Most sorghum fields are blooming or beginning to bloom. Insect pests of grain sorghum in bloom include Sorghum Midge and aphids. We have not found high numbers of sorghum midge in the fields yet. I expect the midge populations to increase with time and later planted sorghum will be more prone to having problems with sorghum midge.
Other potential insect pests of grain sorghum include aphids, stink bugs and headworms.
We continue to find corn leaf aphid, yellow sugarcane aphid and the sugarcane aphid in sorghum fields. Sugarcane aphids are in low numbers in many fields across the area.
The sugarcane aphid has not yet been studied enough to have confidence in an economic threshold (ET) but we can make some educated guess as to where the treatment threshold should be. The sugarcane aphid does not inject a toxin into the plant like the Yellow Sugarcane Aphid.
We have seen that this aphid will develop into high numbers on 1-2% of the plants before spreading to the plants in the rest of the field.
In the Lower Rio Grande Valley, Extension Entomologists are suggesting a treatment threshold of 40% infested plants. An infested plant is a plant containing one or more sugarcane aphids. While this may be the best threshold for the valley, I am not convinced this is the ET we should use in the Mid-Coast.
I suggest one of two methods for determining if a field should be treated.
1. Treat when 40% of the plants contain a certain number of aphids such as 100 aphids.
a. I like this because I don’t want to treat a field having only 40 aphids per 100 plants.
b. The drawback to this ET is the clumping nature of this aphid. Some plants may be overwhelmed before the ET is met.
2. Treat based on the greenbug ET (but remember the SA does not inject a toxin):
a. Preboot treat before entire leaves on 20% of plants are killed.
b. Boot to heading, treat at death of one functional leaf on 20% of plants.
c. Heading to hard dough, treat when aphids cause death of two normal-sized leaves on 20% of plants.
If honeydew production is the concern, treat if aphids are in the head and producing honeydew, but remember, a rainfall event of more than ½ inch could clean up the honeydew.
If I was growing sorghum and sugarcane aphids were in my field, I would get the insecticide I intend to use and put it in my barn. (Suggested insecticides include Transform, Dimethoate and Chlorpyriphos.) I would then continue to closely monitor the aphid population every 2-3 days and treat when I thought the numbers were at a critical level to my field.
The next potential insect pests in sorghum are stink bugs and headworms. Treating for these insects with a pyrethroid may cause the aphids to flare up. This influence in the aphid population may warrant a change from the pyrethroid to and alternative chemistry such as dimethoate for stink bugs, which will have a positive effect on the aphids.