Jhalendra Rijal, 2014 Friends of IPM Award Winner
Graduate Student, Ph.D. category
Not many of us think about protecting grapevines from insects when we’re enjoying a glass of wine, but Virginia Tech Ph.D. student Jhalendra Rijal has made it his mission to help growers find ways to save their vines from the grape root borer. His work with the grape root borer, which has given growers new sampling methods and paved the way for other control options, earned him a Friends of Southern IPM Graduate Student Award.
Grape root borers take two to three years to develop from egg to adulthood. Once hatched, the larvae burrow down in the soil and bore into the roots. They come back up to the soil surface to pupate, and adults emerge 35 to 40 days later. Growers usually do not know that borers are present until vines begin to die. By the time that growers realize they have a problem, it’s too late for control to make much of a difference.
The organophosphate chlorpyrifos is the only insecticide registered to control grape root borer and typically has been used as a rescue treatment against severe infestations, creating a toxic barrier to newly hatched larvae seeking roots in the soil. However, many growers are reluctant to use chlorpyrifos in this manner due to perceptions of its negative impacts on soil biodiversity and associated effects on vine health, berry quality, and ultimately, wine quality. Also, for southern-most states such as Florida, where peak adult emergence activities can extend to October, chlorpyrifos is not an option because of its 35-days pre-harvest interval (PHI) restriction.
So in 2009, Rijal began searching for ways that might alert the grower to the presence of borers in the soil. First, he examined the level of infestations in 48 different vineyard blocks by using pupal cases as a sampling tool and found a substantial variation in the degrees of borer infestation among those blocks. Then, he started examining the different variables such as grape rootstocks, cultivars, age of the vines, insecticide use, irrigation, soil-related and other factors fto see how they contributed to the level of infestation among these vineyards. His examination showed that two environmental variables, water holding capacity and clay/sand ratio of the soil, were most strongly associated with the infestation status. Interestingly, none of the horticultural and cultural factors were significant in explaining the differences among the blocks sampled. By using the two environmental factors, Rijal was able to develop a risk prediction model, which may enable grape growers to predict the relative risk of grape root borer infestation.
“Using two variables in the model, we can predict the likelihood of the vineyard to be infested by the grape root borer,” he said. “We can also use it for new vineyard sites. The grower can use the model to see if there is any risk to establishing a vineyard. If you have the information about soil, clay-to-sand ratio and water holding capacity, you can tell whether the plot will likely be heavily infested (i.e. >10 pupal cases/100 vines) or not. This information should help guide grower decisions about appropriate levels of monitoring grape root borer in their vineyards.”
Toward developing a sampling package for grape root borer population assessments, he also used the pupal case sampling data to determine whether grape root borer infestation is grouped in certain area(s) within a vineyard block or not, and found that grape root borer were indeed aggregated under high infestation.
“This is very crucial information for developing a reliable and accurate sampling package that will ultimately be used to assess borer populations,” said Rijal.
The next challenge was to find an easier way to sample the number of pupae needed to determine the level of infestation. To measure the threshold, growers count the number of pupal cases on the ground near their vines. However, without knowing when the larvae were returning to the surface to pupate, growers had the cumbersome job of walking through their field every week during the growing season and looking for pupa cases.
Over a period of several months, Rijal monitored fields that he knew were infested to determine when peak pupating periods occurred.
“We found that the peak periods of pupae exuviae was 3 or 4 week periods from the second or third week of July through the first week in August,” he said. “If growers sample during a 3 week period during that time, they will get 63 percent of the total population, so they can calculate the season total. It’s very practical and adoptable for growers.”
With the results of his research, Rijal and his advisor, entomologist Chris Bergh, were able to put together an efficient sampling package for growers, with recommendations on when and how to conduct grape root borer infestation status sampling.
Once Rijal and Bergh found ways to help growers predict and assess borer populations in their field, they wanted to find ways to help control borers and give growers more reliable and environmentally less harmful options to chemical insecticides. For that, Rijal conducted several lab-based experiments to study food finding behavior of newly hatched larvae. Since root borers begin searching exclusively for grapevine roots as soon as they hatch, Rijal wanted to know whether the roots produced something (i.e. chemical compounds) that attracted the larvae from the distance in the soil. Based on a series of tests using grape root, grape root extract and volatiles directly collected from the roots, he found that grape root borer larvae are attracted to all of these sources from the distance, confirming the role of grape root-specific volatile compounds. Further, characterization and identification of the exact volatile compound is ongoing in the USDA Agricultural Research Service research lab in Beltsville, Maryland. If he and other researchers could isolate and identify those compounds, they could begin developing a chemical trap to lure borers into before they reached the plant roots.
“Now, we need to analyze the chemicals to get the right compound to use in pest control,” said Rijal. “There are precedents in the literature for the use of host-plant compounds to disrupt food-finding by other root-feeding insects and we could employ a similar strategy for grape root borer as well.”
Rijal also studied grape root borer larvae movement behavior in the soil.
“This is important information for the ultimate development of behaviorally based management tactics,” Rijal added. “We found that the larvae can move horizontally and vertically, up to 120 cm in the soil. They can survive for more than 48 hours without food and can perceive a grape root stimulus from 5 cm away.”
Rijal accepted his Friends of Southern IPM award March 4 at the Southeastern Branch ESA meeting in Greenville, SC.
His Master’s degree work was also focused on IPM-based management of chickpea pod borer by the use of insect pathogenic fungi. He also led organic pest management trainings and other outreach activities for vegetable growers.
Before his doctoral work at Virginia Tech, Rijal worked in Nepal as a Plant Protection Officer, where he worked on applied research and extension of plant protection technologies to the growers including conduction of IPM-Farmer’s Field School.
Sadly Rijal will not see the final fruits of his work on food finding by grape root borer as a doctoral student, as he has recently left for the University of California at Davis, where he will be doing post doctoral work on the mint root borer, another belowground insect.
“It’s been a great experience working with Virginia vineyard growers,” he said. “Many growers are very interested and ask for details about how to pursue the sampling. It’s important for me to have their cooperation.”