Cotton IPM: Module 2

Module 2. Observing Rule One – Grow a Healthy Crop

Now that the introductory part is out of the way we are ready to move on to the main part of our course – designing a practical IPM approach for a cotton growing area with which you are familiar.

For those of you who participated in agle@rn network’s Introduction to IPM, you will find what you learned there quite useful and you might want to review that course for an overview of the underlying concepts. In this course, again, we will introduce various techniques that together comprise a “Cotton IPM toolbox” and talk about preventative approaches as well as interventions a grower may decide to use in order to maintain profitability. Unlike the introductory course, however, these tools will not be dealt with individually. In the following lessons of this course we will attempt to show how these tools might be integrated into a more holistic crop protection strategy that can be implemented throughout the growing season and even during the times before and after the cotton crop is actually in the field.

This is because IPM is a whole-season approach to managing pests. Decisions made before, during and after the cotton-growing season have an impact on pest management throughout the year. In an earlier lesson we talked about four IPM components. In reality these merge into one another and are integral within a farmer’s annual plan. For example, the farmer cannot decide to deep plough or change the variety after sowing. The fallow ploughing and the variety influence levels of pest damage. Clear felling a wood lot can destroy the nesting sites of the birds that eat the caterpillars in the crop. The 100 cash units a farmer might get from selling the wood will not pay for the 1000 cash units she paid for the insecticides she had to buy because the birds had gone away.

In an attempt to make your analytical work easier, we have divided the agricultural year into 5 critical time periods, a scheme based on work done by New South Wales Agriculture and the Australian Cotton Cooperative Research Centre. Two periods focus on ‘off season’ management and three deal with management through the growing season. For each of these periods we list and describe a range of potentially useful tools that have worked in various circumstances around the world.

Interested students can learn more about the 5 critical periods and Australian cotton IPM “best practices” by reading the excellent online publication linked below. While many of the recommendations this document contains may only be relevant to Australian cotton we feel that participants from other areas may want to learn more about what is going on there and think about Australian practices that might also be locally relevant.

Lesson 2.1: Period 1 – Post Harvest and ‘Off-season’

A successful cotton IPM program does not only concern the time in which the crop is actually in the field. The best way to get a head start on pests is as soon as the previous crop is harvested. Particular activities that have been shown to be effective during this period are pupae busting, planting off season rotation crops (mainly N. India and Pakistan), management of cotton regrowth, off season weed management and growing of refuge crops for beneficials.  Australian growers commonly grow a late generation trap crop during this period but this is not common elsewhere.

Pupae Busting

Helicoverpa armigera survive during the offseason (winter in Australia, summer in India…) as pupae in the soil. These insects have survived the sprays of the previous season and so may be resistant to insecticides.  Post harvest cultivation in cotton can therefore be an important management tool. Commonly known as ‘pupae busting’, this aids in the mechanical destruction of pupae in the soil and their exposure to birds and the elements. This operation should be done as soon as possible after harvest as it also increases the chance of rain events sealing up the pupal emergence tunnels, preventing any surviving moths from emerging.

Don’t forget that the stubble of other crops may also harbour Helicoverpa pupae and it is a good idea to practice pupae busting activities on these as well.

Selection of Rotation Crops

In areas where sufficient water is available in the off-season for another crop, it may be possible to break the life cycles of various pests by sowing a rotation crop that is less susceptible to the pests and diseases affecting cotton. The most commonly used rotation crop in South Asia is wheat (which has been shown to improve soil structure).  Pulses (horse gram, green gram, lathyrus), may also be sown into the stubble according to local customs.  Legume crops sown by Australian growers include faba beans, soybeans, hyacinth bean, field peas, horse gram, green gram and cowpeas.

Pest control in rotation crops also has implications for the cotton crop to follow. For instance, use of broad-spectrum insecticides, such as pyrethroids or organophosphates to manage Helicoverpa or aphids in rotation crops may have a detrimental effect on residual beneficial insect populations in the crop locality.

Management of Stubble/Regrowth

Regrowth of cotton (volunteer plants) after harvest can provide a food source for Helicoverpa, pink boll worm, spider mites, and aphids. Regrowth should be controlled by cultivation or slashing.

