Research

by Kay Ledbetter, Texas A&M AgriLife Communications

AMARILLO – While sugarcane aphids have been difficult to suppress in past years due to their natural traits and limited insecticide options, a Texas A&M AgriLife Research study shows resistant sorghum varieties and beneficial predators could provide a solution.

Dr. Ada Szczepaniec, AgriLife Research entomologist at Amarillo, recently authored  “Interactive effects of crop variety, insecticide seed treatment, and planting date on population dynamics of sugarcane aphid and their predators in late-colonized sorghum” in the Crop Protection journal. The full article can be found at https://bit.ly/2IknvD4.

This research demonstrates that a commercially available resistant sorghum variety provides an adequate protection against this pest in the central High Plains. It also shows aphid predators already present are readily attracted to aphid-infested sorghum, Szczepaniec said.

This research was supported by funding from U.S. Department of Agriculture’s National Institute of Food and Agriculture, USDA’s Agricultural Research Service, Areawide Pest Management of the Invasive Sugarcane Aphid in Grain Sorghum, United Sorghum Checkoff and Texas Grain Sorghum Producers Board.

Szczepaniec said biological control of aphids was significantly improved on the resistant sorghum with a combined top-down and bottom-up control approach.

Currently, sorghum fields require weekly inspection for sugarcane aphids with scouting efforts intensified to twice weekly once they are detected to ensure timely insecticide applications, she said. Insecticides are recommended at 50-125 aphids per leaf on susceptible sorghum or once 20-30 percent of plants are infested with evidence of substantial honeydew.

“Once densities exceed 500 aphids per leaf, it is difficult to suppress sugarcane aphids and, left unmanaged, they can reach densities exceeding 10,000 per plant,” Szczepaniec said.

Population size, sorghum growth stage and host plant resistance are the key determinants of the damage intensity to sorghum, which is caused by direct aphid feeding injury, she explained. By removing plant nutrients and injuring sorghum throughout its development, sugarcane aphids can decrease yield, reduce seed weight and lower grain quality.

In Szczepaniec’s study, the three-way interactions among sorghum variety, seed treatment and planting date were tested. Populations of sugarcane aphids reached outbreak densities across treatments in 2016, but their numbers were much lower in 2017.

The conventional planting date for sorghum in the Texas Panhandle is June 1, and sorghum planted in May is considered early planted. Using these timing guidelines, half of the plots were planted May 11 and May 18 in 2016 and 2017, respectively, and half were seeded  June 11 and June 8 in 2016 and 2017, respectively.

Two planting dates, resistant and susceptible varieties, and treated and untreated seed were included in each plot. Individual plots were embedded within larger sorghum fields of the DKS 44-20 without seed treatment to avoid edge effects associated with smaller experimental plots. No insecticides other than the seed treatment were applied to the plots.

Szczepaniec said while the pattern of sugarcane aphid population dynamics and timing of colonization differed between the two years and depended on planting date, susceptible sorghum free of the insecticide seed treatment consistently had the highest aphid densities.

Host-plant resistance emerged as the main driver of sugarcane aphid population dynamics and aphid-predator interactions in late-colonized sorghum, she said.

“While this has been demonstrated previously with university-developed lines, this is the first report demonstrating effectiveness of the commercial resistant variety of sorghum against sugarcane aphids,” Szczepaniec said.

Crop varieties resistant to arthropods have been employed in suppression of invasive aphids with significant success in the past, she said. It is also evident sugarcane aphid populations increase extremely rapidly in late-colonizing sorghum, so early control, especially by natural enemies, is crucial to successful suppression of this pest.

“Conclusions drawn from this work are likely limited to aphid-sorghum-natural enemy interactions in post-bloom sorghum, which may differ significantly when sugarcane aphids colonize sorghum in vegetative stages,” Szczepaniec said.

Future research should examine the synergistic interaction between host-plant resistance and biological control in sorghum colonized in reproductive stages, she said. Also, predators should be integrated in the thresholds in order to improve the long-term sustainability of managing sugarcane aphids in sorghum.

Texas A&M Entomology Assistant Professor Gabe Hamer is working to make it easier for training the next generation of medical entomologists by mounting arthropods in resin.

In a paper that was recently published in the Journal of Insect Science featured with the journal’s cover image  and on Entomology Today, Hamer’s lab created protocols for encapsulating medically important arthropods in resin for educators to use in educating the public on various arthropods that can vector dangerous diseases, such as Chagas disease, Lyme disease, and Rickettsiosis.  The work was led by graduate student Justin Bejcek, who earned his BS in Entomology and will graduate with his MS in Veterinary Public Health this Spring.

The idea for the project came after Hamer was looking for a way to make it easier to have quality teaching specimens of arthropods available as a training tool for veterinary and medical entomology students that are entering the workforce to help them to properly identify these arthropods.

Hamer said that most common tools used in training for identifying include photographs, illustrations, and specimens pinned or in vials preserved with ethanol. According to the paper, the photos and illustrations are limited in what they can show in relation to size and proportion and that pinned and ethanol preserved specimens can become damaged and degrade due to time and excessive handling.

“It is great to be able to hand someone a real insect embedded in resin,” he said. “This is especially important when we are trying to educate students or the public on what insects look like that feed on blood, and are thus important disease vectors, compared to look-a-like insects that do not feed on blood. Even with the best macro-photography pictures available, this just isn’t the same as having the real specimens in your hand.”

The objective of the project was to develop a resin embedding protocol to help maximize the quality while reducing the cost of making each of the castings. For the past 4 years, the team has experimented with polyester and polypropylene casting molds and used triatomines, ticks, mosquitoes, spiders, fleas and true fly adults and larvae.

The result of the experiments done on different resin types culminated in a protocol that includes having the insects stored in ethanol and dried for a specified amount of time before resin casting. Then they used a two layer resin process using Petri dishes for the molds.

The insects in resin were then cut, sanded, and polished to make sure that the surfaces were flat and optically clear to allow for better viewing of the specimens inside.

Hamer said several of the specimens have been in use in the Veterinary Entomology, Medical and Urban Entomology courses to help the students better identify vector insects and has been asked by people outside the university to provide insects for them.

“The extension groups certainly appreciate the resin bugs,” he said. “We occasionally receive requests for the resin insects, but the demand exceeds our ability to supply.”

Hamer said the best part about making the specimens is that it helps to teach people about medically important arthropods and the diseases they vector.

“The best thing about producing our resin bugs is that it enhances our outreach and extension abilities with regard to arthropods of medical importance,” he said.

He said that the hardest insects that they had mounted were mosquitoes, but that they will be working to improve the process.

“We have attempted to mount mosquitoes in resin which didn’t work well.  We still don’t have a great mounting approach for the long-term preservation of mosquitoes while allowing close scrutiny for the identification of morphological characteristics,” Hamer said.  “We need to keep brainstorming and working on innovative ways to resolve the mosquito mounting challenge.”