Insect pest management in cotton

Insect pests that feed on plant structures that directly produce yield, such as growing tips and fruiting structures, are generally the greatest problem in a cotton crop. These pests include Helicoverpa and mirids. The cotton plant has an indeterminate growing pattern (fruit are present for most of the season), which allows some pests to develop through several generations.

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Genetically modified cotton

Until the introduction of genetically modified (GM) cotton, the main insect pests of cotton were the larvae of Helicoverpa armigera and H. punctigera. These pests were generally controlled with synthetic insecticides, which also controlled the majority of other cotton pests.

Now, over 90% of the cotton area is planted with varieties that have been genetically modified to produce highly specific toxins that kill Helicoverpa larvae and most other caterpillar pests when they feed on the cotton plant. However, sucking insects may require more management on Bt cotton compared to conventional cotton.

The genes used to produce the toxin in GM cotton are obtained from a naturally occurring bacterium called Bacillus thuringiensis (commonly known as Bt). Insect-resistant or genetically modified cotton is often called 'Bt cotton'. The introduction of Bt cotton has resulted in a significant reduction of insecticide use by growers.

A resistance management plan (RMP) is essential to ensure that these valuable traits remain effective. Visit the Cotton Cooperative Research Centre (CRC) website for more information.

Major pests

Helicoverpa

Helicoverpa remains a major pest on conventional (non-genetically modified) cotton. The two species - Helicoverpa armigera and H. punctigera - are very similar in both appearance and the damage they cause to cotton. However, correct identification of the species is important because H. armigera has developed high levels of resistance to insecticides.

Species composition in the crop will be influenced by the time of year and location. In temperate regions, the majority of the H. armigera population overwinter from mid-March onwards and emerge during September/October. Helicoverpa punctigera is usually the dominant species through September, but seasonal variation can lead to early infestations of H. armigera in some years, particularly in northern districts.

Helicoverpa adults are mobile and produce large numbers of eggs, leading to rapid build-up of populations under favourable conditions. Survival is influenced by the number of predators present as well as other mortality factors such as wind, rainfall and temperature extremes.

Damage

In cotton, all stages of plant growth may be attacked but reproductive tissue is preferred. Seedlings can be 'tipped out' when terminal buds are eaten. Chewing damage to squares and small bolls may cause them to shed, and chewing damage to maturing bolls may prevent normal development and can lead to secondary fungal infections such as boll rot.

Monitoring and thresholds

Regular monitoring of the crop for the presence of larvae and/or damage is necessary in order to make timely decisions on control. This is especially important when targeting small and possibly insecticide-resistant larvae.

Visual sampling is the recommended method. Check at least 30 plants or 3 separate metres of row for every 50 ha (larger samples give more accurate estimates). Eggs are not necessarily a good basis for thresholds as not all hatch and very small larvae have high mortality rates. Assess beneficial insect numbers and note parasitised eggs and larvae.

When making pest management decisions in cotton, insect numbers alone may not be sufficient. Plant monitoring (fruit load, yield and maturity) assists in decision making when pest levels are just below threshold or when there are combinations of pests.

Thresholds for conventional cotton depend on crop stage and are:

  • Seedling to flowering: 2 larvae/m or 1 larvae (>8 mm)/m.
  • Flowering to cut-out: 2 larvae/m or 1 larvae (>8 mm)/m or 5 brown eggs/m.
  • Cut-out: 15% open bolls - 3 larvae/m or 1 larvae (>8 mm)/m or 5 brown eggs/m.
  • 15%-40% open bolls: 5 larvae/m or 2 larvae (>8 mm)/m or 5 brown eggs/m.

Thresholds for Bollgard II® cotton are the same for the entire season at 2 larvae (>3 mm)/m in 2 consecutive checks or 1 larvae (>8 mm)/m.

Resistance

Helicoverpa armigera developed resistance against most insecticides in the late 1990s. However, with the introduction of genetically modified cotton, biopesticides and more selective insecticides, insecticide resistance to older chemistry has decreased in recent years. Helicoverpa punctigera has no known resistance to any insecticides but the use of more selective options is encouraged to help preserve natural enemies.

