Cotton Pests and Diseases

Ants

Some species of Australian ants (Hymenoptera: Formicidae) are important predators of cotton pests, particularly under raingrown conditions and in lighter soils where ant colonies can establish within the field. However ants can sometimes form associations with aphids, in which they feed on aphid honeydew, and in return, nurture and defend aphid colonies against other predators.

During an irrigation event, ants present in the crop will re-locate their eggs higher into the plant canopy.

Bacteria

Bacterial disease may also affect larvae of various cotton pests such as heliothis. Commonly, bacterial infections can be observed after artificial infection with Bacillus thuringiensis (Berliner), sold commercially as Bt sprays.

Bacterial Blight (Xanthomonas Axonopodis or X. Campestris Pvmavacearum - Exotic Strains)

Although strains of bacterial blight are alreadypresent in Australia, they are no longer a problemdue to varietal resistance. Exotic strains (races)occur, however, that are ‘hypervirulent' and, ifestablished in Australia, would cause large yieldlosses. The disease is seed borne allowing easydispersal and introduction of new races into newareas. Bacterial blight is spread by high temperature,humidity and rainfall.

The initial symptoms include the undersides of leaveshave angular water soaked lesions. Lesions dry anddarken with age then leaves are shed. Black lesionsspread along stem. Bolls often infected at base or tip.Lesions dry out and prevent the boll opening. The pathogen is capable of symptomless transfer andtherefore could be undetected through quarantine.Symptoms include yellowing or bronzing of leaves,leaves wilt and die; dead leaves usually remain onplant. At this stage, roots are dead and surface iscovered with network of tan fungal strands.

Fungal Diseases

Fungal diseases may affect larvae of heliothis, cotton looper, cluster caterpillar and armyworms. Prolonged, moist conditions favour the development of fungal diseases such as those caused by Nomuraea rileyi (Farlow)Samson and Beauveria bassiana (Balsamo) Vuillemin. Larvae infected by fungus become stiff and are anchored to the plant by fungal threads. Later, they are completely covered in white fungus which produces a velvety layer of spores under moist conditions, green in the case of N. rileyi or white for Beauveria spp. These spores infect other larvae and other species of insects producing fungal threads which penetrate their bodies.

TExAS Root Rot (Phymatotrichopsisomnivore)

Texas root rot is an extremely damaging fungal diseasewith a wide host range. It causes sudden death ofaffected plants, usually during the warmer months. Incotton, infection can result in 100% crop loss. If thisdisease became established in Australia, control wouldbe extremely difficult as management using rotationsand fungicides is usually only partially effective.Symptoms include yellowing or bronzing of leaves,leaves wilt and die; dead leaves usually remain onplant. At this stage, roots are dead and surface iscovered with network of tan fungal strands.

Fusarium Wilt (Fusarium oxysporum F. sp. vasinfectum - Exotic Strains)

Fusarium wilt is a fungal disease. Strains ofFusarium were identified in Australia in 1993however the introduction of new strains (races)would increase the difficulty of management as newresistant varieties would be required.External symptoms can appear in the crop atany stage but most commonly appear in eitherthe seedling phase or after flowering when bollsare filling. Leaves appear dull and wilted beforeyellowing or browning progresses to eventual deathfrom the top of the plant. Seedlings may either wiltand die or survive, but often with stunted growth.Adult plants may wilt and die, especially underconditions of stress. Some affected plants mayre-shoot from the base of the stem. Lengthwisecutting of the stem from affected plants will showcontinuous brown discolouration of the tissue.The internal discolouration is similar to that ofVerticillium wilt but usually appears as continuousbrowning rather than flecks. Sometimes thediscolouration is visible in only one side of thestem. External symptoms do not always reflect theextent of discolouration in the stem.

Blue Disease

Blue disease is a virus specific to cotton that canreduce yield potential by up to 20%. It is spread bya vector, the cotton aphid. It has been associatedwith plants infected with cotton leaf roll dwarf virus(CLRDV) and has similarities with cotton bunchytop, anthocyanosis and cotton leaf roll. It is notknown if the same pathogen causes all these diseasesor if there are multiple pathogens causing similarsymptoms. CLRDV was not detected from Australiancotton affected by cotton bunchy top disease. Cottonblue disease affected leaves tend to be smaller, thick,more brittle and leathery and have an intense greento bluish colour with yellow veins. Reddening of stempetioles and leaf veins can occur in some infections.Leaf edges tend to roll downwards and under andplants become stunted due to a shortening of thebranch internodes and produce many branches,giving a bunchy zig-zag stem habit. Symptoms aremore obvious in plants infected at an early age andstunting is more pronounced. Infected plants alsoproduce smaller bolls and boll shed may occur. Singleinfected plants can be overlooked if overgrown bynearby healthy plants.

