SCA After the Rains: Now What?

We are now concluding five straight days of rain on the southern High Plains, but sugarcane aphids are still with us. I spent some time today collecting infested leaves and examining the aphids under a microscope, and I have to report that I can't find any evidence of the fungi that hammered populations on the Gulf Coast. (Although I will keep monitoring the situation.) Most of the aphid colonies I observed looked just fine, and there were some beneficial insects like syrphid fly and lady beetle larvae feeding on them. Dr. Katelyn Kesheimer, IPM Agent in Lubbock and Crosby counties, took 7 Day After Treatment data in a sugarcane aphid efficacy trial yesterday between rain events, and she reported that there was a slight decrease in aphid numbers on the untreated plots, but nothing to write home about.

So the rains did not really reduce the number of aphids, but, significantly, the cooler temperatures slowed them down. Aphid development and reproduction is slower in cooler temperatures, so the explosive population growth potential is not going to be here until we get significantly warmer. The practical effect of this is that fields that still require treatment, or will require treatment, do not have to be sprayed as quickly as they would be in hotter conditions. This is good for a few reasons, one of which is that it will pay to wait a few days.

We know that our insecticides do not work as well when it is cold, or, put another way, they work better when it is warm. Current predictions put the warmest days next week as Sunday - Tuesday, and then Friday - Sunday. If an application needs to be made, make it during the window of warmest days. Given that we don't really have hot weather in the forecast, it would not be a good idea to cut insecticide rates in the face of these moderate temperatures.

Dr. Kesheimer included a generic formulation of imidacloprid in her efficacy trial because growers are using it due mostly to its relatively low cost and a marketing push. We already have older data that this off-label insecticide does not provide good sugarcane aphid control, and her 7DAT data are reinforcing what we already know. Transform and Sivanto remain the effective sugarcane aphid insecticides.

Sugarcane Aphid Increasing on Late Sorghum

It Is Not Over for the High Plains

Even though it is getting late in the season, sorghum is still at risk from sugarcane aphid, especially later planted sorghum. In Lubbock we are seeing leaves with thousands of aphids, and for the last two weeks many of these have been winged. These aphids have and will continue to ride the winds as they do each year. If this year is like the past three years, the aphids will spread westward and northward. Dr. Ed Bynum in Amarillo is reporting treatable populations in his area. The rains did not stop the aphids, and there is no reason to think they will stop before the first or second hard freeze. Last year we harvested sorghum at the Halfway Experiment Station after first freeze and still had plenty of aphids on the plants and in the heads.

What I am trying to say is that if you have grain or forage sorghum in the field, this is no time to get complacent. The photos below were taken at the Lubbock Research Center this morning before sunrise.

Leaves being killed by aphids, and honeydew darkening the soil where it dripped. 

Leaves on late planted sorghum completely covered by honeydew from the thousands of aphids feeding on the undersides of leaves above. All of the sorghum in this field looked this way. 

Mid-June planted sorghum. The untreated row is on the left, obviously. The row on the right was sprayed with 5 oz. of Sivanto. 

Grain Sorghum: Nearly Perfect Storm in Lubbock and Lynn Counties

After writing in this newsletter last week that fall armyworm was not a significant threat this year, Katelyn Kesheimer, Lubbock and Crosby county IPM Agent, and I visited some fields in southern Lubbock county and south to the middle of Lynn County in the last four days. I take it back; fall armyworm is very numerous in sorghum in these places south of central Lubbock County where my traps are located. We encountered fields at panicle exertion or already booted that had as many as six worms per head, with an average of 2-3 mid-sized worms being the norm. For the most part these were fall armyworms in southern Lubbock County, but corn earworms seemed to increase in frequency as we went south. In a field 6 miles west of Tahoka we were seeing something like the 70% fall armyworm and 30% corn earworm. The age structure of the populations was approximately 45% small larvae, 45% medium larvae and 10% large larvae, but of course this will change quickly. Large larvae are by far the most destructive, and the goal is to treat the field before many of them are present. Kerry Siders, IPM Agent in Hockey, Cochran and Lamb counties, reported in his newsletter tonight that headworms were increasing in his counties, and the majority of these were corn earworms.

Fall armyworm larva feeding on a sorghum panicle in southern Lubbock County yesterday.

Prior to the arrival of sugarcane aphids, control options for caterpillars would have been a pyrethroid, Lannate or Carbaryl. Pyrethroids are not very effective on fall armyworms over 1/2 inch in size, so some area crop consultants are now adding a pint of Lorsban to act as a synergist with pyrethroids. HOWEVER, WE FOUND SUGARCANE APHIDS IN ALL OF THESE FIELDS. The use of a pyrethroid and/or Lorsban would eliminate the biological control agents in the field that are suppressing the sugarcane aphid population.

What we have now in these areas is a real problem. The best control practice would be to go after the worms with a soft insecticide that does not kill the biological control agents that keep sugarcane aphid in check. These insecticides would be Blackhawk or Prevathon. Blackhawk is approximately half the price of Prevathon, but DowAgroSciences has told us that there are no supplies of Blackhawk left in the warehouses because of brisk sales this year in the mid-South. So that leaves Prevathon, which is excellent on both caterpillar species. However, an application of 14 oz/acre of Prevathon, the lowest labeled rate, would cost on the order of $18 per acre + application costs. We cannot recommend less than labeled rates, but area Independent Crop Consultants tell us that 10 oz of Prevathon with 5 GPA by air provides good control of both caterpillar pests. This rate would cost approximately $12.80 per acre + application costs.

If one chooses to follow the pyrethroid + Lorsban path in a field with sugarcane aphids, then it is likely that a follow-up application will be needed for the aphids; at least 5 oz/acre of Sivanto or 1.25 oz/acre of Transform. But this is not a given; the aphids south of Lubbock County do not seem to be increasing as fast as they did in years past. Scouting will be essential.

At this point we do not know what to recommend with so many headworms in the system and aphids in the field; it comes down to economics. There are no inexpensive options here that do not elevate risk from sugarcane aphid, and we can't predict the future with respect to whether sugarcane aphids will require treatment later. (But note that some fields in southern Lubbock County are well over treatment thresholds for both pests.)

We have a sorghum headroom treatment threshold calculator here.  Our written thresholds and scouting procedures are here on page 23.

Another unknown is sorghum midge. The late planted crop is at risk, and with Blackhawk (which is effective on sorghum midge) out of the picture, we will have to resort to pyrethroids, which in turn will increase the risk of sugarcane aphid while not being much use on fall armyworm. Yes, there are no inexpensive answers to this emerging multi-pest situation.