Chinch Bug Damage in Sorghum
Similar to last year, conditions are again favorable for chinch bug infestation in sorghum, especially recently seeded fields. With wheat harvest completed, chinch bugs have the potential to move into sorghum fields looking for a new food source. Last week, I received a report of a sorghum field at the Cimarron Valley Research Station (Payne County) that was destroyed due to chinch bugs. A particular concern is in late planted or double cropped sorghum planted directly into the recently harvested wheat. Chinch bugs attack a variety of forage, lawn, and wild grasses. The principal crop plants damaged are wheat, corn, sorghum, and oats.
Damage
The chinch bug pierces the plant with its mouth parts and sucks out the plant sap. This feeding prevents normal growth and results in dwarfing, lodging, and yield reduction. Severe infestations during early development may cause plants to wilt and die prematurely (Fig. 1 -3). Most injuries are caused by later instars.
Figures 1. Chinch Bug in whorl and damage. OSU
Figure 2 - Chinch Bug in whorl and damage. OSU
Figure 3. Severely damaged sorghum plants on field border.
Life Cycle
Chinch bugs overwinter as adults in various protected areas, particularly among weeds and grasses near fields. Adults emerge in the spring and deposit eggs singly behind the leaf sheath or in the soil at the base of the small grain crop plant. In a few days, the eggs hatch and the nymphs begin feeding on all parts of the host plant from the roots to the uppermost leaves. The nymphs undergo multiple developmental stages (Fig. 4), the last being the adult stage. There are two to three generations per year, the later generations migrating to corn and sorghum when small grain crops become dry. (Fig. 5).
Figure 4. Chinch Bug nymphal instars and adult. NC State Extension.
Figure 5. Chinch Bug Damage in sorghum, next to harvested wheat. OSU.
Description
The adult chinch bug is about 4 mm long and black with opaque wings (Fig 6). The wings vary in length, from as long as the body to 1/3 to 1/2 the length of the body and each bears a distinctive, triangular, black mark. The wingless nymph is smaller than but similar in shape to the adult. The head and thorax are brown; the eyes are dark red; and the abdomen is pale yellow or light red with a black tip.
Figure 6. Adult Chinch Bug. Cornell Extension
Control
The first line of defense against chinch bugs is to plant insecticide-treated seeds. Gaucho (imidacloprid), Poncho (clothianidin)) and Cruiser (thiamethoxam) are all effective seed treatments that should provide three weeks of control (Sorghum Checkoff, 2024).
The worst situation is likely to be in double cropped sorghum that is being planted directly into the wheat stubble infested fields. Under this scenario, the entire field may need to be treated. Where sorghum is planted adjacent to a harvested wheat crop, many times only border rows may be infested and need treating.
Treatment thresholds will vary with plant size, but the most common recommendation for seedlings less than 6 inches is two or more chinch bugs on 20 percent of the plants. Larger plants (> 1ft.) can tolerate more insects, however, can still be damaged by 10 or more chinch bugs/plant. In addition to the leaf blade, look under the leaf sheath and around the plant base to find chinch bug activity.
Foliar insecticide options are limited almost exclusively to the pyrethroids. The two most commonly used are lambda cyhalothrin (Warrior w/Zeon) and Zeta-cypermethrin (Mustang Maxx). Chlorpyrifos (Warhawk, Pilot, Others ) also have reasonably good activity on chinch bugs, but supplies may be limited. Chlorpyrifos is labeled and can be used in 2024. For any of the insecticides, coverage is key, so use plenty of water and direct as much spray as possible to the small plants.
For more information on chinch bugs in sorghum, consult CR-7170, Management of Insect and Mite Pests in Sorghum