What Pests Are Harmful to a Cotton Plant?
Several different types of pests attack cotton plants; some of them even live on the plant throughout the year, going through several generations in just one year. These pests are capable of inflicting terrible damage on the plant, which often results in the death of the plant, and ensuing reduction of the cotton crop.
Cotton Boll Weevil
Having migrated from Mexico during the latter part of the 19th century, the boll weevil is a tropical insect and, as a result, has a limited range in the United States. The adult is 8.5 mm long, and is reddish-brown or gray in color. They have a double spur on the inside of each front leg. This is what the female weevil uses to puncture the young cotton bolls to lay her eggs. The larva is a 13mm-long grub with a white body and a brown head.
Cotton Flea Hopper
The boll weevil lays eggs near the tips of the cotton plant leaves; the eggs hatch in seven days. The tiny nymphs go through five changes before finally emerging as adults a month later. The adult flea hopper is about ¼ inch long with a yellow-green and black speckled body and black spotted antennae. As it feeds on the cotton plant, it injects a toxin into the plant that causes the cotton plant to appear unhealthy and wilted.
Five species of cutworm attack cotton plants; black cutworm, pale-sided cutworm, variegated cutworm, granulate cutworm and army cutworm. Cutworms usually cut the cotton plant off while it is still a seedling, and the result is typically re-planting of the cotton in that area. The worms eat roots and stems as well as tender leaves and cause massive damage. Cutworms over-winter as larva beneath the soil. Eggs are laid in the spring and hatch in about five days. The larva have four sets of tiny legs, and appear greasy.
Adult thrips are winged, very tiny, and suck the juices from the newly emerged cotton bolls. Eggs are laid in the tissues of the plant, and are small with a creamy white color. They hatch in four days, which means an entire generation of thrips is completed in only two weeks. Thrips eat grasses as well as cotton, and usually begin with the grass but move on to feed on cotton when the grass begins to go to seed. Adults and larva both feed on the same plants.
Tarnished Plant Bug
Sometimes called lygus bugs, tarnished plant bugs lay their eggs in the stems of the cotton plant and in the ribs of the leaves. Eggs are laid in the plant tissues, with only the end of the egg showing from the outside. They hatch in eight days. The wingless nymph stage comes next and lasts 11 days. A generation passes in roughly a month’s time. Nymphs are a pale green shade at the time of hatching, with an orange spot on the abdomen. Once they begin feeding they turn a dark green. As they grow older they develop black spots on their backs. The adult insect is ¼ inch long and is a mottled brownish-yellow color.
Eggs are laid on the bottom side of the cotton leaf and hatch within one to two weeks. Three days after hatching the immature insects become fixed in place, having found a food source. They spin cocoons, and shortly after they emerge as adults, they are already mating and laying eggs. The adults and the crawling larvae consume the sap from the cotton plant, which causes the plant to droop somewhat. They secrete a substance on the plant that is known as honeydew. This later grows fungal infestations, which inhibits photosynthesis in the plant.
A new generation of aphids can occur as often as every five days, and all the insects are female. They do not require a male to reproduce; and the young are not hatched from eggs, but are birthed alive. Aphids come in three colors--yellow, green and black--and are pear-shaped. They are 2mm long. Some aphids have wings, while others do not. Black mold forms over the aphid's secretions, which can cause the plant to wilt and die. Aphids suck the juices from the plants in much the same way that white flies do, and cause the same types of damage.
Armyworms are smooth-skinned caterpillars with five pairs of legs. Three types of armyworms attack cotton plants. The Beet Armyworm is green or black with a dark head. They sometimes have three light-colored stripes up their backs, and reach a length of around 30mm. They have small black spots on each side of their segmented bodies, right behind their heads. They eat the leaf down to a skeleton, but also eat the cotton blooms and bolls as well. The Fall Armyworm is either green, brown or black with an upside-down “Y” shape on its head. It eats just about every part of the plant, as well as drilling holes into the stems. This caterpillar gets about 40 mm long and has one black stripe down both sides of its body and one pale yellow stripe on its back. The Yellow-Striped Armyworm is 45 mm long and can be gray, dark gray or black in color. They have a yellow-orange stripe on each side and two black triangles on the back of each segment of their bodies. This caterpillar has an inverted “Y” shape on its head, and usually does its worst damage on young seedlings rather than mature cotton plants, often requiring re-planting.
40 mm long as a caterpillar, the Cabbage Looper has three pairs of legs and several white stripes down its tapering body. They chew huge holes in leaves but do not completely strip the leaf to a skeleton. The adult Cabbage Loopers are gray-brown moths with silvery spots on the center of both wings, which look like the figure 8. Loopers over-winter in the pupal stage, dangling from the undersides of leaves in white cocoons. Adult moths come out of the cocoons in the spring and lay their greenish-white eggs on the leaves. They hatch in a few days and develop into worms within a month.