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Light is not a true climatic factor, but it is interrelated with solar radiation and temperature. As such, it is often included as a component of weather. Photoperiodism (variations in the photoperiod in a 24-hour day-night cycle) exercises a great deal of control over processes directly related to survival of insects. Light intensity greatly influences insect behavior. Many species are active during the hours of full sunlight but remain quiescent at night. Some insects are active during the faint light of dawn and dusk.

Day length can also be used as a signal or trigger by insects to enter diapause during potentially harsh conditions such as summer heat, winter cold, and drought. In some cases, the day length experienced by an insect larva provides information about the progression of the seasons. The ability to vary growth and development rates enables the insect to achieve efficient timing relative to favorable conditions (Leimar, 1996). In the case of Kytorhinus sharpianus (Coleoptera: Bruchidae), a wild bean weevil from Japan, the duration of the various stages in the life cycle from egg to adult vary according to the photoperiod at a constant temperature. The whole cy cle can be accomplished between 75 and 80 days, when the insect receives 15 or 16 hours of daylight per day-night cycle. This period increases dramatically as the hours of daylight shorten to 14 and then to 12. With only 12 hours of daylight, the pupal stage is never reached until longer hours of light return.

Hill and Gatehouse (1992) studied the influence of daylight on many insect pests and suggested the migratory capacity of insects may be influenced more by photoperiod during development than by temperature.

SOME IMPORTANT INSECT PESTS OF CROP PLANTS Aphids

Weather factors, especially temperature and rainfall, play a dominant role in the population dynamics of aphids in all the climatic regions of the world where crop production is possible. Aphids are highly sensitive to temperature changes. Field observations, climate chamber experiments, and computer simulations confirm this fact. Skirvin, Perry, and Harrington (1996) used a model to assess the population dynamics of aphids at various temperatures. The model predicted that an increase in temperature leads to a greater number of aphids in the absence of the predator. However, the presence of predators reduces the number of aphids predicted. Temperatures below 20°C and above 25°C limit the buildup, while an increase from 20 to 22°C enhanced the intrinsic rate of increase of aphid populations (Freier and Triltsch, 1996). A maximum temperature of 45°C in the postrainy season has been observed to be lethal for the sugarcane aphid species (Melanaphis sacchari) in sorghum (Waghmare et al., 1995).

Aphids in the tropics show remarkable adaptability to climate, regulating their population by suitably adjusting their life history strategy development, reproduction, survival, and dispersal. In the northeastern states of India, aphid species, both oligophagous (Ceratovacuna silvestrii) and poly-phagous (Toxoptera aurantii), show a shift in the abundance of their populations in space and time in response to seasonal variation in temperature, which brings about changes in their host's quality. In contrast, the mono-phagous species (Cinara atrotibialis) escapes the hot summer of plains and uplands by occurring exclusively in the milder temperatures of the hills. The three species performed optimally at 20°C with respect to development, reproduction, and adult survival (Agarwala and Bhattacharya, 1994).

Studies on aphids in a cotton crop revealed that a drop in mean temperature to below 25°C when the cotton is in the elongation stage could cause a sharp increase in the aphid population (Chattopadhyay et al., 1996). Mini mum temperature, evening humidity, sunshine hours, and rainfall influenced aphid incidence in both the 45 and 52 standard meteorological weeks. If there were substantial increases in minimum temperature and evening humidity and an appreciable decrease in sunshine hours with occasional rain, aphid infestation was observed at high levels in these weeks. Rainfall was found to be the predominant variable controlling the aphid population in the boll formation stages. When the crop was in the maturity stage, a decrease in maximum temperature and rainfall affected the aphid population in the fourth standard meteorological week. Between the two, rainfall was found to be the predominant meteorological variable. Even a decrease in maximum temperature in the second week alone increased aphid infestation in the fourth week. Cloudiness also plays an important role in an aphid population during the first generation.

In Mediterranean climatic environments, several weather conditions are highly related to the population dynamics of cereal aphids in winter (Pons, Comas, and Albajes, 1993). In northeastern Spain, population dynamics of Rhopalosiphum padi L. and Sitobion avenae F., the species found most frequently, were affected not only by low temperatures causing aphid mortality but also by other factors. Dry and cold weather, through the effect they have on host plant phenology and quality, reduce aphid developmental rates.

In the pastures growing in a dryland farming rotation system in the 300 to 450 mm rainfall region of southwestern Australia (Mediterranean climate), aphids were not found on plants in hot, dry summers but were present from April until November, when the weather was mild and humid (Ridsdill, Scott, and Nieto, 1998). However, late summer/early autumn rainfall is the key factor in determining the aphid population in lupins. This rainfall can be used to forecast an aphid population as it maintains weeds on which aphids build up before they move into crops. Little or no rain at this time results in very little green plant material to support aphids, and hence aphid arrival in crops is late. Thackray (1999) has built a computer model that incorporates data on climate, aphid population, and yield losses at a specific site to forecast aphid arrival in lupins in Western Australia.