In order to prevent insecticide resistance, the cotton industry has developed the Insecticide Resistance Management Strategy (IRMS). This strategy is reviewed annually to delay development of resistance of H. armigera to conventional insecticides. The core IRMS principles include:

  • rotation between chemical groups with different modes of action
  • limiting the time period during which an insecticide can be used
  • limiting the number of applications of one particular insecticide.

Visit the Cotton Cooperative Research Centre (CRC) website for more information.

Cultural control

Post-harvest cultivation (pupae busting) to reduce the overwintering stage of Helicoverpa is one of the most important cultural control practices available. Cultivation to a depth of at least 10 cm will damage or disturb pupae, seal their emergence tunnels and trap emerging moths. Cultivation also leaves survivors open to attack by birds, mice, earwigs, and wasp parasites.

Natural enemies

Beneficial insects can affect all Helicoverpa life stages:

Predators
Parasitoids

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Spider mites

The two-spotted mite (Tetranychus urticae) is the most numerous mite species on cotton. Other species that may occur are bean spider mite and strawberry spider mite. Mites live on the underside of leaves and are difficult targets for aerial sprays. Mites prefer more mature leaves and the highest populations are usually found near the top of the plant on main stem nodes 3-5 below the terminal. Heavy mite infestations on cotton can originate from adjoining fields of early sown corn from where they are carried by air currents.

Damage

Mite infestations in seedling cotton rarely justify control but may be a useful indicator of potential problems and should be monitored closely. The first sign of damage is bronzing of the upper leaf surface near the petiole or leaf fold. As numbers increase, the leaves turn red and become covered in fine webbing, and affected leaves may dry and fall off. High mite populations can significantly affect yield and quality of cotton. The earlier in the season that infestations develop, the greater the potential of yield loss and quality downgrades.

Monitoring and thresholds

Start sampling at seedling emergence and continue at least weekly. At low numbers, mites are difficult to find. Mites are often too numerous to count individually, so infestations can be rated on a presence/absence system. In Australia, the leaves on the third to fifth main stem node (down from the plant terminal) are the best to sample.

The general threshold for mites for most of the growing season is 30% of plants infected. The threshold depends more on when mite populations begin to increase and how quickly they increase rather than the number of mites. To estimate the percentage of yield reduction caused by mites, a chart has been developed by the cotton industry for the different cotton growing regions.

Management

  • Under Australian conditions, mite populations increase less rapidly on the okra leaf varieties than broadleaf varieties.
  • Mite infestations increase after the application of some broad-spectrum insecticides for control of other pests.
  • Control weeds within fields and along field boundaries that serve as over-wintering sites for mites.
  • For chemical options to control mites see the latest Cotton CRDC's pest management guide.

Thrips are the major mite predator in cotton. Others include damsel bug, bigeyed bugladybird beetlesbrown smudge bugapple dimpling bug, brown lacewing adults, and tangle web spiders.

Mirids (Creonitiades spp)

An important sucking insect pest of cotton, mirids are abundant in cotton from early to mid season and occur throughout the cotton growing regions of Queensland and New South Wales. Green mirids are more common than brown mirids but both cause similar damage.

Damage

Adults and nymphs feed by piercing plant tissue and releasing a chemical that destroys cells in the feeding zone, resulting in the following symptoms:

  • localised leaf damage
  • terminal wilting of young plants
  • deformity (broom shaped)
  • shedding of squares and small bolls
  • damage to bolls (up to 15 days old) by causing warty growths inside carpel which causes discoloured lint
  • malformed bolls, delay of maturity
  • reduced lint and fibre quality.

Monitoring and thresholds

Mirids are a very mobile pest and populations can fluctuate rapidly so sampling needs to be done every three days. The beat sheet is the most effective means for estimating mirid numbers. Monitor for number of mirids as well as for tip damage and fruit retention on cotton plants. Adult cause more damage than the first three nymphal instars while the fourth and fifth instars do similar levels of damage as the adults. Heavy tip damage and fruit retention levels can also have an impact on control decisions.

Thresholds based on beat sheet sampling vary between warm and cool areas due to the cotton plants ability to compensate for damage.

  • Warm areas. Early season: 4 mirids per metre. Mid season: 3 mirids per metre.
  • Cool areas. Early season: 2 mirids per metre. Mid season: 1.5 mirids per metre.

Management

  • Control alternative hosts including native weeds before crop establishment.
  • Avoid broad-spectrum insecticides.
  • Spray only when both insects and damage are at threshold levels.
  • For chemical control options see the Cotton CRDC's pest management guide.

