Symbiotic relationship between two invasive species in Texas could pose threat to American sugarcane production.
Invasive species are becoming increasingly widespread and problematic, represent robust taxa, and can cause immediate and irreversible damage to entire ecosystems. Invertebrate pests are of special concern because they may transmit pathogens that threaten the environment, the economy, and can cause extensive agriculture damage through foraging events. Further, their small size decreases our chances of early detection and control, while increasing their own rate of spread, establishment, and potential harm.
The red-streaked leafhopper is a small (3-4 mm) but strikingly beautiful insect whose native range includes the steppes of Europe and Asia, spanning from the Mediterranean to Thailand. It feeds on vascular plant species; and leafhoppers are known to forage from the most dominant plant in the immediate ecoregion. They feed by piercing and taking (via siphoning) nutrients from the phloem of grasses and crops, like sugarcane. In large populations, the red-streaked leafhopper can cause direct and indirect damage to crops. åÊIndirect damage mayåÊoccur if the leafhopper transmits a pathogen to the crop, while direct damage can occur during feeding events (physical damage to the plant). One documented indirect threat posed by this leafhopper in its native range is its ability to transmit phytoplasmas to several crop species. Phytoplasmas are bacterial pathogens that attack many different species of plants, and are spread plant-to-plant as phloem-feeding insects (like leafhoppers) pierce the phloem and inject the phytoplasmas into the plant’s vascular system.
One phytoplasma of great concern is phytoplasma group 16Sr11, the causative agent of sugarcane white-leaf disease (SCWL). The first visual symptom of SCWL is white lines appearing down the mid-rib of the sugarcane plant. As the disease progresses, it causes the plantÛªs leaves to stop producing chlorophyll, after which the crop produces unusable canes and the plant eventually dies. Unfortunately, once SCWL and the red-streaked leafhopper are present, it is impossible to remove the pathogen and its effects from the crop. Within leafhopper populations, the phytoplasma is able to transmit from mother to eggs (ovarial transmission), so the leafhoppers can perpetuate the phytoplasma even if sugarcane is no longer present. In Thailand, SCWL can cause crop losses of up to 100 percent, and the red-streaked leafhopper is one of its top carriers. This insect and the pathogens that it spreads are destructive in other countries, and even pose a potential threat to sugarcane-producing areas within the United States (Louisiana, Hawaii, Florida and Texas).
Several federal, state, and local organizations foster and fund survey programs to detect invasive species. However, survey efforts can be costly, require extensive person-hours, and are often a lengthy process, lasting several months. In 2011, the Texas State University System (TSUS) established the Texas Invasive Species Institute (TISI) at Sam Houston University in Huntsville, Texas, to initiate research and coordinate effective early detection and rapid response to invasive species that currently affect, or have the potential to affect, Texas ecosystems. Our first survey efforts included baseline studies on undocumented invasive pest species in Texas. The invasive red-streaked leafhopper (Balclutha rubrostriata: Hemiptera: Cicadellidae) was one of our first 2011 encounters, and our initial goals were to determine whether the red-streaked leafhopper was firmly established in Texas and Louisiana and, if so, to document its range expansion toward sugarcane crops.
A survey conducted in 2010 first recognized that the red-streaked leafhopper occurred in Texas. It was collected in Bexar County as the dominant insect, making up nearly 85 percent of samples collected. However, its success and distribution across the state was not documented. We started our survey efforts in 2011, which included two Louisiana parishes and 17 Texas counties along the Highway 59 and Highway 190 corridors from Louisiana to south Texas. We found a high prevalence (overall mean 40.2 percent; range 25-89 percent) of the red-streaked leafhopper in 15 Texas counties, and a single specimen in one Louisiana parish. More surprisingly, we began to notice a positive correlation between the red-streaked leafhopper and King Ranch bluestem (Bothriochloa ischaemum var. songarica). Likewise, the red-streaked leafhopper was survey-absent from other grasses.
