Sorghum growers in California and Arizona are in the midst of an invasion by a pest never-before identified in the two states.
The sugarcane aphid – known by that name because it was first spotted in sugarcane – is now a problematic pest in California after first spreading across the South, according to David Haviland, an entomologist with the University of California Cooperative Extension in Kern County.
Haviland and integrated pest management (IPM) advisor Pete Goodell of the UC Statewide IPM program are gathering information on this pest as they seek to quickly disseminate the information to growers who are from days to several weeks from harvest.
Sorghum in California’s Central Valley is predominantly a silage crop, meaning it is chopped for cattle feed. Growers in recent years have switched from corn to sorghum because it requires less water to grow.
Sugarcane aphids were identified in California in August after a pest control advisor (PCA) working for a grower spotted the unidentified bug in the south San Joaquin Valley. Since then, the pest has been confirmed in Kern, Tulare and Fresno counties, according to Haviland.
The pest was also identified in the Maricopa and Stanfield areas of Pinal County, Ariz. Like California, this is the first confirmed discovery of the pest in Arizona. Prior to this the furthest west the pest was confirmed is eastern New Mexico.
According to an informational flyer published by the Sorghum Checkoff, the sugarcane aphid, Melanaphis saccari, was first discovered in the United States in 2013.
Haviland says sorghum was not the bug’s original host, but since 2013 the pest has moved into the crop, perhaps due to natural genetics.
By the end of 2013, the pest expanded from Texas to Louisiana. A year later it had expanded to 14 states, many in the South and Southwest.
Since then the pest has developed significant populations in central Arizona, according to Goodell and just recently was identified in the southern San Joaquin Valley of California.
At a tailgate meeting near Pixley, Calif., Haviland and Goodell interacted with about two dozen growers and their PCAs on what they have learned since the pest was first identified in California one week earlier.
“If this was here last year I think people would have noticed,” Haviland said of the large aphid population found at the field day.
Haviland admits much of what he and Goodell know about the pest has been gleaned from information coming out of the South. This includes chemical treatment programs that California growers cannot completely use since one of the two main products used by growers elsewhere in the U.S. is not labeled for California use.
The grower who farms sorghum where the field day occured said he had treated the field several times in weeks leading up to the field day to little avail. Insecticides included two applications of chlorpyrifos and a tank mix of malathion and dimethoate.
What the California researchers do know, and what has been learned by some of the growers who attended the meeting, is the pest impact to sorghum.
Black sooty mold can develop on plant leaves after sufficient levels of honeydew – the sugary substance excreted by the sugarcane aphid – are present. This inhibits the ability of the leaf to collect sunlight for photosynthesis.
Further damage is caused when feeding by the aphid causes the plant to dry out, reducing moisture levels below what is needed for fermentation as silage. Stressed plants also have a reduced ability to fill a grain head.
One grower said plant yields in sorghum that can be about 25 tons per acre in the absence of the pest can drop to about 18 tons per acre. Moreover, the quality of the forage is significantly impacted, making it unsuitable for lactating cows.
“It’s not milk cow sorghum anymore,” said Tulare County grower Charlie Pitigliano. “It’s only good for dry stock with the problem we have right now.”
Identifying the pest
The sugarcane aphid in the California field is a pale yellow, which differentiates them from light green aphids, which are more easily controlled with insecticides labeled for use in California. Other distinguishing marks seen with a magnifying glass include black cornicles, or tailpipes at the back end of the aphid, and black tips on the feet and antennae.
The sugarcane aphid reproduces prolifically. Goodell says they are live-bearers, meaning they give birth to live offspring. Because they also tend to produce female offspring throughout much of the season, at the time a daughter is born to her mother, the daughter is already pregnant with her own offspring.
This means one-to-three new offspring per day can be born as aphids transform from nymph to adult in five days, Haviland says.
Even more troublesome is that once sugarcane aphid populations reach significant numbers some will develop wings, making their spread that much more efficient.
Haviland says there are beneficial insects that feed on or parasitize the sugarcane aphid, but those natural enemies in the South, and his surveys of beneficial insects in fields where the sugarcane aphid is already established, suggests biological control alone will not be sufficient to reduce crop losses.
The good news, if there is any according to Haviland, is the sugarcane aphid has only a few host plants. These include sorghum, sudangrass, sorghum/sudan hybrids and other sorghum species including johnsongrass. The aphids are not known to affect corn.
Haviland admits that he and Goodell are still learning about the sugarcane aphid.
“You have two entomologists who’ve worked in sorghum now for seven days,” he told the gathered field day audience.
There is much yet that UC researchers need to learn about the sugarcane aphid, and as such, are asking growers with the pest to help them.
How will it overwinter in the San Joaquin Valley in the absence of sorghum? Will it, for instance, move into johnsongrass? Haviland is suggesting growers have good johnsongrass control during the winter to perhaps reduce overwintering populations.
Neither do they know how the pest arrived in California.
According to information from the sorghum checkoff, Sivanto (Bayer) and Transform (Dow AgroSciences) are recommended products in the South. Transform is not labeled for use in California and has limited Section 18 authorization elsewhere in the U.S.
Haviland describes these products as more expensive than traditional broad-spectrum insecticides, but have the added benefit of being safe on beneficial insects.
The best control strategy appears to be chemical control in combination with biological control, according to Haviland. So far, inspections of California fields have shown that there are many species of natural enemies that can attack sugarcane aphid.
The hope is that growers can use softer insecticides that are more effective against the aphids and will not inhibit the levels of biological control.