Management of Weeds

Weeds are off-season hosts for a number of pests including Helicoverpa, aphids, armyworm and whitefly. However, we also need to remember that weeds are part of the biodiversity of the agricultural environment and can act as reservoirs for natural enemies of beneficial insects and spiders.

Although developed in Australia for Australian growers the site below provides some useful information on weeds in cotton.

Growing of Refuge Crops for Beneficials

Controlling on-farm weeds in fallow and cropped fields in borders and bund areas results in less refuge area for conservation of beneficials. To ensure that beneficials have areas for growth and reproduction it may be advisable to plant refuge crops in the off-season. Alfalfa (Lucerne) sown on bunds may provide fodder and a refuge for beneficial insects and spiders.

This tactic is part of a broader approach that focuses on habitat diversification.

Last Generation Trap Cropping

Cotton farmers in Australia are being increasingly attracted to the idea of ‘trap crops’ which attract Helicoverpa away from cotton into areas where they can be controlled before they infest cotton. Can this approach be attempted in other countries? In southern India a common recommendation is to sow okra early in cotton as an indicator of pink bollworm attack. If soil moisture permits, could we suggest sowing okra on bunds as a late season trap crop for other bollworms?

In Australia, the ‘last generation’ trap crop is planted with the aim of attracting moths emerging from non-diapausing pupae under cotton. Would something similar work in your area?  Could something like fodder sorghum be appropriate?  The trap crop would be planted so that it will be attractive to Helicoverpa late in the cotton season. The trap crop must be sufficiently attractive to change the local distribution of Helicoverpa moths, concentrating them into the trap crop area and effectively concentrating egg laying in the trap crop. This effectiveness of this strategy depends on making the trap crop more attractive than the cotton crop. This implies that a grower must be careful not to apply too much nitrogen which creates excessive cotton growth towards the end of the season and increases its attractiveness to pests.

The trap crop should be carefully monitored to ensure that it doesn’t become a nursery for pests. The eggs and larvae in the trap crop can be destroyed physically or by applying a ‘soft’ insecticide.

While there is currently no established recommended area for trap cropping, many farmers are adopting the figure of 1% of their total farm area as a starting point.

Lesson 2.2: Period 2 – Pre-plant Planning

Just before planting a cotton crop, a range of decisions need to be made that have serious implications for managing pests during the growing season. Main concerns at this time include nutrient management, varietal selection, water management (in irrigated systems), planting a first generation trap crop, good seedbed preparation and seed treatments.

Nutrient Management

The amount of nitrogen available to the crop affects pest management as well as potential yield and maturity. Too little nitrogen will prevent the crop from achieving its yield potential. Too much nitrogen often creates excessive cotton growth toward the end of the season. This makes the crop more attractive to Helicoverpa and other potentially serious late pest problems. Some agronomists advise soil, or (better) leaf analysis sampling to determine how much nitrogen to apply.

While nitrogen is important, a successful cotton grower must also be concerned about other nutrients.

A recent study by agronomist Mans Lanting of AME Bangalore measured the nutrient content of the lint that is harvested from the fields. His research showed that every 100 kg of cotton lint harvested removes:

element or compound N P2O5 K20 MgO CaO S Fe Mn Zn Cu B
kg of element or compound 15.4 5.3 11.2 9.3 20.4 2.5 0.5 0.025 0.04 0.007 0.02

Note the comparatively high quantities of MgO and CaO. The plant needs these materials to maintain the quality of the lint. They are essential components of the staple.

Based on his work, Mans Lanting has developed some relevant recommendations for low external input sustainable agriculture (LEISA) in cotton. He has the following advice for small farmers in southern India which may be equally relevant for other locations.

  1. Rotate cotton with sorghum or maize.
  2. Incorporate crop trash in non-pink bollworm areas. Burn trash in the field where pink bollworm is endemic and incorporate the ash.
  3. Incorporate fym* or composted biomass @ 1.2 T per 100 kg expected lint yield (with 20% added) (on an acre basis).

Example: if a farmer expects to harvest 500 kg per acre he or she should incorporate 5 x 1.2 T = 6 T and add a further 20% = 1 x 1.2 T = a total of 7.2 T or 20-25 cartloads.

