.
“These proteins are different from those used in currently registered insect-resistant crops and offer the potential to provide the next generation of insect-resistant crops,” says Meade. “We hope to be able to offer producers a broad insecticidal spectrum and resistance management option.”
Meade also says progress is being made in understanding how to effectively use these genes and proteins. However, the research is still in the early stages of product discovery and development.
“Our research group at Dow AgroSciences has been — and will continue to be — active in communicating our progress to the research community,” he says. “We also plan to communicate our progress to producers as we reach meaningful milestones.”
David Bowen, an entomologist at the University of Wisconsin, Madison, who was involved with discovering Photorhabdus, says the potential for using this new bacterium is great since it has very interesting and unique insecticidal properties.
“We have found at least seven different types of insecticidal proteins produced from these soil-borne bacterium carried by the nematodes,” says Bowen. “These bacteria carrying nematodes are very prevalent and I'm sure there may be more unique and useful strains to uncover. We're just scratching the surface and we're trying to better understand how they work since they're a diverse group of bacteria — more complicated than Bt toxins.”