Cotton Leaf Curl Disease (CLCUD)

CLCuD, sometimes referred to as Gemini virus, cancause yield losses of up to 35% in cotton. It is spreadby a whitefly vector. There are at least seven differentbegomoviruses and several different DNA satellitemolecules associated with CLCuD. A cotton plantneeds to be infected with at least one begomovirusand one satellite to develop CLCuD.Symptoms of CLCuD are seen on leaves andinitially appear as a swelling and darkening of leafveins, followed by a deep downward cupping of theyoungest leaves then either an upward or downwardcurling of the leaf margins. Leaf-like structures(enations) on the veins are common and vary in sizefrom only a few millimetres in diameter to almostthe size of a normal leaf. These larger structures areoften cup-shaped.

Indian Green  Jassid (Amrascadevestans)

Indian green jassid is a sap-sucking insect pest thatcan cause yield losses of up to 25%. While several‘jassid' species are found in Australian cotton .The damage they cause is relatively minor, rarely if everaffecting yield. Green jassids inject a toxin as they feed that causes leaves and bolls to drop and canstunt plant growth. Elsewhere green jassids can bemanaged using resistant varieties and insecticides.Hairy-leafed varieties are used in parts of Africa andthe sub-continent where cotton is hand harvestedto provide effective resistance against green jassids.Such varieties are not suitable for mechanical harvestas the leaf hairs cause excessive leaf trash in thecotton lint.

Bugs

Bigeyed Bug

The huge eyes on the bigeyed bug, Geocoris lubra Kirkaldy provide easy identification. The cylindrical, white eggs are laid singly on terminals and under leaves. Adults and nymphs are fast-moving, daytime hunters of a range of soft-bodied insects and mites which they kill with piercing/sucking mouthparts. Numbers in cotton are highest during flowering. Germalus sp. is a slightly larger, related species also common in cotton but the two may be distinguished by colour. Geocoris is dark brown to black whereas Germalus is brown and green.

Brown Smudge Bug

The habits of the brown smudge bug, Deraeocoris signatus (Distant) are not well known, though it has been found to feed on heliothis eggs, aphids and the apple dimpling bug. The maroon coloured nymphs and the adults are very aggressive predators of mites. It is most abundant in unsprayed cotton in December, January and February.

Damsel Bug

The damsel bug, Nabis kinbergii Reuter inserts its eggs singly into soft plant tissues so that the circular emergence caps protrude above the surface. Nymphs are small and wingless but otherwise similar to the adults. Nymphs and adults use piercing/sucking 'beaks' to feed on many insects including eggs and larvae of moths, mites and aphids. They are usually present in any kind of crop infested with caterpillars. Maximum numbers occur in unsprayed cotton at the same time as maximum numbers of heliothis larvae. Adult damsel bugs may be found on weeds and crops during winter.

Glossy Shield Bug

Larvae including those of loopers and heliothis are the usual food of the glossy shield bug, Cermatulus nasalis (Westwood) and its nymphs. The black eggs which are laid in 'rafts' of 50 or more have short, white spines around the rim. Nymphs are dark red and brown with the early instars being bright red. Maximum numbers are found in unsprayed cotton during midsummer. In early summer they can be found in linseed and lucerne crops infested with heliothis. This species is easily confused with the brown shield bug.

Pirate Bugs

Pirate bugs, Orius spp. prey on thrips and eggs of other insect pests in cotton. They often appear in large numbers at flowering when thrips are in abundance.

Predatory Shield Bug

The predatory shield bug, Oechalia schellenbergii (Guerin-Meneville) is another species which feeds with piercing/sucking mouthparts mainly on caterpillars. Adults can be found at any time of year on weeds or crops infested with caterpillars. The black eggs are laid in 'rafts' usually in multiples of 14, with long, white spines around the rim. The nymphs are dark red and brown. Adults and nymphs are usually most abundant in cotton from December to February.

Red Eyes Bug

Leptocoris mitellata, the red eyes bug. This species may be confused with cotton stainers.