*fym = farmyard manure

In addition farmers should incubate the fym/compost with a mix of 1 kg azotobacter (cotton), 1 kg phosphobacteria, 5 kg trichoderma and 48 kg cotton seed cake.

  1. Add sulphate of potash @ 15 kg per 100 kg expected yield, gypsum @ 20 kg per 100 kg expected yield.
  2. At 45 DAS on sandy soils only: Top dress with nitrogen – apply urea @10 kg per quintal expected yield. Work in the same amount at 75 DAS.

In many situations farmers have limited access to farm yard manure. In such cases he or she would need to use various combinations of mineral fertilizer and compost. The important factor to be aware of is that the fertilizer application needs to compensate for the nutrients removed at harvest + 20%.

Varietal Selection

The variety should be matched to the region and likely pests and diseases. It pays to talk to seed company representatives, local extension agents and knowledgeable farmers to get a better idea about the varieties available and how their characteristics as varieties differ in susceptibility to pests. For example, there is some evidence that okra leaf varieties have a degree of resistance to both Helicoverpa spp. and spider mites. More information on the various traits for resistance.

Physiological traits of cotton important in resistance

Pest Resistance Factors Comments
Helicoverpa
  • Shiny (glabrous) leaves
  • Earliness
  • Nectariless trait
Hairy leaves are preferred for oviposition. Plants with short duration avoids attack Seems to work best in combination with other factors. Assumed to be because the plant offers no food source for the moths.
Pink bollworm
  • Nectariless trait
  • Okra leaf trait
Same as above and seems to modify the microclimate within the plant or permit insecticide penetration. Plants with this trait may have lower yields than those with normal leaves
Spider mites
  • Resistance in G. barba-dense but not in hirsutum.
  • Leaf thickness and cuticle thickness.
  • Chemical content of leaves
Mechanism not known. Polygenic. For instance, genotypes with low sugar and high phenol content are more resistant.
Jassids
  • High leaf hair density
  • Greater length.
Thrips
  • High leaf hair density
Appears that pubescent leaves need not be combined with another factor such as a thick epidermis or a chemical factor
Whitefly
  • Hairy leaves
  • Okra leaf shape
  • Open canopy

Those interested in host plant resistance to insects and a framework for understanding the various options available in cotton might want to read this linked paper by George Teetes.

Plant Resistance to Insects: A Fundamental Component of IPM

Bt Cotton

No discussion of varietal selection and host plant resistance in this day and age would be complete without something on Bt cotton. ‘Bt’ is short for Bacillus thuringiensis. It is a bacterium that lives in the soil. It has the remarkable ability to produce protein crystals that are highly toxic to many caterpillar species as a stomach poison. There are many strains of Bt and their degrees of toxicity and the spectrum of species they affect vary considerably. Formulations of the actual product (crystals) have been used for many years as conventional sprays where selective caterpillar toxins are needed. (N.B. related species of Bacillus are toxic to other kinds of insects).

Bt cotton is a cotton plant that has been genetically engineered to contain genes for producing these toxic crystals. It then becomes an insecticidal plant. While it is popular with farmers in countries where its use is legal and results in significantly lower uses of chemical pesticides, it has not been universally accepted. Many consider the dangers it poses to outweigh its benefits.

Interested persons can visit the following Websites for more information on Bt cotton.

Intercrops, border crops and trap crops

Small holder farmers in southern India have found that the enhancement of biodiversity in and around their fields provides multiple benefits. The practice has spread to larger cotton holdings in Guntur District.

Border crops of maize or sorghum provide a physical barrier to a field of young cotton plants (the cereal grows faster). Wind currents tend to carry flying pests higher over the barrier so that fewer land in the field. They also form a chemical barrier that masks the odours given out by the cotton plants that attract moths seeking egg laying sites.

Inter crops (short duration pigeonpea, sorghum, soybean, mungbean, dwarf cowpea, castor) attract beneficials to the fields. They are all harvested after about 70-80 days so do not interfere with the cotton. The beneficials are released into the cotton at harvest.

