|White Grub: Phyllophaga crinita (Burneister)|
|Description and Biology: The adult white grub is a May or June beetle, but only the larva, "C"-shaped with a white body and tan or brown head, damages sorghum. The larvae feed on sorghum roots, sometimes deep in the soil. A fully-grown larva is 20 to 25 mm long. Dark-colored, nondigested food can be seen through the smooth, shiny, transparent, last abdominal segment of the larva. Two rows of minute hairs on the underside of the last segment distinguish white grubs from similar-looking larvae. White grubs overwinter as larvae so are present in the damaging stage when sorghum is planted in the spring. Depending on the species, a life cycle may require one, two, or more years.|
|Symptoms and Damage: Damage results from larvae feeding on roots. The most obvious and significant damage occurs during the spring soon after sorghum plants emerge from the soil. Seed germination occurs and a satisfactory stand is established, but damage to roots causes seedlings less than 15 cm tall to die. Stand loss can occur within seven to 10 days after plants emerge in severely infested fields. One white grub can destroy plants along 0.3 to 0.5 m of a row. Infested plants not killed as seedlings are severely stunted and may never produce grain. A third kind of damage is root pruning by overwintered as well as current-season larvae. Injured plants may produce panicles after such damage but frequently do not have sufficient roots to prevent lodging. Occasionally, lodging is increased by secondary stalk rot organisms.|
Monitoring: Three important considerations when sampling for white grubs are: 1. the insect overwinters in the last larval instar and is present in the soil when sorghum is planted, 2. the presence of larvae must be determined before sorghum is planted, and 3. effective control measures cannot be applied after the crop is planted. To determine the presence and abundance of white grubs in a field before planting, soil in a 32 cm2 area should be excavated with a shovel and examined for grubs. Temperature affects the depth white grubs are in the soil. During cool winter months, white grubs may be 32 cm or more deep in the soil. However, in spring grubs become active and may be only 7 to 12 mm deep. One half or more grub in each sample of soil can cause economic damage to sorghum. White grubs sometimes are seen during disking or other soil preparation activities. Sampling soil for white grubs is the only way to accurately assess abundance.
Management: Planting sorghum in a field where a nongrass crop was grown the previous year is the most important cultural management tactic against white grubs. Preplant application of registered insecticides is effective but expensive because the insecticide must be broadcast and then disked into the soil. Some suppression of white grub abundance can be achieved by using an in-furrow or band application of insecticide at planting.