Habitat Diversity and Predatory Insects
in Cotton IPM: Case Study of Maharasthra
Cotton Eco-System

O.P. Sharma, R.C. Lavekar*, K.S. Murthy and S.N. Puri**
National Centre for Integrated Pest Managment
IARI Campus, Pusa Complex
New Delhi, India

Sustainable agriculture has developed over centuries of trials and errors and stood rest of time (Altieri, 1991). They have "man-made ecological sustainability" (Zadoks, 1993) but in most cases, the economic sustainability is required to keep pace with rising population and food supply. Cotton (Gossypium spp.) crop in major cultivated areas is subjected to infestation by a wide range of pests (diseases and insects). Among insects the major ones being Helicoverpa spp. and bollworm complex apart from the boll rots leading to low quality lint production (Fitt, 1989). Management of these pests heavily depends exclusively on insecticides which in due course of time has graduated from organochlorine to synthetic insecticides. Due to significant assured success based on chemicals, research on natural enemies of pests and its field application received a back seat. Quest to harvest more and large scale farm mechanization led to adoption of modern farming system which undoubtedly heavily rely on, monocultures, zero tillage and on chemical pesticides has added up new pest probems. India being diverse both type of cultivation viz., monoculture as well as strip cropping is in practice respectively at northern India (Rajasthan, Punjab & Haryana) and central India predominantly in Maharasthra. Bollworm complex out break is more in monocropping than mixed cropping and has led to indiscriminate application of chemical pesticides which has been over simplified by tractor mounted spray equipments. Apart from causing temporary reduction in yield losses it has created socio-economic problems. Although pesticides will continue to be a component of pest management, following major obstacles deter their frequent use.

Resistance to pesticides
Pesticide-induced pest problems
Lack of effective pesticides
Fewer new pesticides, and
Human and environmental health concerns

The development of a strategy that may conserve and maximize the abundance as well as effectiveness of natural enemies will be crucial in the management of insect pests. There are many classical examples which have shown that increased habitat diversity in crops can increase population densities of locally available predators to enhance biological control of pests. Predatory beetles (Campoletis sonorensis) have been reported to respond to volatile chemicals emanating from specific plant tissues and few (Macrocenrus grandii) are attracted to volatile chemicals from undermanaged plants (Nishide, 1956, Vinson, 1975, Elzen et al. 1983, 1984, Whitman & Eller, 1990, Udayagiri and Jones 1992). The search for the "right" habitat is imperative because like all living organisms, insect parasitoids and predators have requirements for resources, other than hosts. However, these other sources may or may not be found in the same habitat in which hosts are found. Optimal microclimatic conditions for a given parasitoid, nectar sources, or pillar may exist in some host habitats (crop systems) but not others. One assumes that the habitats in which parasitoids find hosts also provide other needed requisites at optimum levels. There is little empirical or experimental data, to support this to be true, even for unmanaged eco-systems. The objective of consumed biological control is to ensure that the occurrence of as many essential parasitoid resources and hosts coincide in time and space.

The conservation of natural enemies by the direct enhancement of vegetation diversity has been a subject of intense study for many years (Root, 1973 and Andow, 1991). Earlier it was hypothesized that lower levels of herbivores in diverse agroecosystems were a result of higher levels of natural enemies "enemies hypothesis" (Root, 1973). Much needed attention was given on the nature of the relationship between pest, crop and non-crop plants, and their physical environment. As a result habitat manipulation seeks to manage these relationship to enhance the impact of natural enemies on pest population. Indeed this approach is one of the key elements in the use of indigenous natural enemies in IPM. Conservation which involves protection and maintenance of natural enemy population has proved crucial for maintaining local / native natural enemies in ecosystems. Review revealed that conservation involved modifying pesticide application practices so that they occur only when the pest population exceeds specified levels, however Hull and Beers, (1985) advocated that conservation of natural enemies can also be achieved by changing the active ingredient, rates, formulations, timing, and location of pesticide applications or by maintaining refuges. According to Tauber et al (I985), "it is probable that the most dramatic increase in the utilization of biological control in agricultural IPM systems could come through the judicious use of selective pesticides in conjunction with effective natural enemies in location specific cropping systems.

While knowledge of pesticide selectivity is available it is inadequate to generally allow such precise usage. As long as key pests cannot be controlled biologically, culturally, or through host plant resistance, agricultural chemicals will be essentially needed.

The aim of this study was to quantify the impact of habitat diversity on abundance of predators and egg parasitoids in cotton fields as a step towards the conservation and establishment of these natural enemies before the pest built-up and their migration to adjoining crops. Present investigations have been made in order to evaluate the utility of locally grown alternative crops (refuge crop for strip-cropping) as refuge for increasing activity of predatory and parasitoid insects.

Materials and Methods

Field studies were conducted to determine whether the maize interlaced with cowpea grown on border serve as a cover crop serving as a source of predatory and parasitoid of cotton ecosystem. Field trials was conducted for two years in succession (1996-97& 97-98) at Cotton Research Station, Nanded (MS) apart from testing it on large scale in farmers field (Barad) as well as at village level (Ashta) during 1997-98 (5 acres) and 1998-99 (450 acres) respectively. Initially during 1996-97 maize crop mixed with cowpea were planted as 10 or 11th row, but as adjustment and feed back from farmers the same was shifted as border rows in subsequent years. Predatory insects were sampled weekly by taking visual counts at random on all the rows. The number of eggs, grubs and adults of Chrysoperla spp , Coccinella spp, spiders and other natural enemies were recorded .