Note that their wings are generally dark, rather than being orange with a distinct dark spot. L. mitellata has a wide host range including fruit trees and many garden plants. They are attracted to light and will often shelter in large numbers around houses. Little is known of the behaviour of these species in cotton. If they are present, ensure bolls are monitored for signs of plant bug damage.

Seed Eating Bug

Graptostethus servus, seed eating bug (Note that their wings are generally dark, rather than being orange with a distinct dark spot.cf pale cotton stainers) G. servus is a pest of sorghum, soybeans, mungbeansand cowpeas. They are attracted to light and will often shelter in large numbers around houses.Little is known of the behaviour of these species in cotton. If they are present, ensure bolls are monitored for signs of plant bug damage

Tarnished Plant Bug (Lygus Lineolaris)

The tarnished plant bug is a pest of over 250 plantspecies. In cotton, its feeding causes seed abortion,stem or leaf wilting and poor seed germination.It has 2-5 generations per year and can thereforequickly build up to high levels.

Beetles

Green Carab Beetle

Beetles belonging to the family Carabidae are general predators which vary greatly in size and colour. Green carab beetle, Calosoma schayeri Erichson is a large, active beetle hunting caterpillars and other slow-moving prey. It flies at night and is attracted to lights. If handled roughly these beetles give off an unpleasant smell. The larvae are predaceous also but have not been found in cotton fields.

Green Soldier Beetle

The green soldier beetle, Chauliognathus pulchellus W.S. Macleay sometimes occurs in large, flying swarms of low density. Some swarms seem to be mating aggregations while others seem to be mass dispersal flights. Large numbers of these beetles can arrive in a crop in a very short time. It is now thought that the adults of these beetles are plant feeders although they are not classified as a pest of cotton. The larvae are predacious although they have not been found in cotton fields.

Red and Blue Beetle

The distinctive red and blue beetle, Dicranolaius bellulus (Guerin-Meneville) is extremely abundant in a range of habitats in some years and is rare in other years. The adults are partly carnivorous and search actively over cotton plants during the day. They eat eggs and small heliothis larvae and probably other slow-moving insects. In rice crops, adults have been found to feed on pollen anthers and stamens and are therefore sometimes referred to as pollen beetles. The egg, larval and pupal stages occur in the soil.

Ladybirds

Adults and larvae of ladybirds are important predatory insects in cotton, especially early season and when chemicals have not been used against pests. They are voracious predators of aphids and, under most conditions, ladybirds, along with lacewings and hover flies, stop aphid populations from increasing explosively. Ladybirds are also important predators of mites, heliothis eggs and small larvae. Ladybird eggs are generally yellow, spindle-shaped and laid standing on end in groups. The larvae are characteristically tapered, often with prominent tubercles, whilst the adults are round and highly convex in shape.

The most common ladybirds found in cotton are: transverse ladybird, Coccinella transversalis Fabricius, striped ladybird, Micraspis frenata (Erichson) and the maculate or three banded ladybird, Harmonia octomaculata (Fabricius). Minute two-spotted ladybird, Diomus notescens (Blackburn) is another common predator in cotton, particularly of mites. The tiny mite-eating ladybirds, Stethorus spp. are also good mite predators.

White Collared Ladybird

This new ladybird species Hippodamia variegata was first recorded by Dr. Franzman (QDPI) in Gatton in November 2000 and has spread rapidly since then. It's not known how they entered Australia, but researchers suspect that it was either accidental or through deliberate introduction for pest control. The species is native to Europe and has spread to many other regions around the world.

Transverse Ladybird

The Transverse ladybird or Coccinella transversalis is commonly found in cotton crops, and are voracious predators of aphids, mites and Helicoverpa eggs and small larvae.

Earwigs

Earwigs have distinctive pincers at the 'tail' and hide on and in the ground during the day. Adult female guard their oval, white eggs and young nymphs in burrows in the ground. The common brown earwig Labidura truncata Kirby is a nocturnal predator of caterpillars. It also attacks heliothis pupae in their chambers in the ground. Adults have wings folded beneath the slate-blue covers on the thorax and can fly.

Black Field Earwig

Black field earwig, Nala lividipes (Dufour) also commonly occurs in cotton fields. It is smaller and entirely black in colour and can be a pest of seeds and seedlings.

Flies

Hover Flies

Adult hover flies (Syrphidae) are harmless but mimic bees and wasps with black and yellow bands across their abdomens. They feed on nectar and pollen of flowers. They fly swiftly and hover. White, oval eggs are laid on their sides near colonies of aphids. Larvae are blind, slug-like maggots which feed voraciously on aphids. The brown, tear-shaped puparia are found under leaves or in ground litter. Other fly larvae like members of the Chamaemyiidae family are also predators of aphids. They are similar to the hoverfly larvae but are smaller and have 2 distinct spines on one end.