There is potential to use lucerne as a trap crop for mirids. Lucerne should be established two weeks before cotton.

Research has shown that the addition of petroleum spray oils to some chemicals improves product efficacy at lower (half) label rates and helps preserve natural enemies. Research has also demonstrated that the addition of salt (5g/L of water) to dimethoate greatly improves that product's efficacy against mirid nymphs and adults at rates as low as 33% of the registered rate, while lessening the spray's impact on natural enemies. Note that previous registrations of dimethoate were suspended on 6 October 2011, and replaced with permit PER 13155. Visit the APVMA website for more information. A number of beneficial species are known to feed on mirids yet none are recognised as regulators of mirids in cotton populations. Adults, nymphs and eggs are eaten by damsel bug, bigeyed bug, predatory shield bug, and a number of spider species.

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Aphids

The cotton aphid (Aphis gossypii) is the most common aphid pest. Green peach aphid is an occasional pest in the early season and cowpea aphids can colonise cotton but rarely become a problem.

Damage

Aphids cause early to late season damage to terminals, leaves, buds and stems, and are known to transmit Cotton Bunchy Top (CBT) diseases. The honeydew secreted by aphids can contaminate lint once bolls begin to open.

Monitoring and thresholds

Aphids tend to be patchy in distribution so sample in several locations. At each location collect 20 leaves from different plants (one leaf per plant) and choose main stem leaves from 3-4 nodes below the terminal of the plant. Sample for winged and wingless adults as well as nymphs. Non-winged adults indicate that the population has settled in the crop. Aphids prefer new growth so focus on terminals and recently expanded leaves. Start sampling at seedling emergence until defoliation and sample weekly. Where hotspots of aphids are evident, also monitor cotton for CBT.

Aphid numbers are determined by a scoring system. Seasonal aphid scores are obtained by using the table in the Cotton CRDC's pest management guide.

Significant yield loss can occur if aphid populations are allowed to develop to high levels (>90 % of plants infested) for a period of 2-3 weeks. Thresholds depend on the estimated yield loss which, when compared to cotton price and control costs, helps determine whether control is warranted. When open bolls are present thresholds should be lowered to 50% of plants infested or 10% of plants if honeydew is present to prevent contamination of lint.

Management

  • Control weeds that serve as alternative hosts for aphids, including marshmallow, capeweed, thistles, nightshade, bladder ketmia and thornapples.
  • Ratoon and volunteer cotton should be controlled as they are winter hosts and may also carryover CBT disease.
  • Cotton aphid has developed widespread resistance to a number of insecticides, so adhere to the cotton IRMS.
  • For chemical control options see the Cotton CRDC's pest management guide.

Whitefly

Silverleaf whitefly (Bemisia tabaci B-biotype) (SLW) and Bemisia tabaci Q-biotype are the most serious whitefly pests in cotton due to their resistance to many insecticides and rapid reproduction rate. Bemisia tabaci Q-biotype is a recent arrival in Australia (2008) and currently not as widespread as SLW. How this biotype will affect cotton compared to SLW is not yet known. Others found in cotton are greenhouse whitefly (Trialeurodes vaporiorium) and eastern Australian native whitefly (Bemisia tabaci).

Whiteflies are found in all cotton growing areas of Queensland. SLW is more prevalent in warm areas. Hosts include cotton, sunflower, tomato, rockmelon, sowthistle weed. In cotton growing areas, SLW normally start to reach pest levels in December through to cotton harvest. While SLW are capable of vectoring serious cotton viruses, no such viruses are present in Australia. SLW is highly resistant to some insecticides and has the ability to develop resistance to newer insecticides if resistance is not carefully managed.

Damage

SLW feed on plant sap and cause plants to wilt, drop leaves and under severe pest pressure, plants may die. Excretion of honeydew contaminates lint. One of the sugars in SLW honeydew has a lower melting point than those found in aphid honeydew and creates problems during processing. Honeydew promotes sooty mould, which reduces potential crop yield by blocking sunlight and reducing assimilation of nutrients for plant growth.

Monitoring and thresholds

Sample once a week from first flower (777 degree days (DD)) and then twice a week from peak flowering (1300 DD). Sample from a minimum of 2 sites in your sample area (maximum of 25 hectares). Count the number of adults per fifth node leaf (sample one leaf per plant).

