Saturday, August 26, 2017

Sorghum: Sugarcane Aphids, Headworms and Midge

We have been closely monitoring sugarcane aphid numbers at the Lubbock Experiment Station and hoping the rain would knock them down. Unfortunately this has not happened, and our untreated plants in bloom now have 500 - 2,000 aphids per leaf and the lower half of the canopy has severe leaf damage. There are plenty of winged adults, too, and these will be flying off to infest other fields. With all of the late sorghum planting after failed cotton, there is now a very wide range of sorghum maturities out there and the younger plants are still subject to the full force of the aphid. High Plains insecticide action thresholds for each growth stage are presented on page 5 of our SCA Management Publication. There is also a statement about re-treatment thresholds.

However, we now have a 2-3-axis threat because cotton bollworm/corn earworm egg laying has really picked up and is now at a level of concern in both cotton and sorghum, and sorghum midge can still injure crops yet to complete bloom. In sorghum, cotton bollworm and fall armyworm comprise the headroom complex, and these insects feed directly on the developing kernels and can cause significant yield loss. Our treatment thresholds are based on the size of the worms, number of plants per acre, cost of control and market value of the grain, and these thresholds are presented on page 22 of Managing Insect and Mite Pests of Texas Sorghum. While cotton bollworm numbers are high, thankfully fall armyworm numbers are fairly low.

We think our High Plains bollworms are still susceptible to pyrethroid insecticides even though there has been some weakness in susceptibility in south Texas. A headworm population that is predominately cotton bollworm (but not fall armyworm) can be taken out with pyrethroids - EXCEPT that using them will eliminate most of the biological control in the field and stimulate a sugarcane aphid and/or yellow sugarcane aphid population increase. 

If a field reaches treatment threshold for either headworms or sorghum midge then insecticides should be applied to protect yield. However, if sugarcane aphids are present in the field then the choice of insecticide is important. We have some "soft" insecticides for headworms that will not remove the beneficial insects that are important for aphid control. Unfortunately, except for Blackhawk insecticide, this is not the case for sorghum midge, and any "hard" insecticide application (for either pest) should be followed up by careful monitoring of aphid populations. Insecticide options in these multi-pest situations are presented in "Insecticide Selection for Sorghum at Risk to Sugarcane Aphid Infestations".