The two top insect responses in the “One Question Survey” were Cutworms and Aphids.
Cutworms are the larval stage of moths, several species of which damage corn, grain sorghum and cotton. The cutworms species may include Army cutworm, black cutworm, granulate cutworm, pale western cutworm, variegated cutworm.
Cutworms are dingy, gray-black and smooth-skinned (greasy appearance) robust bodied caterpillars that may reach 1-1/2 inches long. When disturbed, the caterpillars curl up tightly into a C-shape. Larvae overwinter and become active in the spring, feeding on a wide variety of plants. Adults are dingy, gray-brown colored moths with light or dark spots on the wings and wingspans of from 1 to 2 inches. (Source)
Cutworms are more likely to be found in fields with abundant winter weeds. Search under these weeds to see if cutworms are present. If cutworms are present it would be advisable to band an insecticide at planting. Fields which have been free of live vegetation for four to six weeks are much less likely to experience a cutworm problem. However, there have been cases in the past when cutworm moths have laid eggs fairly late resulting in small cutworms on seedlings. The black cutworm has been the species most often found this time in the season; whereas, on larger corn the pale-sided cutworm was found a few years ago cutting brace roots.
Aphids may be found on corn, grain sorghum and cotton plants as soon as they emerge from the soil. Several different species of aphids can be found and it is important to be able to distinguish between them.
The aphids of grain sorghum are pictured on the right. More information on identification of these aphids can be found HERE.
Corn Leaf Aphids are rarely found in populations that reduce yield. Instead, they usually benefit the sorghum by attracting beneficial insects that feed on aphids such as lady beetles, syrphid flies and green lacewings. The corn leaf aphid is typically found in the whorl of the plant.
The Yellow Sugarcane Aphid is one we have dealt with for many years. This aphid injects a toxin into the plant which can cause leaf death. Yellow sugarcane aphids are found on the undersides of lower leaves on the plant. The leaves they are on can often be spotted from several feet away due to yellowing or purpling of the leaf.
Sorghum plants should be inspected beginning the first week of plant emergence and twice weekly until plants have at least five true leaves. Yellow sugarcane aphids most damage small sorghum plants. Very small sorghum seedlings (one to three true leaves) often are damaged significantly after being infested for a week or less. As plants grow larger, they better tolerate aphid feeding. Source
Economic Injury Levels are based on control cost and crop value and vary depending on the number of leaves on the plant. (See tables at the bottom Source)
The Sugarcane Aphid has been in Texas since 2012.
Sugarcane aphids (SCA) are different from Yellow Sugarcane aphids in both appearance and injury. SCA are usually found on the underside of leaves lower in the plant canopy but can be found on all parts of the plant.
While I have not found them near economic levels prior to boot in the past few years, they are early season pests in other sorghum producing areas of the state. I received notice SCA have been found this week in the Lower Rio Grande Valley on sorghum at the V8 stage (8 leaf collars visible). This finding is about 3 weeks earlier than last year.
Monitoring sorghum fields for aphids should begin as soon as the plants emerge and continue until harvest. Several economic thresholds for SCA are effective. I think all three of these will work in the mid-coast of Texas.
- 50-100 aphids per leaf (field average)
- 20% of leaves checked have 50 or more SCAs
- 20 – 30% of plants are infested with localized areas of honeydew and established colonies are present