Over the last several years entomologists in Texas and across other southern states have been fighting a war against the sugarcane aphid, a menacing and persistent pest that threatens grain sorghum production. Though some success has been reported from development of specific types of pesticides, entomologists and integrated pest management specialists have been searching for more cost effective methods of control.
Thanks to a visiting Master’s student from Mexico, researchers in south Texas are now focusing on a new control method that may prove the most beneficial in the on-going war against the aphids.
According to Texas A&M AgriLife Extension, sugarcane aphids are currently one of the most important insect pests of grain and forage sorghum in Texas. SCA were first found feeding on sorghum near Beaumont in extreme southeast Texas in 2013. This sorghum-feeding SCA biotype developed because of a genetic change in the existing U.S. population, or was introduced into the U.S. from elsewhere.
That same year, the sugarcane aphid was also found in sorghum in the Rio Grande Valley and the Texas Gulf Coast, as well as in north Texas, southern Oklahoma, Louisiana, and Mississippi.
Sugarcane aphids survived the 2013 winter in south Texas and quickly spread throughout other regions of the state and in 12 other southern states during the spring and summer of 2014. In 2015, the troublesome aphids spread through Texas into Oklahoma and Kansas, eventually infesting 17 states. This area encompasses 90 percent of all U.S. sorghum acreage.
Extension specialists say the aphids can be economically controlled to prevent crop loss and harvest difficulties associated with the honeydew it creates, but growers must assess infestations frequently and use that information to properly time any needed insecticide applications, which can be costly. In recent years a number of SCA resistant hybrids have been developed, which helps in better managing aphid infestations but has failed to provide complete control.
Dr. Robert Bowling, an entomologist and assistant professor and Extension specialist in Corpus Christi, is working with Margarita Martinez, a Master’s student from Mexico, studying entomopathogenic fungi in the sugarcane aphid. Bowling says her research is producing some exciting results. New research indicates the potential for a new tool to use in SCA control and management, one that is greatly needed as the sugarcane aphid continues to propagate into new areas.
Identification and feeding habits
According to a Texas AgriLife fact sheet, sugarcane aphids are pale yellow, gray, or tan in color. The cornicles or tailpipes, feet, and antennae are black. The aphid feeds on the underside of sorghum leaves and though initial colonies consist of just a few aphids, they can eventually cover much of the lower leaf surface. These aphids do not feed on the upper leaf surface. They produce large amounts of honeydew, which collects on the tops of leaves below, making them sticky and shiny and problematic at harvest as equipment quickly becomes gummed up. When scouting for aphids, the shiny honeydew can help detect them. Sugarcane aphids can move into the plant's panicle over time and during heavier infestations.
Infestations on pre-boot sorghum can cause significant grain loss, but infestations present during grain development can also reduce yields. Large infestations can stunt the plant’s growth and cause uneven panicle emergence from the boot. Infestations in forage sorghums also kill leaves, slow growth, and reduce forage yields.
What is an Entomopathogen?
Dr. Bowling says entomopathogens are either bacteria, fungi or other microorganisms that infect insects and eventually kill them under the right conditions. The type of entomopathogens Bowling and Martinez have been studying are fungi species.
"Entomopathogenic fungi infections begin by coming into contact with their host, in this case the sugarcane aphid. This happens when an aphid comes into contact with a spore," Bowling says. "Spores are living fungal particles that travel by wind. Once contacting sugarcane aphid and under favorable environmental conditions, the spores grow string-like threads called mycellium. It is the mycellium that penetrates the exoskeleton of the aphid."
He said the fungus grows inside the aphid, spreading throughout the body while attacking vital organs. Once the host is fully consumed, the fungi exits through the exoskeleton to release more spores and the cycle will be repeated when environmental conditions favor the pathogen.
"What Margarita has been doing in the U.S. for us here in Texas over the last few months is awesome work...[she has] collected random samples of sugarcane aphids from volunteer sorghum along the Coastal Bend of Texas and then under specific laboratory conditions has been able to find entomopathogenic fungi living inside these aphids."
Bowling says after [Martinez] found fungal species living in the aphids she isolated them and began rearing them in colonies to determine what species they might be. She believes the two species of fungi found so far that could affect the sugar cane aphid are Lecanicillium lecanii and an Isaria sp.
"We are in the process of getting a final verification, [but] the significance of this is that both fungal entomopathogens have been found in infecting sugarcane aphid in Mexico. Almost all the aphids Margarita and I collected showed the presence of fungal entomopathogens," he said.
Bowling says widespread occurrence of these entomopathogenic fungi among sugarcane aphid in South Texas may make it possible for researchers to evaluate environmental conditions necessary for an epizootic, or outbreak of the pathogen. He says it may be possible to predict sugarcane aphid colony collapse when environmental conditions favor an epizootic and adjust strategies for managing the aphid in South Texas sorghum.
"We suspect epizootics [to be] a possibility for sugarcane aphids to collapse across large areas following an extended period of cool and wet conditions," he added.
Research on the value of entomopathogenic fungi continues in the Coastal Bend with hopes of developing yet another tool to help control sugarcane aphid outbreaks.