Abstract

Sorghum as fresh foliage with high nutrition value in animal feeding is used as alternative forage crop in the summer in southwest dairy industry. The sugarcane aphid (SCA), Melanaphis sacchari (Zehntner) (Hemiptera: Aphididae), is a new invasive pest of Sorghum sp. including forage and grain sorghum, Johnsongrass, and Sudangrass. This aphid was first found in Arizona in 2016 and has become a serious pest since. High reproductive potential, wind dispersal, and ability to overwinter on living annual and perennial hosts of Sorghum sp. appear to be the main factors in its impressive geographic spread in all southern states in the USA from East to West coasts. Heavy infestations, often with thousands of SCA per leaf, can cause significant decrease in yield and forage quality. Infested leaves become sticky and shiny from honeydew and coated with sooty mold fungus which hampers harvesting operations and can become unsuitable for livestock feed. The University of Arizona Cooperative Extension Forage IPM Program examined the efficacy of in-seed bed injection and foliar applications of certain selective vs broadspectrum insecticides for forage sorghum system in the low elevation desert from 2016 – 2019. These results provide the firsthand information of management of SCA in forage sorghum in the region.

Identification and Damage

Materials and Methods

Foliar Treatment Trials

Insecticide efficacy study was conducted at the University of Arizona Maricopa Agricultural Center (MAC) using a randomized complete block design with six insecticide treatments and an untreated check (UTC) in 2017, and seven treatments plus UTC in 2018. Plots measured 4 row by 30 ft. with 7 ft alleys, and each treatment was replicated four times. The insecticides used in 2017 trial were: Beleaf 50™ SG (2.85 oz/A), Transform 50™ WG (1.5 oz/A), Sivanto Prime™ (7 oz/A), Lorsban Advanced™ (32 oz/A), Majister™ (36 oz/A) and Dimethoate™ 4E (16 oz/A). In 2018, the treatments were Centric™ (2.5 and 3.4 oz/A), Sivanto Prime™ (4 and 7 oz/A), Transform™ (1 and 1.5 oz/A), and Dimethoate 4D™ (16 oz/A). Insecticide treatments were applied on August 11, 2017, and August 28, 2018. All plots were relatively equal in terms of their SCA infestation. The SCA populations in 2017 trial were assessed by collecting five leaves from the median height of randomly selected five plants approximately ten feet (3 meters) apart on a weekly basis. A brushing machine (Bioquip™) was used at the lab to sort out SCA by removing them from leaves onto a dish detergent-filmed, circular, clear glass plate. Aphids were then counted under light microscope to estimate SCA populations per leaf. Mechanical yields of each plot were harvested on 09/18/2017 and 10/19/2018 from each plot using a two-row Hesston 7155 chopper. The green chopped harvests were weighed and recorded for each plot as lb. per 200 sq. ft., then converted to tons per acre.

Seed-bed Injection Trial

Similar setup of plot as the foliar application trial have been used for this trial. At planting (5/30/2018), each planter has been equipped with a short boom ending with a single-hole orifice plate nozzle for seed-bed application of Sivanto Prime™. The insecticide rates that used were 2, 4 and 8 oz/A. Another set of seed-bed treatments were combined with foliar application of Centric™ at 2.5 oz/A. Foliar application was carried out on 8/28/2018. The SCA populations were assessed using the United Sorghum Checkoff guide (watch video below) . Five plants per plot were randomly selected and each rated according to SCA infestation. Mechanical yields were harvested on 10/19/2018 using the same method described above. About 5.5 inches of rain were recorded at MAC weather station between 9/13 and 10/20/2018.

Statistical analyses were conducted using JMP 11 software. Means comparisons were determined using Student's t-test at LSD < 0.05.

Results

Foliar Treatment Trials

2017:

Some chemistries such as Sivanto Prime and Transform 50WG provided the best control for SCA about a week after treatment; lowering the population of SCA under 170 per leaf (Fig. 1). In the second week Sivanto further controlled the SCA population under 70 per leaf, and continues to control SCA below 500 per leaf. Transform acted quickly on SCA for about two weeks only, as well as Lorsban (Fig. 1). All insecticide treated plots resulted in higher yields compared to the untreated check. Plots treated with Sivanto Prime, exhibited significantly the highest hay yield. About 30% of average yield increase recorded on Sivanto treated plots compared to untreated plots. Lorsban and Transform treated plots gave yield advantage of 16% and 15%, respectively over the untreated check (Fig. 2).

2018:

Both rates of Centric and Sivanto Prime at 7 oz/A provided significantly better protection against SCA compared to the rest of treatments. Bothe rates of Transform and Sivanto Prime at 4 oz/A lowered the SCA population more significantly than the UTC. There was no significant difference between Dimethoate treatment and the UTC (Fig. 3). There were no significant yield differences among treatments and between each treatment and the UTC (Fig. 4).

Seed-bed Injection Trial

The SCA population results for seed-bed application trial are included in figure 5. All rates of seed-bed application of Sivanto Prime alone and with Centric foliar application significantly reduced SCA populations compared to the UTC and the foliar application of Centric (2.5 oz/a) alone. The late treatment lowered the SCA population more significantly than the UTC. Yields from all seed-bed treatment + foliar application and the seed-bed application of Sivanto Prime at 8 oz/A showed significantly higher yields than the rest of treatments (Fig. 6).

Best Practices for Managing Sugarcane Aphid:

• Plant early to avoid infestations. Our trials showed that planting in late April instead of late May can mitigate SCA population and scape its damage to forage sorghum crop in Arizona.

• Control Johnson grass and other sorghum species around field

• Scout early and often. Do not let SCA populations develop to large numbers. Treat when 20% of leaves checked have 50 or more SCAs

• Use efficacious insecticide; that is soft on beneficials. Avoid pyrethroids for other pests if possible. Spray coverage is key for SCA control.