Thresholds are based on a scoring system. Score each leaf as either un-infested (0 or 1 adult per leaf) or infested (2 or more adults per leaf). Calculate the percentage of crop infested and the crop age in day degrees as thresholds are relative to crop development stages. Visit the Cotton CRC website for more information on whitefly thresholds and managment.

Management

  • Control alternate weed hosts and consider planting cotton away from other hosts such as soybeans, sunflower and cucurbit crops.
  • Develop an area-wide management strategy with neighbours including a tight planting window so that whitefly does not migrate to successive plantings.
  • Avoid early season broad-spectrum insecticides that kill beneficial insects and subsequently flare whitefly populations. Natural enemies play a vital role in the management of SLW.
  • For chemical control options see the Cotton CRDC's pest management guide.

Admiral - an insect growth regulator (IGR) for the control of whitefly. IGRs prevent insects from moulting to the next development stage (i.e. eggs do not hatch, juveniles do not become adults and females are infertile). Admiral is a suppressor not a quick knockdown insecticide, so it may take several days to see SLW numbers decline after spraying. Only use Admiral once per season so that resistance does not develop.

Natural predators include ladybird beetles and bigeyed bugs. Two small parasitic wasps, Eretmocerus hayati (released in 2005) and Encarsia formosa (released in 1930s) are important parasitoids of SLW. Both wasps are present in cropping regions but may not occur at levels that control SLW.

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Minor pests

Thrips

Thrips including the tobacco thrip (also known as cotton seedling or onion thrip), tomato thrip, and western flower thrip are common early seedling pests but their damage is usually cosmetic and rarely affects yield or earliness of cotton.

Damage

Nymphs and adults cause early season damage to terminals, leaves, buds and stems. Infestations are more serious in cool dry conditions when seedlings are unable to grow away from damage. Severe damage results in destruction of the growing point (tipping out) which leads to lateral branching. Seedling death can occur due to severe infestation but this is often associated with damping off diseases.

While considered a pest, thrips also predate on spider mite eggs. In recent seasons western flower thrips have been abundant on terminal growth later in the season.

Monitoring and thresholds

Sample weekly from emergence until the crop has six true leaves for number of thrips/plant, checking for both nymphs and adults (presence of nymphs indicates that the population is actively breeding). Check the number of thrips on 20-30 plants for every 50 ha of crop. Look for symptoms of tip damage which appear as extensive blackening of the small leaves within the terminal. Adults are highly mobile and nymphs are very small so counts may vary with the time of day. The damage potential also varies with crop vigour and temperature.

For seedlings to six true leaves the threshold is 80% reduction in leaf area and 10 thrips/plant (adults and nymphs). Thresholds may need to be lowered in adverse conditions such as cold weather, herbicide damage and damping off disease.

Management

  • Consider that thrips are natural enemies of spider mites. Use chemical control only if thrips damage exceeds the threshold.
  • Cotton crops planted next to cereal crops are more at risk of thrips infestations.
  • Thrips prefer cooler temperatures and populations decline if temperatures rise above 30ºC.
  • Western flower thrips are resistant to some insecticide groups.
  • For chemical control options see the Cotton CRC's pest management guide.

Predators of thrips include pirate bug, lacewing larvae, brown lacewing, and ladybird beetle.

Sucking bugs

Green vegetable bug (Nezara viridula)

Green vegetable bug (GVB) is an emerging pest of cotton. In the past, sprays applied for other pests have also controlled GVB, however the adoption of genetically modified cotton and greater implementation of IPM has resulted in reduced insecticide use and GVB populations are increasingly found in cotton crops. The crop is most susceptible to damage from flowering onwards. Both nymphs and adults cause damage to bolls. External damage is visible through feeding marks and internal damage shows up as wart-like growths and lint staining. Damage to bolls older than 18 days is uncommon with the preferred age about 12 days and less.

Monitoring and thresholds

Crops should be inspected for GVB twice weekly in early to mid morning from boll set to boll maturity. Beat sheet sampling is the most efficient monitoring method as infestations tend to be patchy. Monitor as many sections of the field as practical. GVB nymphs are more difficult to sample accurately as their distribution is extremely clumped, particularly during the early nymphal stages. Regular damage assessment from boll set to boll maturity is recommended - randomly select 14 day old bolls and look for the presence of warts or stained lint. Damage symptoms cannot be distinguished from those caused by mirids.

