Sugarcane Aphid Update: Recent rain has temporarily relieved the drought conditions for many south Texas farmers. This rain was a little too late and a little too light to help some sorghum and corn but it will help many other fields to harvest. It does look like grain harvest may start a little earlier this year when compared with what we think of as an ‘average’ harvest window (harvest is underway in the Lower Rio Grande Valley). This is a testament to the mild winter conditions that led to early planting of most crops in south Texas.
Over past two weeks or so the sugarcane aphid has reached thresholds in sorghum fields from Kleberg up to Refugio and surrounding counties. Infestations seem to be somewhat scattered with some fields experiencing heavier infestations that other nearby fields. I have had calls from several farmers saying that their fields have very few sugarcane aphids but their neighbors fields have exceeded thresholds and these fields are being sprayed. There are reasons for this but most have to do with overwintering sugarcane aphids and their proximity to sorghum fields.
Many of the infestations we are currently experiencing are from overwintering sugarcane aphid moving into surrounding fields. This became very apparent from the heavy infestations of sugarcane aphid on the borders of sorghum around the area. I surveyed a number of fields over the past two weeks where sugarcane aphid populations along field edges were averaging over 2000 (two-thousand!) aphids per leaf but 300 feet into the field populations were averaging fewer than 100 aphids per leaf. There has been a very clear edge effect on sorghum in Kleberg, Jim Wells, Nueces, San Patricio, and several other counties. Although sugarcane aphid could be found on any field field edge bordered by Johnsongrass or volunteer sorghum, a number of fields were infested along the northern field edges. I think this was unexpected by many folks. Keep in mind that we had several cold fronts in April with some strong north winds. Our surveys did show that winged sugarcane aphids had developed on overwintering hosts by the end of March. It is not a stretch for the winged aphids to be blown onto the northern edges of fields during these cold fronts. It was not until late-April/early-May that environmental conditions (hot and very dry) became favorable for rapid population growth by sugarcane aphid. The apparent lack of field-wide infestations (much like we saw last year) leads me to believe that we did not experience the migratory flight from Mexico and the Lower Rio Grande Valley I had expected. Sugarcane aphid activity was very light in the Valley this year (and I am certain the sorghum producers were glad of that!).
Economic Threshold for Sugarcane Aphid? Scouting Susceptible Grain Sorghum for Sugarcane Aphid – Using Aphid Counts vs. Aphid Presence and Honeydew
There has been some interest in using a combination of percent plants with aphids and presence of honeydew in deciding whether or not to apply an insecticide for management of sugarcane aphid. I would advise you to use extreme caution when considering this scouting approach. Where we have looked at this approach using percentage of plants with greater 10 aphids/leaf or greater than 25 aphids/leaf, there was no consistent yield response to those levels of infestation and I am not aware of any research that has shown that relationship. In fact, there was less yield loss with >25 aphids per leaf than with >10 aphids per leaf. By using this method, growers run the risk of treating too early (and possibly unnecessarily) or too late if they incorrectly evaluate or assess honeydew and whether aphid colonies are established or not.
I believe the most reliable decision aide is counting or estimating aphid numbers on a per leaf basis (see scout card here). With current grain prices of around $3.80 per bushel, economic thresholds are between 50 and 100 aphids per leaf for treatment costing $10 to $15/acre for the upper and lower gulf coast regions of Texas. Scouting is still of utmost importance. Current recommendations are to scout fields at least weekly, once sugarcane aphids are detected. Checking all sides and at least 100’ into the field is important to gauge whether or not the entire field, edge, or no treatment is needed.
Resistant hybrids have been discussed previously and it is believed that different hybrids have varying degrees of resistance/tolerance to sugarcane aphid damage, and thus would have different thresholds. Scouting resistant hybrids is still important although treatment decisions may currently need to be based more on experience as we are still working on threshold establishment for resistant hybrids.
How will the recent rain effect the sugarcane aphid? That remains to be seen but, in the past three years it seems that the sugarcane aphid populations have collapsed about 10 to 14 days following major rain events. Higher relative humidity coupled with lower temperatures certainly mean the environmental conditions are favorable for an epizootic. There is no shortage of sugarcane aphid on sorghum around the area. The one unknown is whether or not fungal spores are at the necessary concentration are in contact with the aphid. It is something I will be watching over the coming days.
