Feeding Sugarcane Aphid and Sooty Mold Infested Sorghum
By Mike Kriegshauser¹, Jennifer Chaky, M.S.², Bill Seglar, DVM³, and Sandy Endicott, M.S.⁴
- The sugarcane aphid has become a severe pest of North American sorghum production over the last few years and has spread across most sorghum areas within the United States, Mexico, and throughout Central America.
- Aphid populations can grow exponentially due to their live-birth reproductive behavior.
- Aphids deposit a significant amount of “honeydew,” a sticky, liquid excrement. Honeydew may serve as a food source for saphrophytic fungi, such as sooty molds, which turn the plant leaves black in color, reducing the photosynthetic capacity of the plant.
- Sugarcane aphid infestations followed with sooty molds result in protein and energy nutrient reductions of sorghum forage fed to cattle.
- Sugarcane aphid/sooty mold infested forage sorghum may have reduced palatability in the beef herd.
The sugarcane aphid, Melanaphis sacchari, has become one of the most important insect pests of sorghum in the southern United States, Mexico, and throughout Central America. First documented in 1977, the sugarcane aphid became a serious insect pest in sorghum in 2013 and is capable of causing significant damage and reduction to grain yield in sorghum with losses of up to 100% (Catchot et al., 2015). Sorghum production intended for cattle feed will need to be managed closely to ensure quality feed is harvested.
Sugarcane aphids: A winged adult, non-winged adults, and nymph.
Sugarcane aphids can reproduce without mating. Most sugarcane aphids are female and give birth to one to three live, pregnant offspring daily. Nymphs pass through four stages and can reach reproductive adult stage in five days, resulting in exponential growth rates under favorable conditions. The life span of the female is around 28 days with a range of 10-37 days.
Infested sorghum leaf with all stages of aphids present.
Sooty Mold Complex
Aphids leave behind “honeydew,” a sticky, liquid excrement. The honeydew may serve as a food source for saprophytic fungi, such as sooty molds, which can turn the plant leaves black in color, thereby reducing the photosynthetic capacity of the plant.
Sooty mold on sorghum resulting from a heavy infestation of sugarcane aphid.
Sooty mold growth is often composed of a complex of fungi; the outward appearance of which is typically a dark mold growing across plant surfaces. Six sooty mold samples from the 2016 growing season were submitted for analysis to the Pioneer plant diagnostic lab in Johnston, Iowa and to Kansas State University (KSU). These submissions represented locations from Lincoln County, Kansas, and Hale and San Patricio counties in Texas. Three different fungi were observed in all six samples: Capnodium, Cladosporium and Alternaria. In addition, the sample from San Patricio County, Texas also contained some Leptoxyphium.
Sooty mold fungi are saprophytic and survive, in these instances, superficially on the honeydew secretions of the sugarcane aphids feeding on the sorghum. They are not technically pathogens and therefore are not directly affecting the health of the plants. Another important factor to note is that mycotoxins, various toxins produced by some species of fungi, are not known to be produced by the species of fungi observed from these sooty mold samples.
Animal Behavior and Feed Selection
Grazing beef cattle have been observed to preferentially feed on other available grass or broadleaf plants while avoiding aphid infested sorghum plants. When feeding on the aphid infested sorghum, the cattle preferred the stalks over the leaf material, probably due to lower amounts of sooty mold present. When given the choice between sorghum hybrids, the cattle chose the hybrid that had less sooty mold present (more aphid tolerance).
Sooty mold on forage sorghum stalks from a heavy infestation of sugarcane aphid.
Chopping sugarcane aphid infested sorghum can be challenging due to the honeydew presence. Several growers reported reduced ground speeds due to buildup of “sticky” material on the cutter bars and whole harvesting head. The exit chute of the chopper also had to be cleaned periodically to keep the forage flow accurate.
Best Ensiling Management Practices
When ensiling sugarcane aphid/sooty mold infested forage sorghum, it is imperative to intensify the best silage management practices that include:
- Harvest at the proper maturity
- Harvest at the proper moisture
- Chop at the proper length
- Pack firmly
- Cover securely
- Proper rate and method of feed out
Photo on left is sugarcane aphid/sooty mold infested silage, note the improper cut, poor packing and face management. Photo on right is an ideal bunker face.
Feed Quality of Sugarcane Aphid Infested Sorghum
Sugarcane aphid infestations followed with sooty molds result in protein and energy nutrient reductions of sorghum forage fed to cattle. Aphids utilize plant nutrients for substrate while sooty mold can result in musty smelling forage that can depress cattle palatability.
Nutrient demands by gestating beef cows substantially increase during the last 60-90 days of pregnancy. Table 1 shows megacalories net energy for maintenance (NEM) and crude protein (CP) requirements for 1,300 pound (600 kg) beef cows based on 24 pounds (11 kg) of dry matter intake (DMI) for the first two trimesters and 26 pounds (11.8 kg) DMI the last trimester of pregnancy.
Table 1. Nutritional requirements of gestating beef cows.
Grazing beef cows on forage sorghum normally provides all of the energy and protein nutrients required during first six months of pregnancy. During the last trimester of pregnancy, supplemental protein and energy often are required to keep up the cow’s body condition and complete fetal development of the calf.
The image below shows two sugarcane aphid and sooty mold infested forage sorghum samples from a field in Kansas in 2016. The sample on the left was infested to a lesser degree than the sample on the right.
Samples of sugarcane aphid/sooty mold infested silage.