Results and Discussion

Population dynamics of predators of Helicoverpa spp and other bollworms assessed from the study plots during different years are given in Table 1. Population of predatory beetles (Coccinella transversalis (Fab.); Adalia bipunctata (Linn.), lace wings (Chrysopa spp), reduviid & pirate bugs Coranus triabeatus (Hozwath) and spiders (Lycosa spp., Aranews spp.) recorded on the maize/cowpea were significantly higher than on the adjacent cotton. Studies on dispersal and occurrence frequency revealed that the population of these predators and parasitoids seemed to decline with increasing distance from border row of cover crop. In general, the predator numbers recorded on cotton at different distances away from cover crop (maize & cowpea) were lower.

The results indicated that the interplant maize and cowpea has acted as a source of the predators to cotton crop. Among the reported factors that contribute to higher level of natural enemies in diversified agroecosystem were availability of diverse microhabitats, greater availability of food sources (such as prey, nectar and pollen), alternative hosts and shelter all of which encourage colonization and population buildup of natural enemies. The refuge or source function of the border maize and cowpea may be attributed to the abundance of floral nectar and alternate prey (aphids) shelter, mating and oviposition sites harbored in the border crop compared with monoculture cotton having lesser bio-diversity. Volatiles emanating from plant tissues had been reported (Elzen et.al, 1984., Udayagiri and Zones, 1992 ) influence the harbourage which may have also played vital role. With depletion of food resources, shelter, mating, oviposition sites etc., higher numbers of the predators have inclined to move from the border to forage the aphids or whitefly affecting the adjacent cotton crop. Further their movement is increased due to abundantly available food attractants which has also been demonstrated by other researchers (Mensah & Harris, 1994, 1995; Mensah, 1997) and or absence of feeding deterrents. However, various explanations have been given for the decrease in insect populations under intercropping conditions (Altieri et al. 1978, Matteson, 1982 and Ezueh and Taylor, 1983).

The magnitude of these effects depend on whether the border crop is well established and colonized by predatory and parasitoid insects long before the cotton crop attains flowering stage. According to Carbett and Plant (1993), an inter planted vegetation may act as a source of natural enemies, when natural enemies colonize strip vegetation before crop germination, but may act as a sink when crop and interplanted vegetation germinate at the same time. In this study, maize, cowpea being fast growing crop could establish slightly before the cotton. Unless and until natural enemies are present and are well established in sufficient numbers before the initial pest arrival they cannot respond fast enough to manage the pest before crossing the ETL. The presence of maize, cowpea crop within the cotton system has significantly enabled establishment of high population of beneficial insects in the cotton field on time, prior to the arrival of sucking pests (Aphids & whitefly) and Helicoverpa spp. This has also enhanced the efficacy of the beneficial insects and enables their use as basic component in a IPM system. Lower insect attack is reported to be one of the many factors that maximize productivity in intercropping as has also been demonstrated by Amoako-Atter et al. 1983. The present study also indicated higher lint yields due to intercropping (331kg/ha), in the dry land conditions.

Table. 1. Status of beneficial insects in IPM and non-IPM trial

Conclusions

Habitat or environmental manipulation has proved another form of conservation and augmentation of natural enemies. Cropping system has been successfully altered to augment and enhance the effectiveness of a natural enemy. It has been observed that adult parasitoids and predators have been significantly benefited from source of nectar and the protection provided by refuge (hedgerows, cover crops and weedy borders). Observation also suggests that mixed planting and the provision of flowering borders can increase the diversity of habitats and provide more effective shelter and alternative food source to predators and parasites. Intercropping or mixed cropping which involved simultaneous growing of two or more crops on the same piece of land, is one of the oldest and most common cultural practices in most tropical developing countries (Karel and Nudnguru, 1980), which needs to promoted based on scientific utilities.

Use of maize and cowpea on border in cotton to enhance the population of beneficial insect (coccinellids and chrysopa) in the cotton system, especially in the crop season as indicated in present study, is very important to the management of key biotic factors such as Helicoverpa spp. This is because the pest has been observed to rapidly infest cotton crop through migration from other sources available in the vicinity (sunflower, tomato & pigeonpea). The conservation of natural enemies is probably the most important and effective which is also readily available biological components available with all the farmers. Natural enemies, which occur in all production system, from kitchen, garden to commercial fields. They are adapted to the local environment and to the target pest, and their conservation is usually simple and very much cost effective. With very little effort their activities can be enhanced and demonstrated through extension workers. Lacewing (Chrysoperla ), ladybird beetles (coccinellids), hoverfly larvae, and parasitized aphid mummies can be easily observed in aphid infested colonies. These natural controls are important and need to be conserved and considered while making pest management related decisions. Implementation of IPM or ecological based pest management relies on the knowledge that, stability in biological systems relies on feedback between organisms. For example, as the number of one organism increases, the number of predators, parasites and pathogens that attack that the organism also proportionally increase and provides density dependant relationship. In the most stable system, the amplitude of these oscillations is minimal. This means that potentially damaging species usually are never abundant enough to become actual pests.

Based on issues of heavy dependence on chemical pesticides and its repercussion, it is essential to plan future to be less reliant on broad-spectrum pesticides. By conserving the eco-system the fundamental goals of IPM (safety to humans and environment, assurance of profitability for the farmers, and long term durability) can be achieved which will undoubtedly become a base for sustaintainable agriculture. Learning how to conserve natural enemies in the agroecosystem will be an effective way to increase the use of biological control in agriculture.

References

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