Thresholds depend on the sampling methods used:

  • Visual sampling: 0.5 adults/m from flowering to harvest.
  • Beat sheet sampling: 1 adult/m from flowering to harvest.
  • Boll damage (14 day old bolls): 20% damage.

Nymphs of the GVB are less damaging than adults and their numbers must be converted to adult equivalents.

Natural enemies

GVB eggs are frequently parasitised by a tiny introduced wasp, the green vegetable bug egg parasite (Trissolcus basalis). Parasitised eggs are easily recognised as they turn black. GVB nymphs are attacked by ants, spiders and predatory bugs. Final (fifth) instar and adult GVB are parasitised by the recently introduced tachinid fly (Trichopoda giacomellii).

Pale cotton stainer bug (Dysdercus sidae)

Pale cotton stainer bugs generally arrive in cotton around the time of first open boll. Adults mate soon after arrival resultng in an expanding population of developing nymphs. Cotton stainers are recognised as occasional pests of cotton, but rarely cause economic damage and are often controlled by broad-spectrum insecticides applied for other pests. They cannot survive in temperatures above 40°C and need free water for survival. In mild seasons they may need to be managed especially in genetically modified cotton crops where fewer insecticides are used.

Cotton stainers feed on developing and mature cotton seed, resulting in loss of seed weight, oil content and seed viability. Feeding on bolls up to two weeks old can kill developing seed and cause boll shedding. Where bolls are not shed, lint yield may be reduced and 'tightlock' around the damaged seed prevents lint from fluffing out as bolls open.

Cotton stainers can continue feeding on seed throughout boll development and also feed on the mature seed of open bolls, creating bald patches (less lint). Yellow staining on lint may be evidence of watery faeces as bugs feed in the open bolls. Pale cotton stainer bug damage is similar to that of green vegetable bugs and includes a black spot on the outside of the boll, warty growths inside the boll and brown discoloured lint

Monitoring

Beat sheet sampling is a suitable method to monitor bugs but visual searching in the lower canopy compliments monitoring methods. Sample at multiple sites as distribution is often patchy. Stainer bugs tend to hide in the heat of the day and are not easily observed at this time.

First instars are usually found on the soil or in the lower canopy and tend to remain in clusters. Older nymphs can be found in the lower to mid canopy and can be seen feeding in open bolls. Once stainer presence is confirmed, monitor bolls for signs of damage by cutting open bolls of varying ages to confirm damage.

Natural enemies

The role of natural enemies of pale cotton stainer bugs has not been studied in Australia. Natural enemies such as tachinid flies (parasitic flies) and predatory bugs such as assassin bugs have been reported in Africa.

Cotton harlequin bug (Tectocoris diophthalmus)

The cotton harlequin bug (also known as hibiscus harlequin bug) feeds on many species belonging to the hibiscus plant family (Malvaceae), including ornamental hibiscus species and cotton. It is sometimes a minor pest of cotton, feeding mostly on young shoots, piercing the stems and sucking the sugar-rich juices intended for shoot growth.

It has been known to cause the introduction of a fungus, which rots the cotton boll. Nymphs feed on seeds in very small bolls or in opening bolls. Generally damage occurs only in autumn.

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Beetles

Wireworms

Wireworms are larval stages of beetles that live in the soil and generally feed on organic matter. True wireworms (Agrypnus variabilis)  prefer wet soil whereas the eastern false wire worm (Pterohelaeus darlingensis) prefers drier conditions proected by stubble and weeds.

Wireworms generally cause problems in spring especially in newly developed fields, fallowed fields (with heavy trash cover) and in fields following summer crops such as soybeans. They bore into germinating seeds and chew through young seedlings just below the soil level. Damage can occur for up to four weeks after planting and heavy infestation results in patchy stands that may need replanting.

Wireworms are difficult to sample as they move into the soil when dry and only emerge after rain or irrigation, and seedling damage is often the first indication of their presence. Control measures must be applied before or at sowing. The most effective control is achieved by modern seed dressings. In-furrow band application of granular or liquid insecticides are also effective. For chemical control options see the Cotton CRC's pest management guide.