As a side-note, I will be hosting a graduate student from Mexico starting in mid-August through October. She is working toward a Master’s degree and her thesis project is fungal pathogens of sugarcane aphid. She will identify field collected diseased aphids and identifying pathogens (entomopathogens and saprophytes). If you suspect entomopathogens in sugarcane aphid colonies and would like a confirmation please place specimens in ethanol and send them to me. Please include collection information (location, date, collector, and GPS coordinates would be nice).
How are insecticides performing against sugarcane aphid? Transform and Sivanto are performing up to expectations. I have seen these products perform above and beyond expectations where field edges were heavily infested by sugarcane aphid (aphids populations averaging 5,000 to 10,000 per plant). These field edges were, for the most part, very clean after two weeks post spray.
Are there any watch-outs for applications? I have had several consultants tell me that large aphid populations could be found on the bottom leaves of sorghum a couple of days following an insecticide treatment (either Sivanto or Transform). The common theme around all of these observations was dense canopies either associated with the hybrid and/or narrow row spacing. Both insecticides must come in contact all leaves to be effective. In all cases the final volume (10 gpa or less) was not adequate to penetrate the canopy. Prior to treating sorghum for sugarcane aphid assess the canopy and consider row spacing to determine total volume necessary to penetrate the canopy. If the field is on narrow rows or the hybrid has produced a dense canopy use no less than a final volume of 15 gpa, lowering the spray boom as much as possible, and consider increasing the pressure. Research from 2016 showed hollow cone nozzles offer better penetration through the canopy compared with other nozzles when higher spray volumes are used. If hollow cone nozzles are on the boom already it is suggested that they would be preferred over other nozzles to gain the penetration needed to reach the lower canopy.
Biological Control: Another common occurrence in conjunction with sugarcane aphid is an abundance of natural enemies. The two most common natural enemies has been a complex of lady beetles (ladybug) and an parasitic wasps in the family Aphelinidae (these parasitic wasps turn sugarcane aphid into black mummies). They appear to have been effective at suppressing sugarcane aphid populations prior to the rapid populations increases seen over the past couple of weeks.
How are the sugarcane aphid tolerant hybrids performing against sugarcane aphid? I have heard that some fields planted to sugarcane aphid tolerant hybrids have been sprayed for sugarcane aphid. It seems like a lot of these instances were hybrids with “moderate tolerance” to sugarcane aphid and not those classified as “highly tolerant” to sugarcane aphid. Large populations of sugarcane aphid are present on a field housing a strip trial of ~30 sorghum hybrids. I counted aphids in several of the hybrids and the “highly tolerant” hybrids were no where close to reaching threshold levels for sugarcane aphid while the two hybrids that were either moderately tolerant or susceptible had exceeded the threshold (see Table 1). I was in this field earlier in the week and the “highly tolerant” hybrids have yet to reach the economic threshold wilt the moderately resistant and susceptible hybrids are being killed by the aphid. It is a very impressive look at performance of these sorghum hybrids exposed to heavy aphid infestations. In several instances sorghum hybrids “highly tolerant” to sugarcane aphid had very few to no aphids while neighboring plants of susceptible hybrids were being killed by huge populations of sugarcane aphid. As always, treat all sorghum as though it is susceptible to sugarcane aphid (i.e. scout all fields for sugarcane aphid) but the sorghum “highly tolerant” to sugarcane aphid looks very impressive against the aphid.
This is the 5th season south Texas sorghum producers have faced the sugarcane aphid and, in many ways, this year has been as unique as the previous four. But, so to has been the environment. The one thing that seems very consistent is that rapid sugarcane aphid population growth is really driven be hot and dry conditions. This is well documented in the literature. For folks in areas that received significant rain over the past week or so, keep and eye on the sugarcane aphid populations. If you happen to see the populations collapse I would certainly interested in knowing. You can drop me a line in area below this newsletter or on Facebook.
For more information on the sugarcane aphid and other field crops topics check out our new website at: (http://betteryield.agrilife.org/).
Robert Bowling, Ph.D.
Assistant Professor and Agrilife Extension Entomology Specialist
Texas A&M Agrilife Research and Extension Center at Corpus Christi
10345 Hwy 44
Corpus Christi, TX 78406