For Enakshi Ghosh it was love at first sight.
“I was taking a tour at the National Bureau of Agricultural Research Insect Resources Museum in India, and the director showed me the tiny egg parasitoid under the microscope, and I cannot tell you how cute that wasp was,” said Ghosh, a Fulbright-Nehru Postdoctoral Researcher at Colorado State University’s Natural Enemy Ecology Lab in the College of Agricultural Sciences.
After that, Ghosh was hooked and knew her research would focus on parasitoid wasps and their invaluable service to agriculture.
“Everybody loves bees because they’re pollinators and cute, and they fear wasps because they think all they do is sting,” Ghosh said. “But wasps are very important, too.”
In addition to also being excellent pollinators, parasitoid wasps — think the microscopic variety rather than the big yellow jacket kind — are excellent at pest control, particularly against some of agriculture’s greatest enemies.
Ghosh — along with CSU agricultural biology professor Paul Ode and postdoctoral researcher Ryan Paul — recently published a study analyzing how host plant chemistry enhances herbivore cellular immunity with differential effectiveness against two parasitoid species in the journal Functional Ecology.
A balanced diet
Insect herbivores must simultaneously balance the effects of plant defensive chemistry with the top-down effects of natural enemies. At the intersection of these effects are herbivore immune systems, a trait that has been overlooked in studies of plant-insect interactions, Ghosh said.
Cruciferous plants — such as cabbage, rapeseed, horseradish and mustard — have a special defense strategy against insects called the “mustard oil bomb.”
While it sounds nefarious, it’s actually where the plant stores glucosinolates — chemical defenses that react with an enzyme when caterpillars feed on it and damage the plant tissue. The higher the amount of glucosinolates, the more toxic the plant is to the caterpillar.
Selecting plant varieties based on how much of a “mustard oil bomb” can be produced by them has been a key strategy for growers in keeping away pest insects in agro-ecosystems, Ghosh said. But while this is effective against most pests, the cabbage white butterfly — one of the world’s most invasive and costly pests to commercial growers of cruciferous plants — is immune.
Ghosh likens it to the way a specific diet can improve or worsen the human immune system.
“Caterpillar immunity depends on the plants they eat,” she said. “We found that the cabbage white butterfly caterpillar has a cool evolutionary mechanism in which they actually utilize the toxin from plants to improve their immune system. This enhanced immunity can give them better protection against many natural enemies.”
An unexpected hero
Among those enemies are parasitic wasps, minute insects that lay their eggs inside or on the caterpillar where the developing wasp acquires all the nutrients from the caterpillar’s body, eventually killing them.
Not all parasitic wasps are successful biocontrol agents, Ghosh said. One reason is because the caterpillar’s immune system is composed of blood cells called hemocytes. These immune cells form hardened capsules around the parasitoid eggs, ultimately asphyxiating the parasitoid.
But wasps can lay single eggs (solitary) or multiple eggs (gregarious) and attack a specific caterpillar group (specialist) or multiple caterpillar groups (generalist).
In the case of the cabbage white butterfly caterpillar, researchers have identified that its immunity does not protect it from specialist or solitary parasitoids, Ghosh said. These particular parasitic wasps can inject venom or viruses with their eggs that are capable of dismantling the caterpillar’s immune system.
Using the right wasp for the job
As biocontrol agents are used to control pest insects, knowing both the parasitoid identity and the concentration of plant toxin will play a critical role in the success or failure of those agents, Ghosh said.
One example she gave compared mustards (which have a high toxicity) and collards (which have a low toxicity). While both types of parasitoid wasps would be effective in fighting an invasion of cabbage white butterflies in a collard field, that same invasion in a mustard field would require the release of a specialist parasitoid wasp because the generalist’s defense mechanisms would be rendered useless by the caterpillar.
This type of fundamental research allows growers to take ecoimmunology into consideration when determining the best biocontrol agents while also reducing the need for pesticides, Ghosh said.
Taking the research out of the lab and greenhouse into the field is a critical next step to the study, Ghosh said.
“In the field we have many other contributing factors,” she said. “So doing the field work will be crucial. This way, holistically, we can develop integrated pest management.”
In addition, Ghosh said she hopes to improve the acceptance of the much-maligned wasp, at least the parasitic kind, as a natural form of pest control.