Redshouldered leaf beetle (Monolepta australis)

Redshouldered leaf beetle is a very minor pest of cotton, damage is not common and usually confined to the edges of fields. Larvae feed on plant roots while the adult beetle chews terminal buds, leaves, squares and surface of bolls.

Leafhoppers (jassids)

Both the vegetable leafhopper (Austroasca viridigrisea) and cotton leafhopper (Amrasca terraereginae) can affect cotton. Adult and nymphs suck sap and inject toxins. Leafhoppers feed on the upper surface of leaves. The damage appears as a pale stippled effect. They occasionally damage seedlings and new growth. Leafhoppers can transmit diseases such as viruses and mycoplasma.

The current threshold for control through the early season is 50 leafhoppers per metre of row.

Minor caterpillar pests

A number of caterpillar pests can attack cotton plants. Most of these pests are sporadic pests and rarely need control unless large populations are present. For insecticide options to control specific caterpillar pests see APVMA. Many caterpillar pests use weeds as their main host plants before moving into cotton. Control of these weeds will assist in minimising pest infestation.

Rough bollworm (Earias huegeliana)

Presence of rough bollworm in cotton is often associated with the presence of the weed bladder ketmia, Hibiscus trionum. Direct boll damage occurs when larvae tunnel into bolls. Larvae can also be found in squares. Primary growing points can be damaged by larvae tunnelling into the main stem. This can occur at any growing stage including in seedlings.

The northern rough bollworm (Earias vittella) is of minor importance in northern Australia, widely distributed throughout Asia and is an important pest of cotton in India.

Cotton tipworm (Crocidosema plebejana)

Outbreaks of cotton tipworm are associated with winter and spring rainfall and cool temperatures which favour growth of Marshmallow weed Malva parviflora, the main host of tipworm. When marshmallow dies off in early summer, tipworm move into cotton.

Larvae tunnel into terminals destroying the single stem habit. Subsequent multiple branching can lead to excess vegetative growth at the expense of reproductive growth and crop development can be delayed. At high numbers, squares and bolls can be damaged.

Cotton looper (Anomis flava)

Loopers are leaf feeders. The cotton looper prefers older leaves and defoliation of cotton progresses upwards on the plant. Up to 80% defoliation has been recorded from large infestations of cotton loopers.

Pink spotted bollworm (Pectinophora scutigera)

The pink spotted bollworm is present in Central Queensland. Larvae tunnel into large squares, flowers and bolls and often complete their lifecycle within the one structure. Mature larvae can also pupate on surface thrash and effective burial of cotton crop residues reduces subsequent infestations.

The pink bollworm (Pectinophora gossypiella) is a major pest overseas but currently restricted to north western Australia. It could cause serious damage if it spread to eastern Australian cotton regions.

Light brown apple moth (Epiphyas postvittana)

The host range of light brown apple moth includes many broadleaf weeds as well as lucerne.  It can be found infesting cotton seedlings in cool seasons. Young larvae feed by tying terminal leaves together with webbing.

Cotton leaf perforator (Bucculatrix gossypii)

Young larvae of the cotton leaf perforator (first to third instar) feed between the upper and lower leaf surface (mine) while the fourth instar feeds directly on the leaf. The fifth instar causes the most obvious damage by feeding on the lower surface, leaving windows and holes in the leaf.

Cluster caterpillar (Spodoptera litura)

Cluster caterpillar was a serious pest in Northern Australia cotton growing areas but it is a minor pest in Queensland and New South Wales. Young larvae skeletonise leaves at night; larger caterpillars consume whole leaves. Caterpillars can also destroy squares and flowers. Large infestations can lead to larger areas being defoliated.

Lesser armyworm (Spodoptera exigua)

While often present in low numbers in young cotton, lesser armyworm generally prefers weed hosts. Large numbers can cause heavy losses in seedling crops by defoliating plants and re-sowing may be necessary. Mature larvae can be mistaken for Helicoverpa.

Cutworms (Agrotis spp.)

Cutworms attack cotton seedlings at or above ground soil level. They feed in the late afternoon and at night, and can destroy seedling by chewing through stems or eating entire leaves. Insecticide treatments should be applied in late afternoon and evening

Other pests

A number of other pests can infest cotton from time to time. Most never in occur in numbers that require control. Other pests include:

Further information