Trap crops (marigold and sunflower for Helicoverpa, castor for Spodoptera) sown at low density around the outside of field reduce ovipositon within the field.   Okra (lady’s finger or bhindi), a botanical relative of cotton, has a special role as a border crop. It attracts pink bollworm and can either be used to tell the farmer that pink bollworm is present or it can take on the role of the trap crop. ‘Stung’ pods turn over at the tip. Two batches are usually sown. One at the same time as the cotton, the other two-three weeks later. Growers must ensure trap crops do not become future nurseries of Helicoverpa and must practice effective control in the trap crop. This may involve timely destruction of the crop itself. Because the trap crop will not be harvested for yield, bio-pesticides like Bt and virus formulations may be well suited because a fast knock-down is not required.

Women family members commonly take ownership of the intercrops and find them valuable for feeding the family and as a source of cash. The taller plants double up as bird perches.

In Australia, lucerne (alfalfa) planted around the field supports a residual population of predators and parasites.

Seedbed Preparation

A feature often mentioned by leading cotton growers in achieving an early crop is a good seedbed. Desired characteristics include non-cloddy soil and firm, high, well shaped beds. This helps achieve earliness by providing the seedling plants with the best chance to grow and not be held back. Some research in Australia has shown that cotton shows better seedling growth when sown into standing stubble of previously harvested wheat or sorghum.

Seed Treatment

Many growers have been able to delay the build up of sucking pests for 30-40 days by treating their seeds with a neonicotinoid insecticide but the decision to use this tool should be made carefully. Although this insecticide is quite selective and does not affect beneficial groups early in the season, a personal observation is that seed treatment with a neonicotinoid can be so effective that the ‘clean’ crop delays the build up of beneficials. Having said this, however, this procedure is less likely to impact on beneficials than a foliar application of a broad-spectrum insecticide and should be considered to be a ‘best practice’.

  1. Seed may be treated (or said to be treated) with a neonicotinoid insecticide before it is purchased. Farmers may or may not wish to accept the risk that the treatment is going to be effective.
  2. Farmers may also wish to treat the seed with a fungicide – this should not create a compatibility problem although there are obvious limits as to how much pesticide powder a seed can carry.

Lesson 2.3: Period 3 – Planting to Flowering

This is a critical time for making informed decisions about pest management tactics in cotton because it is when predators move into the crop. The good news is that this is also the time when the crop is most resilient and able to recover from pest damage, often without yield loss or delayed maturity. Pest control during this stage focuses on four criteria:

  • Damage to the crop
  • Monitoring pest abundance
  • Pest thresholds
  • Predator to pest ratios

Specific approaches during this stage include early planting, toleration of non-economic early season damage, installation of bird perches, augmenting beneficial populations, nutrient management, pest, beneficial and plant growth sampling, observing action thresholds and application of chemical or biological control products. Details on each of these follow.

Early Planting

Growers usually plant as early as is practical and possible. Preseason soil preparation (perhaps with the first monsoon showers) to optimise soil structure and seedbed tilth, will facilitate early planting. Dryland farmers who sow too early run the risk of the rains failing, but will often do this because they fear late season Helicoverpa attacks much more. Late planted cotton runs the risk of declining yield potential (due to falling temperatures at the end of the season) and late infestations of H. armigera, which are difficult and expensive to control.

Toleration of Non-economic Early Season Damage

The compensatory capacity of cotton provides the opportunity to reduce spraying in the critical early season phase, thereby allowing beneficial insects to survive and contribute to the control of pests. It has been shown that cotton plants can recover from most early season damage with no reduction in yield or delay in maturity. For example, it has been shown that young cotton plants can tolerate up to 80% reduction in leaf area (leaves less than 1 cm in length) up to the six-true-leaf stage without yield or maturity being affected. Note that seed treatment with a neonicotinoid affords about a month protection, and a further two weeks spray-free period should be possible.

Installation of Bird Perches & Beneficial Insect Augmentation and Inoculation

A key practice in cotton IPM involves keeping a favourable balance between friendly organisms and pests.  Farmers in India have had considerable success in erecting bird perches (minimum of 10/acre or 25/ha) that allow birds such as drongos to sit 1m above the height of the crop will result in enhanced predation of any large bollworm larvae that have escaped the other IPM components. Increasing natural enemy numbers – Birds

Other related practices include collecting and installing paper wasp nests (augmentation) and releasing mass reared natural enemies (inoculation).  For example, in some localities it is possible to purchase commercially raised organisms, some of the most important of which areTrichogramma chilonis and Chrysoperla carnea which feed on the eggs of Helicoverpa, Earias and Spodoptera.

