Identifying and managing cotton pests
Larvae of the cotton bollworm moth Helicoverpa armigera cause severe damage to cotton crops reducing yields in many of the major growing regions, including the USA, Mexico, Brazil, India and China. The adult female moth lays between 1,000-1,500, ribbed, spherical eggs in one cycle with the larvae emerging about three days later. The larvae attack the plant causing damage to the blooms that may open prematurely and stay fruitless while seedcases (bolls) may fall off or produce reduced amounts of cotton lint of inferior quality.
Under optimum temperature conditions the cotton bollworm moth lifecycle is just 35-40 days, meaning it is capable of producing three to four generations per crop.
Identification: The adult moth is about 35-40mm across with yellow/orange fore-wings, that bear a single dark spot and paler second wings with a distal stripe. Eggs of about 0.6mm are visible near flowers, bolls and on leaves. The emerging larvae are identifiable by dark, green stripes.
Management control: Pheromone and light traps attract the moth enabling early detection and population estimates for chemical control. Biological control includes the Trichogramma pretiosum fungus which attacks eggs.
Cotton boll weevil
Anthonomus grandis is a major pest of cotton and highly-mobile, travelling distances of over 70km in strong wind currents. The bug infests crops in North, South and Central America where it attacks the developing bolls, damaging the cotton lint inside. The disease has cost growers in the USA hundreds of millions of dollars in yield losses and control.
Identification: The adult weevil is grey/brown and about 5mm long, with a 3mm-long protrusion from the head, while the larvae are white grubs with brown mouthparts. Egg laying damage appears as puncture marks at the side of flower buds which are covered by a secretion that hardens to form a tough blister.
Management control: Crop rotation and any other factors that get crops growing quickly - good seed beds, early planting in good conditions, and treated seed - will help the plant reach a more mature and robust stage by the time the weevil is active.
Aphis gossypii is found wherever the crop is grown throughout the world. In most regions the aphid uses secondary host plants during its lifecycle. Sexual reproduction in the autumn, yields numerous eggs which overwinter to hatch once spring temperatures rise above 10c. Early signs of an infestation are yellowing leaves but as aphid numbers grow, the symptoms rapidly worsen, and foliage begins to pucker and curl. Without any control the pest will eventually colonise the whole plant, depositing honeydew which then attracts a black, sooty mould. Where this is the case, plant growth will be stunted and in the most extreme infestations, may even die.
Identification: The winged female is small (less than 2mm) and ranges in color from its yellow legs, pale-green abdomen and black head and thorax. Wingless females are various shades of green with yellow legs and antennae. Likewise, nymphs show color variations from green, tan and grey with a dark head, thorax and wing pads. Eggs are yellow initially but quickly turn gloss black. Females can reproduce asexually (parthogenesis) with up to 50 generations a year.
Management control: Clearing secondary host plants, where possible will reduce the aphids’ habitats for egg laying and overwintering. This includes destroying volunteer cotton and plant trash. Monitoring for population thresholds should begin at seedling emergence and is recommended at weekly intervals in some growing regions. Counting the number of aphids on leaves can then be compared and converted to likely damage scores using standard data tables produced by some authorities. Biological controls include various parasitic species of beetles and their larvae, certain wasps and lacewing.
Spider mites which infest cotton include species such as Tetranychus urticae, T turkestani and T pacificus. These are found in numerous cotton growing regions including South America, South East Asia and Africa with variations in presence or dominance of each of the three species. Mites can spread across long distances and readily cross borders on imported plants, on clothing or in strong wind currents.
Identification: Mites are more closely-related to spiders than they are to insects and have eight legs, are just 0.5mm long and can be green or red depending on the species.
Species may feed on the upper or lower leaf surfaces and the intensity of the damage can cause defoliation.
Management control: Weekly sampling should begin at seedling emergence and the frequency should be increased in hotter, drier weather. The mite has many predators which can be introduced including: thrips, certain ladybird types, damsel bugs, brown lacewing adults, and tangleweb spiders. Spider mites can survive on almost any broad-leaved weed so clearance of preferred secondary hosts such as turnip weed, deadnettle, and sowthistle, will reduce carry-over to the next cotton crop.
Both Green Creontiades dilutus and Brown Creontiades pacificus are flying insects found in Australia and Eastern Asia where they damage cotton plants by introducing toxins during feeding. The adult and nymph stages release chemicals when their mouthparts pierce plant tissues. This destroys cells and can cause branching in the cotton shrub when growing points are killed. Bolls, buds and flowers may also be destroyed resulting in significant yield and quality losses. Mirids lay eggs by inserting each one into the plant tissue with an oval cap left protruding above the leaf petiole surface. The full lifecycle, from laying to adult, takes around 21 days and the adult may then live for a further four weeks.
Identification: The winged adult Green Mirid is pale green, about 7mm long with red markings, while the pear-shaped nymph has red/brown tips on its antennae. Brown Mirids differ because they have a brown head and thorax and are slightly larger at about 8mm long. At the nymph stage their antennae have red/brown and white stripes.
Management control: Sampling to establish infestation levels should be carried out on a weekly basis initially with an increase to twice-weekly at peak fruit production when the crop is more vulnerable. Lucerne may be used as a trap crop to draw the mirid from the cotton crop while removing plant trash and weeds such as wild turnip and thistles, will limit the opportunity for overwintering.