King Ranch bluestem is an invasive perennial grass with the same native range as the red-streaked leafhopper (although it is unknown whether the red-streaked leafhopper is associated with the bluestem in its native range). King Ranch bluestem was imported into the United States and planted in south Texas (King Ranch) in the early 1900s because of its high forage potential for livestock. åÊThen, following the 1970s Clean Water Act, the Texas Department of Transportation planted King Ranch bluestem over thousands of miles of Texas highway and byway roadsides to prevent soil erosion. Since then, it has aggressively spread, and now outcompetes native grasses that are more nutritious for livestock as well as roadside wildflowers. Through intentional and unintentional spread, King Ranch bluestem is currently present in the majority of the state and in several other states including Oklahoma and the countryÛªs top sugarcane producer, Louisiana. åÊ
Since our initial survey suggested a positive relationship between King Ranch bluestemåÊand the red-streaked leafhopper, we refocused our project goals and surveyed a broader range of possible host plants as well an increased our geographic survey breadth. In 2013, we surveyed the insect fauna on more than 10 grass species including invasive grasses such as King Ranch bluestem and Johnson grass (Sorghum halepense), and native grasses such as purple-top (Tridens flavus) , silver bluestem (Bothrichloa saccaroides) and big bluestem (Andropogon gerardii) within 52 additional Texas counties and 2 Louisiana parishes. åÊAdditionally, we focused on the three major sugarcane-producing counties of south Texas (Hidalgo, Willacy and Cameron).
We found a high prevalence of the red-streaked leafhopper in 13 new counties including those known to grow sugarcane. Furthermore, we only found the red-streaked leafhopper when we sampled from King Ranch bluestem, confirming our original hypothesis. åÊ
During our survey efforts, we observed King Ranch bluestem to be the most common grass abutting sugarcane fields, and although we readily collected the red-streaked leafhopperåÊfrom King Ranch bluestem up to the very edge of sugarcane fields, we found no evidence of the red-streaked leafhopper within crop boundaries. Since we know the red-streaked leafhopper transmits the pathogenic phytoplasma that causes SCWL to sugarcane crops in their native range, and that the pathogen can persist through several generations, we used standard molecular (PCR) techniques to see if they were carrying the pathogen to sugarcane growing areas of Texas. All samples tested were negative for the phytoplasma.
In 2014, we initiated a continuation and further expansion of our survey efforts and documented the leafhopper in five out of 12 new counties. Fifty-eight of 93 Texas counties that were dominated by King Ranch bluestem contained a high prevalence of the red-streaked leafhopper and one Louisiana parish contained one individual leafhopper. In both states, there was a positive correlation between the invasive red-streaked leafhopper and the invasive King Ranch bluestem. We conducted molecular tests on a subsample of leafhoppers and did not find the phytoplasma that causes SCWL (even in areas adjacent to sugarcane fields). It is uncertain whether the phytoplasma is currently carried by the leafhopper in Texas but continual testing is necessary. Since it is documented that red-streaked leafhoppers can maintain the virus for generations, even in the absence of sugarcane; testing for SCWL should occur in red-streaked leafhopper populations regardless of their proximity to sugarcane fields.
Although the density of King Ranch bluestem populations is significantly reduced in east Texas and Louisiana, small stands were noted near the heart of the Louisiana sugarcane producing areas. If the association that we observed between the insect and the grass is important for maintaining the red-streaked leafhopper, then that may explain why the insect has not readily dispersed into Louisiana, but is widespread and established throughout Texas. The relationship between these two established invasive species may be advantageous to our understanding of invasive species and their relationship with a familiar host plant, but in a new environment.
We observed a gradient of King Ranch bluestem establishment along the Texas gulf coast regions, east Texas, and southwestern Louisiana. That is, south Texas (and the associated Gulf Coast regions) appears to have the highest rate of King Ranch bluestem encroachment. However, this gradient diminishes eastward through the eastern regions of Texas and western Louisiana. The red streaked leafhopper mirrors this gradient, suggesting some relationship. If the red-streaked leafhoppers in the United States started carrying SCWL to our sugarcane crops, especially in Louisiana, there could be devastating economic effects. Predictive modeling and impact analyses have shown this insect/phytoplasma complex would cost the country more than $70 million dollars in five years, with the financial losses growing exponentially each year.
The overwhelming presence of the red-streaked leafhopper in many of our samples suggests that it outcompetes native leafhoppers in King Ranch bluestem-infested grasslands. Couple the large populations of King Ranch bluestem and red-streaked leafhopper with the leafhopperÛªs ability to transmit a devastating phytoplasma to an economically important crop, and this insect/grass/phytoplasma complex clearly becomes a potential threat to sugarcane crops in the Texas and Louisiana.We plan to continue red-streaked leafhopper surveys as well as molecular detection efforts for SCWL disease, study the coexistence of King Ranch bluestem and the red-streaked leafhopper, raise awareness through education and outreach, and look for creative management solutions of this complex before sugarcane white-leaf disease arrives.
Autumn Smith-Herron is the campus director of the Texas Invasive Species Institute, located at Sam Houston State University in Huntsville, Texas.
Ashley Morgan has been a research associate and education/outreach coordinator at TISI for the past five years.