Whitefly (Bemisia Tabaci B-Type or Q-Type)

Whitefly feeding results in a sticky residue, sooty moulds, reduced boll size and poor lint quality.Although the B-type whitefly is present in Australia there is a risk of other B-type strains and other biotypes, e.g. Q-type, entering the country with different insecticide resistance profiles. Whiteflies are also vectors of damaging exotic viruses such as cotton leaf curl disease.

Lacewings

Green Lacewings

At least two species of green lacewings, belonging to the genus Mallada are found on cotton. Their eggs are unmistakable, being white, oval and attached in groups to the undersides of leaves by long, flexible stalks.

The larvae are squat, six-legged, voracious hunters which camouflage themselves by lodging corpses of their victims on their backs. They have large, hollow, sickle-shaped jaws which they drive into softbodied insects or eggs before sucking up the contents.

The pupal cocoon is also camouflaged with corpses. Adults have a slow, fluttering flight and are not predaceous as they usually feed on nectar. Greatest numbers are found on cotton in mid to late season, especially if aphids are present. Larvae also feed on mites.

Large numbers of green lacewing eggs are often found in commercial cotton crops, but larvae are seldom seen. This is thought to be due to: the camouflaged nature of the larva making detection difficult; high mortality due to the use of broad-spectrum insecticides; and the often high level of egg parasitism by Telenomus sp.

Brown Lacewing

The brown lacewing, Micromus tasmaniae Walker lays cream, oval eggs singly attached by one side to the underside of leaves. Larvae have sickle-shaped, hollow jaws like green lacewings. They differ from green lacewings by being longer and thinner and by not camouflaging themselves. Larvae are voracious predators of all insects and adults have been seen to eat heliothis eggs. Adults fly at night and are attracted to light.

Spiders

Flower or crab spidersFlower or crab spiders, (family Thomisidae) have crab-like legs and are often brightly coloured. They specialise in hiding on flowers where they are well camouflaged and ambush small insects attracted to nectar and pollen. They make no web but females cover their eggs with a silken shelter, usually under leaves.

Jumping Spiders

Jumping spiders (family Salticidae) have two particularly large, forward-facing eyes which give them excellent binocular vision. They rest during the night and actively roam plants during the day jumping on prey detected by eyesight. They are generalist predators but have been observed eating jassids and small heliothis larvae. Except for the first pair, the legs are relatively short. Most species on cotton are predominantly dark or grey in colour.

Lynx Spiders

Lynx spiders, Oxyopes spp. are active hunters usually seen near the tops of plants. They do not make a web but use their good eyesight to locate prey, then jump on it from a distance. Their legs bear numerous characteristic spines. Their abdomens are narrow and pointed and usually pale brown to green and striped. As in almost all spiders, the young disperse by 'ballooning' from tall plants where the silk is played out into the breeze until the pull lifts and carries them away. Most spiders arrive on cotton in this way in early summer and do not mature and reproduce until the crop is flowering. Lynx spiders are generalist predators like most spiders and have been observed eating small helicoverpa larvae and jassids and are known as important predators of Mirids (Cotton Flea hopper) in the United States.

Nightstalking Spiders

Small nightstalking spiders, Cheiracanthium spp. are pale yellow with a faint grey mark on the abdomen. The 'head' of larger specimens may become pale orange (yellow nightstalking sac spider - Cheiracanthium mordax L. Koch). During the day they hide in silken retreats, often under leaves and during the night they stalk over plants searching for slow-moving prey. They will eat heliothis eggs as well as larvae. Mating takes place in midsummer in large silk retreats and the females guard their eggs in the retreats until they hatch.

Wolf Spiders

Several species of wolf spider, many belonging to the genus Lycosa occur in cotton fields. Most have a shallow burrow in the soil as a retreat and range over the ground searching for prey. Their coloration merges well into a background of soil. Some species also climb plants. Large specimens have been seen killing large heliothis larvae and attack Heliothis moths. Females of these spiders carry their eggs in a grey ball attached to the abdomen. Wolf spiders are most easily seen in early summer before the cotton canopy closes over furrows.

