By Sara Hendery

Gall-inducing insects can be valuable natural enemies—especially in tackling invasive weeds—due to their narrow host range and low impact to non-target species. A new study reveals, however, that gall size and abundance can be dependent on elevation, which has important implications for the effectiveness of gall-inducing insects as biocontrol agents.

Infestation by galling insects results in the formation of plant galls, or small shelter-like nodules that provide nutrition and protection for an insect. Evidence shows that persistent and high populations of galls can suppress plant health. This includes invasive weeds, which can spread rapidly and negatively impact native species. Hundreds of thousands of different gall-inducing insect species exist around the world, among them the eupatorium gall fly (Procecidochares utilis).

New research published in April in the journal Biocontrol Science and Technology examines the release of P. utilis against the noxious invasive weed Ageratina adenophora along varying elevations in Nepal. Conditions in Nepal are ideally suited for studying the impact of invasive species, as the country contains unique terrain, including a climatic range that varies from conditions like the Florida Keys to conditions like the Arctic in a geographic span of just 180 kilometers. This is the first study that has analyzed the effect of elevation on P. utilis gall abundance and size. Researchers from Tribhuvan University in Nepal, as a part of a Feed the Future Innovation Lab for Integrated Pest Management project, surveyed the distribution of galls along each major elevation belt in the country, including the highest and lowest elevational range of A. adenophora distribution. Length and breadth of each gall was measured.

The study showed that peak gall abundance on A. adenophora existed at the mid-elevation range, with galls absent at the highest and lowest elevational distribution range of the weed. The results can be explained by the phenomena that species abundance is often highest at the center of its range where the most favorable biotic and abiotic conditions occur.

“Gall abundance is one of the key factors to gall effectiveness as biocontrol agents,” says Pramod Jha, Ph.D., professor at Tribhuvan University and a co-author on the study. “Ageratina adenophora is an aggressive weed that grows very rapidly throughout Nepal, so understanding the dispersal and quantity of its natural enemies is vital to combatting it.”

A. adenophora, also known as the Mexican devil or Crofton weed, thrives in warm, moist climates and is one of Nepal’s most destructive weeds. Invading many of the Southeast Asian country’s tropical and subtropical forests, the weed easily outcompetes native vegetation, decreases the carrying capacity of pastureland, and is poisonous to some livestock, especially horses. Nepal is one of many countries that has experienced its negative impacts—in the 1950s in Australia, the invasion of A. adenophora was so insurmountable that it led to the forced exodus of many farmers from their land.

published in April in the journal Biocontrol Science and Technology examines the release of the eupatorium gall fly (Procecidochares utilis) against the noxious invasive weed Ageratina adenophora along varying elevations in Nepal. Conditions in Nepal are ideally suited for studying the impact of invasive species, as the country contains unique terrain, including a climatic range that varies from conditions like the Florida Keys to conditions like the Arctic in a geographic span of just 180 kilometers. (Photo courtesy of Feed the Future Innovation Lab for Integrated Pest Management)

The Nepal study showed that gall size also varies based on elevation. The average diameter of galls made by P. utilis peaked at mid-elevation range, with diameter of galls decreasing at the highest and lowest elevations.

As both gall abundance and gall size peaked at mid-elevation, these results suggest that mid-elevation sites are optimal for biocontrol of A. adenophora by P. utilis.

“We have seen that elevation is one of the most important abiotic factors that affects gall abundance and gall size,” says Anju Poudel, a Ph.D. candidate at Tribhuvan University and lead author on the study. “Knowing that both gall abundance and size peaked at mid-elevation, we can surmise mid-elevation has prime conditions for the growth and fitness of P. utilis, thus, enhancing its effectiveness.”

In addition to examining the impact of elevation on galls, the study also demonstrated the general affect galls have on plant life. Gall inducing insects are capable of intercepting or redirecting nutrients of the host plant to the galls themselves, making the galls “nutrient sinks” that the insect feeds on. As evidenced by the study, the concentration of three nutrients—nitrogen, phosphorus, potassium—were higher in galls than in the A. adenophora stem section below the gall and normal weed stems without the gall, further solidifying that galling by P. utilis reduces the vegetative and reproductive growth of A. adenophora.

The spread of invasive weeds puts at risk human and animal health, economies, and environments. Almost half of threatened species are at risk due to invasive species, and the risk is higher than ever due to increases of climate change, human mobility, and globalization. Nepal’s biodiversity in particular is some of the richest in the world, with over 100 different ecosystems traversing a short horizontal span.

Poudel says Nepal’s unique conditions are exactly why studying the impact of P. utilis on A. adenophora is valuable and urgent.

“Developing countries are especially vulnerable to the spread of invasive weeds, with less infrastructure to manage the threat,” she says. “Without understanding and implementing measures like biocontrol that can offer a safe and economical pathway to tackling them, some of the most biodiverse hotspots in the world will be threatened, as well as the food security and livelihoods of those who depend on them.”

Sara Hendery is the communications coordinator for the Feed the Future Innovation Lab for Integrated Pest Management at Virginia Tech. She holds an MFA in nonfiction from Columbia College Chicago. Email: saraeh91@vt.edu.