Recommendations for use of these insects are:

  • Trichogramma chilonis – release 50,000/acre (125,000/hectare) at 10 days intervals during the growth phase.
  • Chrysoperla carnea – release 2 larva / plant during the growth phase.

Nutrient Management (Australia)

Some farmers have had success with monitoring the need for supplementary nitrogen application. This provides the opportunity to apply extra nitrogen, if required, in time to be effective. Assessing nitrogen need can be done in a variety of ways including petiole nitrate tests, chlorophyll meters or leaf color charts.

Pest, Beneficial and Plant Growth Sampling

Crops should be checked visually for pests, beneficials, and for damage or fruit retention at least 2-3 times a week. Thorough, objective sampling that takes account of the variability in pest densities across a field is essential to IPM. This is a time consuming process that can lead to ‘operator fatigue’. There are no fixed rules, except that it is necessary to get a representative sample of the whole field or farm and you should not cause more damage to the crop than the pests.

If we are dealing with fields of up to 2 ha, most people would take a transect from one corner to the opposite one and then go the other way. A plant would be taken at random every, say, 20 or 30 paces depending on the size of the field. This practitioner paces out the distance, throws his hat over his shoulder and selects the plant where the hat lands. The sample unit depends on the circumstances. Eggs per leaf, or plant, bollworms per plant, damaged bolls, number of leaves with jassid burn per plant, number of aphid colonies per plant etc. It is usually necessary to count the buds, flowers, squares and bolls. It may be necessary to collect plant parts and take them away to search – for instance with pink bollworm, where they may invisible within a boll or flower.

If the fields are bigger it may be necessary to sample several 0.5 – 2.0 ha blocks at random from within the fields.

How many samples per field or block? The simple answer is 20!! That is the voice of experience. But this should be established experimentally first by applying this formula:

  • Take a number of samples – and 20 is a good place to start – work out the mean and the standard error of the mean of whatever parameter you are most concerned.
  • Number of samples needed to keep the standard error of the mean within 10% of the mean (the convention acceptable to statisticians) = [standard deviation/(0.1 x the mean of the sample)]2.

There are other less fatiguing ways to sample pest populations and involve the use of pheromone or yellow sticky traps.

Pheromones

Pheromones are chemicals released into the environment (atmosphere or water) that influence the behaviour of another member of the same species. They are a subset of the group of substances called semiochemicals. Pest managers normally think of pheromones in terms of a female insect attracting males when she is ready to mate. However, there are many more examples.

  • Mammals marking their territory with a scent from a special gland or with urine.
  • The males of some mammalian species secrete a pheromone to repel other males during the mating season.
  • Ants follow a chemical trail between a food source and their nest.
  • Queen honey bees secrete a pheromone that suppresses the sexuality of the worker bees.
  • The females of a few species of insect release a pheromone that attracts males and other females into mating aggregations.

The pheromones of many pest species (mainly Lepidoptera, also some Coleoptera) have been identified, synthesised and made available commercially. The resultant technology is based on the exposure of the pheromone chemical or blend of chemicals to the atmosphere in a manner that results in the slow release of the material, usually over a minimum period of a month. The pheromone is dissolved in an organic solvent and absorbed into a rubber or plastic ‘septum’. This is simple but effective.

The pheromone septum is placed in a trap that catches the males as they fly around the pheromone source. Traps are most commonly set up at the rate of 2 per acre or around 5 traps per hectare.  The design of the trap is usually optimised for each species. It could be a funnel with a bag attached, a net cage or a sticky surface.

There are three applications of this technology:

  1. Monitoring the flight pattern of insects. This is perhaps the most common application. The pheromone traps are placed in a strategic position and are examined regularly to get an indication of whether the target species is flying in the vicinity. This procedure basically results in information that tells a farmer that he may have to make an intervention in the near future. One or two traps per 10 ha are sufficient for Helivcoverpa in cotton. Too many in a small area interfere with each other. This is because the ‘plumes’ of pheromone can fuse and create a confusing ‘signal’ to the male insects. The traps should be a minimum of 100 m apart.

Two additional points need to be held in the mind when considering this tool.