Thrips

Although thrips are one of the most common early seedling pests in most cotton growing districts, their damage is usually cosmetic and rarely affects yield or earliness. Severe damage, resulting in plant tip loss or seedling death, tends to occur only under extreme infestation levels or in combination with cool weather. Control is warranted under these circumstances. However all thrips species commonly found in cotton are also predators of spider mite eggs. So control of thrips to prevent damage should be carefully weighed against their predatory activity and the impact of insecticides on other beneficials.

The most common species infesting cotton are tobacco thrips, Thrips tabaci Lindeman and tomato thrips, Frankliniella schultzei (Trybom). Plague thrips, Thrips imaginis Bagnall, although previously suggested to be an important species, is found rarely in cotton. Adults are small, cylindrical insects less than 1.5 mm in length, and colours range from yellow to grey to black. They have two pairs of narrow wings fringed with long hairs. The nymphs are less than l mm in length, pale yellow and wingless.

T. tabaci is the most abundant in the early season and is usually a pale straw colour. F. schultze can be abundant on young cotton but more typically dominates once flowering has commenced. This species can vary in colour but in most cotton regions is typically a dark almost

In the 2001/02 cotton season, the western flower thrips, Frankiliniella occidentalis (Pergande), was recorded in cotton for the first time. This species was accidentally introduced into Australia in the early 1990's. It is similar in appearance to F. schultzei exept that it is paler in colour (see picture). This makes it easy to distinguish from F. schultzei later in the season but hard to distinguish from T. tabaci in the early season. The western flower thrips causes similar damage to cotton as our native species and also feeds on spider mite eggs. The strains introduced to Australia have resistance to a range of insecticides, including many organophosphates and carbarmates.

Damage Caused by Thrips

Thrips are pests only during the seedling stage of growth. They infest the underside of cotyledons, young leaves and also the growing point of the plant. They use their mouthparts to pierce the leaf surface and suck out plant juices. Damaged leaves take on a silvery, bleached appearance and younger leaves become distorted in shape. Under most circumstances, this form of damage is cosmetic, plants recover quickly and control is not warranted.

Thrips infestations tend to be serious under cool, dry conditions when seedlings are unable to grow away from the damage. Severe damage can result in destruction of the growing point ('tipping out') leading to extensive lateral branching. Seedling death can be associated with extreme infestations or a combination of desiccation by thrips and damping off diseases. If damage to this extent is allowed to occur, heavy losses of plant stand may necessitate replanting the crop, while severe multiple branching may cause a maturity delay of 7 to 14 days. This delay, if compounded by damage from other pests, may cause a yield loss. Analysis of the effects of thrips on the yield and maturity of cotton over the past 6 years has shown that the risk of yield loss occuring is about 1 year in 10 in warm regions; but is higher at 1 year in 2 in cool regions. Overall, the risk of delayed maturity was low with delays of less than 4 days most common. Only extreme populations (70 thrips per plant) caused delays in excess of 7 days. This information can be used to help make decisions on the need for prophylactic insecticide treatment to the seed or at planting.

Cotton Boll Weevil (AnthomonusgranDIS)

Cotton boll weevil is specific to cotton and causes large yield losses due to damage to developing bollsand subsequent reduction in lint production. In theUSA, control of cotton boll weevil using insecticidescosts hundreds of millions of dollars.

Spider Mites (Tetrany Chid Mites)

Spider mites are in the group that includes ticks. Theyfeed on the undersides of leaves, sucking out thecell contents. Their damage causes a characteristicbronzing of leaves, and if uncontrolled candramatically reduce yield and fibre quality. Severalspecies are found in Australian cotton, the mostcommon of which is the two-spotted spider mite.However, overseas there are a range of other speciesthat have different host preferences, cause moresevere damage or have resistance to some of our keyacaricides.

Melon Aphid (Aphis Gossipyi Exotic Strains)

Aphids damage cotton by feeding on young leaves andbolls which can reduce yield. They produce a stickyresidue that can cover leaves resulting in reducedphotosynthesis and contamination of lint as bollsopen, reducing the crop's value. This species may alsocarry exotic diseases such as blue disease. As well asthe risk of disease, there is a risk that new aphid strainsentering the country will have different insecticideresistance profiles, making control more difficult.

Verticilium Wilt - Defoilating Strains

Australian strains of Verticilium wilt are describedas mild in comparison to the defoliating stratins thatoriginated in North America but are now becomingmore widespread. If established in Australia,management would be reliant on the use of resistantvarieties, with a lag of several years before adaptedvarieties were available.

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