Table 2 shows comparisons for NEM and CP requirements for pregnant beef cows compared to the nutritional value (analysis from Pioneer forage lab) for the sugarcane aphid /sooty mold infested forage sorghum that came from fresh crop samples from Kansas shown in the image above.
Table 2. Nutrient contributions from less infested and more infested sorghum forage.
Data findings revealed that both levels of sugarcane aphid /mold infested sorghum had NEM and CP values that were substantially lower than what is typically seen with normal forage sorghum at 0.60 NEM/DM lb (1.34 NEM/DM kg) and 9.1% CP (NRC 1989 for beef). Laboratory results show that both the less and more infested forage sorghums fail to meet NEM and CP needs for all three trimesters of pregnancy to maintain body weight on beef cows and develop a healthy fetus that will be strong at calving. Therefore energy and protein supplementation is required by the first two trimesters and even more aggressive supplementation is needed during the last trimester of pregnancy.
Other collected data showed that besides having significant molds identified in the crop, Bacillus spoilage microbes were present at high colony forming unit (cfu) population counts. Bacillus population counts are tolerable up to 100,000 cfu/gm forage. In contrast the infested forage counts ranged from 1,000,000 to 10,000,000 cfu/gm forage. Bacillus respiratory activity generates much heat and is the primary microbe that causes hay bale fires. While hay fires are sporadic, a more common observation is the Maillard reaction, where the heat generation from Bacillus results in a portion of CP to be bound to the acid detergent fiber fraction (ADF) of fiber, resulting in a bound protein complex that’s not digestible by cattle. Bound protein values less than 10% are considered normal. In the Kansas samples, bound protein ranged from 23-26%.
Feeding Management Suggestions
Cows exposed to colder temperatures coupled with damp weather conditions have greater energy and protein needs to meet their dietary maintenance requirements. Dry matter intakes may increase up to 40% to meet the nutrient needs. However, the infested forage sorghum may have reduced palatability in the beef herd, thus lowering the cow’s acceptance of the forage and further jeopardizing the ability to meet energy and protein needs.
Working with a nutritionist is advised when feeding compromised forages such as sugarcane aphid/sooty mold infested forage sorghum crops. The nutritional consultant needs to assess the forage quality from each cattle producer’s operation for determining required energy and protein supplementation to maintain beef cow body condition during pregnancy and for development of a healthy fetus. Monitoring body condition scores or weighing cows at various time points while feeding the sorghum crop is essential for making nutritional energy and protein adjustments.
Sugarcane Aphid Management
Best Management Practices
- Control volunteer sorghum and other grasses to remove sources of early infestation.
- Select and plant hybrids with good sugarcane aphid tolerance. Contact your local Pioneer sales representative for the latest information on hybrid characteristics.
- Shorter statured hybrids may allow for better aphid control when applying insecticides due to easier spray penetration deep into the canopy compared to taller hybrids.
- Use an effective insecticide seed treatment such as Cruiser® insecticide seed treatment.
- Plant early.
- Scout fields early and weekly.
- Apply an approved insecticide when the action threshold is reached. Check with your local Pioneer sales representative or University Extension for the thresholds in your area.
- Avoid using pyrethroid insecticides, which are harmful to beneficial insects and may cause aphid populations to rebound rapidly.
- Be aware of pre-harvest intervals for animal feed when applying insecticides. Read the label for complete information.
Insecticides for Sugarcane Aphid
Several insecticides are labeled for use on sorghum in the U.S. Insecticide trials conducted by Texas A&M University have shown that Sivanto™ 200 SL insecticide (flupyradifurone, Bayer CropScience) can provide significant reductions in aphid populations up to two weeks after application. Transform™ insecticide (sulfoxaflor, Dow AgroSciences) was previously used in some states but its registration was canceled by the EPA in November 2015. Transform insecticide had a Section 18 label for the 2016 season from the EPA. Check with your local pesticide supplier for availability.
When applying insecticides by ground equipment, University of Arkansas researchers recommend that insecticides be applied in 10 gallons of water per acre. Growers should consult their Cooperative Extension Service for a current list of registered chemicals in their respective states and updated results on the efficacy of sugarcane aphid insecticides. Read and follow all label directions before applying an insecticide.
Thanks to Dr. Robert Bowling, Texas A&M AgriLife, for providing information and insect close-up photograph.
Field photos of sugarcane aphids on plant leaves are courtesy of the Pioneer Mexico Agronomy team.
Photos of fungi are courtesy of Jennifer Chaky, Pioneer Research Scientist, Pathology.
Photos of sugarcane aphid infested sorghum plants and silage samples are courtesy of Mike Kriegshauser, Pioneer Field Agronomist, Kansas.
Cachot, A., J. Gore, and D. Cook. 2015. Management Guidelines for Sugarcane Aphids in MS Grain Sorghum 2015.
Endicott, S. and M. Rice. 2016. Sugarcane Aphid: Biology and Management in Sorghum. Crop Insights Vol. 26 No. 1. Pioneer. Johnston, IA.
Hall, J.B. et al, 2009. Nutrition and Feeding of the Cow-Calf Herd: Production Cycle Nutrition and Nutrient Requirement of Cows, Pregnant Heifers, and Bull. Virginia Cooperative Extension; Pub. 400-012.
Seiter, N. et al. 2015. Sugarcane aphid, a new pest of grain sorghum in Arkansas. University of Arkansas Cooperative Extension Service.
Sorghum Checkoff. 2016. Pest management.
¹ Pioneer Field Agronomist
² Pioneer Research Scientist
³ Pioneer Senior Nutritionist
⁴ Pioneer Senior Agronomy Manager
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