  • the pheromone trap only attracts males and they may have a different flight schedule to females.
  • even if the male flight pattern is representative of the female flight pattern the there is no guarantee that the females will lay eggs in the vicinity. They could be ‘just passing through’.

Helicoverpa is one of the problematical species in this case. The pheromone has been available for many years but it has never proved as effective as the pink bollworm pheromone for example. Even if large numbers of insects are caught, there is not always a corresponding increase in oviposition over the next few days. It is assumed this is because of points a) and b) above.

  1. Male confusion and interference with mating. If the air above and around a cotton field is flooded with a pheromone during the mating period of the relevant species, the males would not be able to locate the females that are calling to them in that field and mating would not take place. The males would become confused. The theory meets the practice and such schemes are in place in northern Africa for pink bollworm management.
  2. Attract and kill. The idea is that males are attracted to a pheromone lure and brought into contact with a fast acting insecticide.

The two management applications are based on the premise that the males are ‘caught’ as soon as they start to fly, soon after eclosion and that females do not engage in multiple mating with ‘outside’ males.

Yellow sticky traps: Another way to monitor some pest populations (primarily Whitefly) through trapping is by the use of yellow sticky traps.  These can be made with iron board which is triangular in shape. It can be painted in yellow colour. Smear a few drops of castor oil in the evening hours. This attracts the adults and the nymphs of whitefly. This can be placed in intervals at the rate of 3 per acre or 7.5 traps per hectare. (Source: Pesticide Post, Vol. 8, No. 5, September 2000 http://www.ciks.org/sep-20.pdf). A good article on yellow sticky traps for whitefly monitoring in cotton can be found by clicking the following link – http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7401.html

Observing Action Thresholds

Of course, monitoring pest populations and crop damage is useless unless it is used to make a decision about whether or not to intervene with a chemical or biological control product. In some countries clear guidelines have been made available by a government agency or crop protection company that indicate when to act based on the stage of the crop, the density of the pest and how to respond if predators are present. These factors determine the threshold.

Threshold – The point at which an action is taken; often applied to insects; (most are action thresholds – an action is taken when a level of eggs or caterpillars is reached; or can also be an economic threshold, which takes the commodity and treatment cost into consideration).

Thresholds are usually worked out experimentally by establishing how much yield loss is caused by different densities of a pest and the cost of eliminating the constraint in the context of the value of the crop. This can be a long process, but it is effective.

On the other hand, if circumstances permit, thresholds can be established empirically by the ‘what happens if’ method. This is OK provided it is controlled and fully recorded. Divide a series of fields in paired 0.5 ha sections. Establish the pest and predator density in each and make whatever intervention may be called for. Compare the change in density of the pest and predators and the resultant change in crop status over a suitable period. In our current context we would for instance assess the number of buds, open flowers, bolls and damaged bolls (or boll weight) per plant and relate these parameters to the density of pests and predators in control and intervention ‘plots’. If you allow two grams of lint per boll and you know the expected price per unit weight of the lint it is possible to estimate the benefits of the intervention.

Application of Chemical or Biological Control Products

In many cases, even if great care has been taken to control pests through non-chemical means, pest populations reach a point where they start to cause unacceptable economic losses. When this happens, the most common response is to apply chemical or biological control products. Here are some ‘rules’ that may help in decisions about when and what to spray.

  • The most selective control option should be used first.
  • Do not use early season broad-spectrum insecticide sprays, particularly synthetic pyrethroids, carbamates, and organophosphates. This reduces beneficial populations and often leads to outbreaks of spider mites, which increase unchecked by beneficials.
  • When monitoring indicates that the first bollworm eggs are about to hatch, apply mild insecticides such as neem oil and entomopathogens (Bt, and nuclear polyhedrosis disease formulations).  Some farmers in India have also reported success in using sprays made from fresh green chilies and tobacco.  The Centre for Indian Knowledge Systems (CIKS – http://www.ciks.org/) has developed several recommendations on preparing and applying biological control products.  Interested students should click the following link to popup a new window with some of their ideas.
  • If control of an insect pest is warranted, the most selective, effective option should be used. For control of Helicoverpa this will often be a biological insecticide. Selective synthetic insecticides are the next choice.
  • During this period growers should avoid applying two consecutive synthetic insecticide sprays. This is a recipe for creating pesticide resistant pests.
  • Broad-spectrum insecticides are a last resort in this period.

