Growing Magazine - December, 2012
Glowing Leafhoppers Help Facilitate Studies on Curly Top Viruses
Fireflies, sometimes called lightning bugs, don't fly in New Mexico, but that doesn't mean you won't spot some "glowing" insects in the state. Researchers at New Mexico State University (NMSU) are catching leafhoppers and applying fluorescent dust on them before releasing them.
This infected chile plant exhibits leaves that are yellowing and rolling.
Photos courtesy of Rebecca Creamer, NMSU.
The fluorescent dust applications are one of the newest techniques being used as researchers attempt to track just how far the beet leafhoppers (Circulifer tenellus) are likely to fly from the host weeds where they feed before making their way into New Mexico's chile fields.
The leafhoppers transmit the beet curly top viruses (often referred to by the more general term "curly top virus") to crops. A number of elements in the leafhopper's ability to transmit curly top virus have already been identified. Learning the distance leafhoppers are likely to fly will bring researchers a step closer to helping growers protect their crops from curly top virus outbreaks.
Effects of curly top virus
"Curly top viruses infect a very wide range of crops and weeds," said Dr. Rebecca Creamer, plant pathologist at NMSU and part of the team studying curly top virus transmission. The virus has been reported on sugar beets, peppers, tomatoes, dry beans, spinach and melons. The team of NMSU experts collaborates and shares information with a larger group from research institutions throughout the western states in an ongoing effort to find ways to help growers avoid the losses associated with curly top viruses.
Curtoviruses, including beet mild curly top virus, pepper curly top virus and beet severe curly top virus, have wide distribution throughout the arid and semiarid western U.S., so it's a concern for growers in that region. Curly top viruses came to the U.S. as sugar beets were brought into California in the early 1900s. The major crop losses are now in processing tomatoes in California.
"Pepper plants infected at an early stage are highly stunted," Creamer said. "Leaves are yellowed, thickened, crisp and rolled, and plants have reduced fruit set. Plants infected later show somewhat rolled, thickened leaves and stiff, brittle upper shoots and terminals."
While several strategies attempt to control curly top virus on peppers, minimal field resistance has been identified in commercially grown peppers. Insecticide sprays have been used in California to reduce leafhopper numbers, but spraying does not prevent virus transmission. Planting early or late to avoid peak leafhopper flights has been done, since young plants are more susceptible. Heavy seeding of directly planted peppers can be done in combination with late removal of infected plants during thinning. Removal of known weed hosts from crop fields may help reduce crop losses, and this weed identification and removal directs the NMSU team's studies on curly top virus transmission by leafhoppers.
Leafhoppers with fluorescent dust applied glow under UV light as they are tracked for flight distances.
Creamer noted that outbreaks of curly top viruses vary not only from year to year, but also from field to field, sometimes in adjacent locations. "We've seen fields where the crop loss may be about 2 percent of the crop and may be 30 percent in a field just down the road," Creamer said. While even 2 percent loss is significant to growers, 30 percent can be devastating.
Impacts on chile
"Chile is a signature crop to New Mexico," Creamer said. It has a large role in the state's agriculture and is also a major tourism feature.
While other crops contribute more than chile to New Mexico's agricultural economy, both red and green chiles are grown extensively and are major crops. They are sold at fresh markets and processed for canned and frozen chile, as well as salsa.
Host weeds identified
After about 10 years of studying curly top virus outbreaks and leafhopper transmission, a number of issues associated with leafhopper transmission of the viruses have been identified. Researchers have established certain connections and are able to help growers predict when leafhoppers will be more prevalent and thus able to transmit more curly top virus to crops.
Dr. Rebecca Creamer captures leafhoppers for fluorescent dust application to help track their movement.
An infected chile plant shows stunted growth when compared to a healthy plant.
Studies indicated that fields adjacent to fallow land or roadsides with heavy stands of weeds were more likely to have extensive numbers of leafhoppers. Once that determination was made, work began to identify which weeds were more likely to be host plants.
"We found that London rocket, a wild mustard, grew extremely well following our fall rains here in southern New Mexico," Creamer said. With London rocket identified as the winter host plant, further research identified kochia as the most likely host in summer. Although both weeds were identified as hosts, London rocket created the most concern. The leafhoppers feed on the London rocket and deposit eggs on the plant's leaves. Leafhoppers are thought to breed in southern New Mexico and Arizona in the spring and early summer, migrate north and east into cropping areas, and head back to the breeding grounds in late fall.
Close-up of infected chile, which shows signs of curly top virus.
London rocket begins to die off with the dry weather in spring, just as chile plants are growing, providing leafhoppers with a new food source. The virus is transmitted as they feed on the chile plants.
Armed with the knowledge of which plants leafhoppers are most likely to feed on, growers could begin identifying and eradicating those weeds in hopes of avoiding leafhopper infestation.
"Based on the amount of rain in the fall, we are able to predict how bad the leafhoppers will be in the spring," Creamer said.
With those predictions and host weeds identified, the next step is to determine how far the leafhoppers are likely to travel from the host weed sites to chile fields; that's where the glowing leafhoppers come into play.
Curly top virus signs are obvious when infected plants are compared to healthy plants.
"We collect leafhoppers with sweep nets and apply the fluorescent dust," Creamer said. "The leafhoppers are then released, and we begin tracking them as they are caught in sticky traps or again with sweep nets." Placed under UV lighting, the leafhoppers with the fluorescent dust are easily identified. A grid system has been developed to help researchers track the leafhoppers and determine how far they are likely to fly.
"Research has indicated that leafhoppers are able to fly good distances," Creamer said. The information obtained by tracking the leafhoppers brings researchers a step closer to helping growers determine how far chile fields must be from weedy areas to avoid leafhoppers moving from the wild hosts into their chile fields. This same information will help other growers, from major processing tomato producers to sugar beet growers.
Nancy Riggs is a freelance writer. She resides in Mount Zion, Ill.