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Corn and the battle with bugs [Margaret Evans]

A fascinating research study has shown that, when corn seedlings are nibbled by caterpillars, they send out scent compounds to attract parasitic wasps that will come to their rescue. The wasps know that the scent is from the smell of nibbling larvae. They descend on the corn where they lay their eggs in the caterpillars and the young wasps consume their internal organs before emerging from the insect.

This is a pretty dramatic defense strategy by a plant against a destructive insect. The study on this three-way relationship was done by researchers at Martin Luther University Halle-Wittenberg in Germany and the Boyce Thompson Institute (BTI) at Cornell University, with the assistance of a USDA-Agricultural Research Station scientist.

The researchers used 26 corn varieties to identify the genes responsible for sending out an SOS signal. Those scent compounds are called terpenes. But the researchers found that those corn varieties produce terpenes in a variety of mixes and amounts and not all of them are as effective as they need to be to signal help. The locations of the genetic links were mapped on their chromosomes.

“With this information, we have done a mapping analysis so we can find out which gene is important for which volatile,” says Annett Richter, lead author of the study published in The Plant Cell. “It's interesting to know where those genes are localized because then you can cross that variety to lines that are not able to produce them.”

The thought is that, by breeding for the most effective genes for terpene production, breeders can develop corn varieties better equipped to protect against caterpillars.

The next step for Richter is to study how these genes are regulated.

“The aim is to improve the volatile signals of their natural defenses, and now breeders have the opportunity to use those defense genes,” says Richter.

Pest control via diversity

Another study at the University of California, Davis looked at the value of managing farmland on the basis of crop diversity as a key step toward pest control. It all comes back to the vulnerability of monocultures in which vast acres are planted to one crop where a pest insect can draw all the food it needs from one source. But if a field contains a variety of crops it won’t be able to offer the volume of food for insects to thrive.

“A monoculture is like a buffet for plant-eating insects where every dish is delicious,” says William Wetzel, a doctoral student in population biology and lead author of the study published in the journal Nature. “A variable crop is like a buffet where every other dish is nasty.”

According to the press release written by Kat Kerlin, researchers examined 53 species of insects which included caterpillars, grasshoppers, beetles, aphids and flies. Applying some mathematical laws to the data collected, they calculated how plant diversity influences plant-eating insects.

Globally, many small farms grow a diverse mixture of plants. But in most monocultures, the plants are bred to be as identical as possible. To address the needs of larger growers wanting to introduce diversity but still maintain production levels, Wetzel suggests the introduction of a variety of genotypes of the same crop species with different nutrient levels. The parts of a plant consumed by humans (the head of broccoli or corn cob) remain identical but the leaf parts the insects eat could vary. According to Wetzel, this kind of genotype mixing is already being done on some rice and wheat fields to reduce disease spread.

And yet another study done in 2015 at Penn State University’s College of Agricultural Sciences shows that a caterpillar feeding on corn leaves can trick the plant to lower its defenses allowing the insect to eat more. Fall armyworm larvae eat copious amounts of corn leaves and defecate in the deep hollows where leaves and stalk meet. The deception is that the feces, or caterpillar “frass,” have chemical compounds from both the host plant and the insect itself that signal to the plant it’s being attacked by a fungus, not an herbivore.

“The plant perceives that it is being attacked by a pathogen and not an insect, so it turns on its defenses against pathogens, leaving the caterpillar free to continue feeding on the plant,” said Swayamjit Ray, a doctoral student in the intercollege plant biology program. “It is an ecological strategy that has been perfected over thousands of years of evolution.” It cannot protect against both at the same time.

The research, which was supported by the US Department of Agriculture, was published in Chemical Ecology.

Vol.103 Issue 1
JANUARY 2017

Bright Greens