Lesson 2.4: Period 4 – Flowering to One Open Boll Per Metre

This is the critical period where the crop is setting the fruit that will contribute to the yield and maturity of the crop. Helicoverpa will probably be the main target throughout Asia and the Pacific Rim. Cotton strainers, other boll worms and sucking pests can be a problem too. The aim through this period is to continue to maintain beneficial insect populations and use selective insecticides where necessary while ensuring that the crop is protected to prevent yield loss or delay in maturity. Some of the main tactics employed during this period include water management, planting a last generation trap crop, monitoring pest and predator populations and crop damage, and application of chemical or biological control products.

Water Management: Water stress is particularly dangerous during this stage and lack of water will weaken the plant and make it more susceptible to pest damage. Monitor soil moisture and irrigate if possible.

Planting a Last Generation Trap Crop (Only Where Irrigation is Possible): This is the time to plant the last generation plant crop discussed in the first section of the lesson on post harvest tactics.

Monitoring Pest and Predator Populations and Crop Damage: Monitoring should continue during this period to assess populations and damage to alert growers to the need for interventions with chemical or biological products. Pheromone traps may be of assistance, but daily field visits are needed if farmers are to observe the eggs that herald the onset of an invitation. Remember that Helicoverpa eggs hatch in only three days.

Application of Chemical or Biological Control Products: When pest levels reach threshold levels there is usually little choice but to apply chemical or biological products. The general strategy will be to start with the softer materials (i.e. those with the least impact on the beneficials), such as botanicals and entompopathogens, insect hormone analogues, fermentation products and if these are not effective move on to ovicides (such as profenophos and methomyl).

Resistance Management: Care should be taken to ensure that farmers do not apply the same materials in succession and that no material is applied more than three times in season.

You should note that if a broad spectrum pesticide has been used, the effectiveness of beneficial insects will be much less and more pesticides will probably be needed.

A key concept regarding the use of chemical control products at this time is the rapid growth of the plants. This results in what is known as ‘growth dilution’ of any insecticides applied – quite simply, new plant tissue has not been exposed to the insecticide and small leaves with sufficient spray deposits become large leaves with insufficient deposits.

This issue is particularly important for the compounds that require ingestion. Application of such compounds in particular should aim to ensure as thorough coverage as possible to increase the likelihood that larvae will encounter and consume spray deposits. Particular attention should also be paid to combinations of insecticides that may be incompatible in terms of inducing pesticide resistance.

Lesson 2.5: Period 5 – One Open Boll Per Metre to Harvest

At this time of the season the crop has set much of its primary boll load and is either preparing for the later flushes or is finishing the maturation of its fruit – depending on so many factors. Nevertheless it is still important to continue to monitor pest populations in the crop. As crop growth stops, growth dilution diminishes and stomach poisons become more effective. If insect immigration permits, the strategy should be to attempt to reduce insecticide applications. However, Helicoverpa is potentially most damaging during this period and appropriate steps need to be taken to protect the crop. The farmer’s income is still in the boll – it is not yet in the bag.

Water Management: It is recommended to let the soil dry out during this period. This will help to reduce regrowth and the attractiveness of the crop to pests. Farmers in cyclonic areas are at the mercy of the weather gods and may welcome rain to swell the later flushes.

Last Generation Trap Crop (Australia): The trap crop planted during the last phase should be carefully monitored and the pests it harbours managed with a biological agent or by cultivation if necessary.

Monitoring Pests and Application of Chemical or Biological Control Products: During this period thresholds will generally go up and many pests can be safely ignored after a certain stage of plant development. For example, once the crop reaches 30-40% of bolls open, control for Helicoverpa spp. can generally cease. Mite control is no longer necessary once the crop exceeds 20% of open bolls (Australia).

However, where more flushes are required, vigilance for egg laying must continue. It is still possible for heavy immigrations and farmers may feel incapable of dealing with them when they find an average of 1-10 eggs laid on a terminal leaf each night over a period of a week or more. In these circumstances, the outcome can be disastrous to the farmers and, if the other options have been used up, a ‘strong’ pyrethroid, carbamate or organophosphate can be applied as the method of last resort.