The pea leafminer (Liriomyza huidobrensis) is a pest of plants including peas, beans, potatoes, and spinach, and has spread from South America to Central America, Europe, Africa, and Asia. A new study on the species examines how temperature, host-plant range, and competition affect the pea leafminer’s range and how those factors can aid in predicting its invasive risk in other areas. (Photo credit: Merle Shepard, Gerald R.Carner, and P.A.C Ooi, Insects and their Natural Enemies Associated with Vegetables and Soybean in Southeast Asia, Bugwood.org)

By John P. Roche

As average temperatures rise globally, the ranges of many species will be affected. Climate-induced shifts in the ranges of invasive species will be particularly important because of the high economic and ecological impacts of these species. And predicting the extent to which temperature increases could affect the range of invasive species could offer practical benefits, such as allowing for more focused population screening and control.

A recent study on an invasive species of leafminer from South America, published Wednesday in the open-access Journal of Insect Science, examined how temperature, the range of host plants, and competition with a closely related species affect the range of the fly. Then, using their findings, the researchers made predictions about how global warming might affect expansion of this species.

The study, led by Genoveva Rodriguez-Castañeda at the University of Texas at Austin with colleagues in Guatemala, The Netherlands, and Sweden, examined factors that influence the range of the pea leafminer, Liriomyza huidobrensis. These flies are called leafminers because their larvae burrow through leaves between the upper and lower leaf surfaces, leaving tunnels or “mines.” The mines harm plants by impairing photosynthesis. Among 456 species of leafminer flies, L. huidobrensis is one of only three species that are serious pests to crops. L. huidobrensis is particularly damaging to peas, beans, potatoes, and spinach.

The pea leafminer originated in South America, then spread to Central America, Europe, Africa, and Asia. It is not found in the United States.Several factors have allowed the pea leafminer to spread rapidly around the globe: It eats a wide range of plants, it produces multiple generations quickly, and it has been spread in plants in an increasingly globalized produce trade.

The geographical range of a species is limited by its ecological tolerances to conditions in its environment. Rodriguez-Castañeda and colleagues reasoned that, if you determine these ecological tolerances, you might be able to better predict the risk of invasion of an invasive species moving to a new region as a result of a change in conditions. And knowing thermal tolerances, in particular, is valuable because it can help in predicting potential expansions of invasive species as a result of climate change.

In their study, the researchers looked at how the range of L. huidobrensis is shaped by temperature tolerance and presence of another species, the vegetable leafminer, Liriomyza sativae. They raised populations of L. huidobrensis in the lab and looked at success under different temperature regimes. They found that the upper threshold of temperature for successful development of L. huidobrensis was 28 to 29 degrees Celsius. They also collected L. huidobrensis in the wild and did not find any individuals in locations where the maximum temperature in the warmest month of the year was above 27 C. This temperature limit keeps L. huidobrensis from going below elevations of 1,000 meters in Guatemala.

The authors also examined which environmental factors affected occurrence of L. huidobrensis and L. sativae. They found that a mild maximum temperature in the hottest month of the year was the best predictor for the occurrence of L. huidobrensis. They found that precipitation in the coldest quarter of the year and the average temperature in the warmest quarter were good predictors for the occurrence of L. sativae. In addition, the absence of L. huidobrensis was a good predictor for the presence of L. sativae. L. huidobrensis has a narrower niche than L. sativae and is limited by high temperatures. L. sativae has a broader niche and is limited by low temperatures and by the presence of L. huidobrensis. As a result of the temperature tolerances, L. huidobrensis lives on plants in the highlands. As a result of interspecific competition with L. huidobrensis, L. sativae lives in the lowlands. In both China and Indonesia, L. sativae lived in mountain areas but was displaced to living only in lowland areas when L. huidobrensis arrived in the mountain habitats.

The pea leafminer feeds on more than 300 plant species, but this study found that reproductive success of L. huidobrensis is higher on some host plants, such as sweet peas and snow peas. The study also found that wasps that lay their eggs in the L. huidobrensis larvae—species known as parasitoid wasps—significantly reduce emergence of L. huidobrensis.

The study’s discoveries about the temperature tolerance of L. huidobrensis led the authors to conclude that it is unlikely that L. huidobrensis will invade and become established in the United States. They made this conclusion because most produce from South America enters the United States through Miami, and temperatures in Miami are above the temperature threshold for L. huidobrensis.

Genoveva Rodriguez-Castañeda says that two discoveries in the study stood out as most significant. “One was that the distribution of L. huidobrensis, a species that has long been regarded as a tropical invader, is primarily limited by high temperatures that seem mild during summers in temperate regions—that is, 29 degrees Celsius, which is approximately 84 degrees Farenheit,” she says. “The notion that distribution of montane tropical species is affected by high temperatures is one that keeps escaping science and makes scientists focus on cold tolerance of species. And, in this case, this focus on cold tolerance of species has skewed accurate predictions of the species’ future distributions with climate change.”

Rodriguez-Castañeda says the other most significant finding was that limits of temperature tolerance affect the interspecific ecology of species. “Typically we are familiar with studying interspecific interactions in a single location, and we assume that the same strengths and rules apply across species’ distribution range,” she says. “Here, we found that the outcome of competition between two leafminer species depends on the temperature of the location at which they co-occur.”

Regarding future directions for this research, Rodriguez-Castañeda suggests that, whereas most of the physiology of L. huidobrensis is already known, a great deal remains to be discovered about the species’ ecology. “One future research direction,” Rodriguez-Castañeda says, “could be to test if the distribution of L. huidobrensis is shifting towards areas with cooler temperatures as a response to warmer temperatures locally, regionally, and globally. It would be worthwhile to conduct studies of L. huidobrensis, snow peas, and the main parasitoid species at various points along a temperature gradient to test which temperatures show the greatest success of L. huidobrensis, of snow peas, and of the main parasitoid species.” This could further inform control efforts and help make predictions about changes in range as a result to temperature changes.

John P. Roche is an author, biologist, and science writer dedicated to making rigorous science clear and accessible. He has a Ph.D. in biology, has published more than 180 articles, and has written and taught extensively about science. For more information, visit http://authorjohnproche.com.