Studies in Britain, representing temperate climates of the world, suggest temperatures have an important role in the flight phenology of aphids. The first migratory period appeared to be more strongly correlated with winter temperature than summer temperature for most of the prevalent aphid species. Warm winters will probably lead to advances in the first migratory period and large intervals between the adjacent migratory periods. The study indicates climate change will lead to more frequent and severe attacks of many aphid species and the virus diseases they transmit (Zhou et al., 1996).

Armyworms

Economic infestations of armyworms in many parts of the world have revealed that precipitation is the primary factor influencing pest populations. However, temperature and the availability of weather transport systems were the most important climatic factors governing pest abundance (Pair and Westbrook, 1995; Rose, Dewhurst, and Page, 1995; Tucker, 1994; Stewart, Layton, and Williams, 1996).

Findings from studies in Africa (Rose, Dewhurst, and Page, 1995) show that the onset of the first outbreaks of an epidemic is caused by oviposition at high density by S. exempta concentrated by wind convergence at storm outflows. The sources of these insects seem to be low-density populations, which survive from one season to the next at sites receiving unseasonal rainfall. Some areas in Tanzania and Kenya are particularly prone to early outbreaks that are potentially critical for the initiation of a subsequent spread of outbreaks downwind throughout eastern Africa. These areas have low and erratic rainfall and are near the first rising land inland from the coast. Below-average rainfall prior to the development of outbreaks increases the probability of their occurrence. Their subsequent spread is enhanced by storms downwind, which concentrate insects in flight, and by sunshine during larval development. Persistent wet weather reduces the spread of outbreaks. Seasons with many outbreaks often had rainstorms separated by dry periods during the rainy seasons.

Grasshoppers

Temperature sensitivity determines the geographical distribution of grasshoppers, with generalist species widespread and thermally specialized species restricted to warmer habitats. For all the species that are thermal specialists, variation in their sensitivity to temperature is a good predictor of their distribution. The developmental and reproductive responses to different rearing temperatures of grasshoppers, examined in a laboratory experiment, showed that growth and development rates increase with temperature for each species. Nymphal development, adult mass and size, and egg production rate also increases with temperature (Willott and Hassall, 1998).

Examination of life history variations among many populations of the grasshopper Chorthippus brunneus, from around the British Isles, revealed a relationship between grasshopper life histories and the climates of their ancestral sites (Telfer and Hassall, 1999). The grasshoppers from cooler sites are heavier at hatching, and those from northern sites grow faster and develop through fewer instars, attaining adulthood earlier at the expense of adult size. Adults are larger in warmer, sunnier, or more southerly locations.

A study of rangeland grasshopper population dynamics in Wyoming (Lockwood and Lockwood, 1991) has indicated that the climate in December to January has predictive value with regard to population dynamics the following spring. However, when temperature and precipitation during hatching and early development of grasshoppers (April to May) are the control variables, forecast of the observed population is better.

Prolonged drought conditions suppress the population of grasshoppers. Kemp and Cigliano (1994) monitored the abundance of the rangeland grasshopper (Acrididae) species at various sites in Montana during the period 1986 to 1992, which included an extreme drought year (1988). Significant post-1988 reductions were observed in rangeland acridid species abundance in the eastern and south-central regions of Montana, where drought intensity had been increasing during the previous 20 years. In the north-central region, which also experienced the 1988 drought but showed no long-term drought trend, a postdrought reduction in overall acridid species abundance was not observed.

The black cone-headed grasshopper lives in hot, arid environments. The sexual dimorphism of the species suggests the larger females may have an advantage in water storage over the males. Both sexes were able to depress their internal temperatures below the higher temperatures of their environment by evaporative cooling. The males lost proportionately more water by evaporation, produced drier feces, and may have been more constrained by water availability. The females appeared to be more profligate with their water reserves, which supports the theory that large body mass may be an advantage to an insect in the desert (Prange and Pinshow, 1994).

Locusts

Locusts are able to travel long distances and colonize new habitats. Therefore, their distribution is variable in time and space and can occur within a large area. The pest is feared for both its destructive capacity and its constant threat to the region. It is capable of sudden appearances and severe devastation to standing crops. The most affected part of the world is the continent of Africa, where in some years the infested area may cover several million hectares. Distribution and sequence of rainfall is the principal determinant of locust population increase over several generations and of the concentration of widespread populations into highly mobile and destructive swarms.

Survival and populations are greatest with an increased frequency of sufficient rainfall, where rainwater is enhanced by runoff and flooding, and where vegetation and soils provide suitable habitats. However, excessive rainfall affects the first three nymphal stages, limiting its development (Hunter, 1981; Montealegre, Boshell, and Leon, 1998). Locusts move in swarms from one area to another where rain has fallen. Wind direction and speed at 850 hpa (about 1.5 km above the earth's surface) and convergence zones determine the paths of the locusts' movements.

In the Indian subcontinent, monsoon rainfall and, to a certain extent, winter-spring rainfall play a role in the resurgence, establishment, and termination of locust plague upsurges (Chandra, 1993).

Temperature affects the rate of development, body size, and adult color (Gregg, 1983). Capinera and Horton (1989) and Nikitenko (1995) suggest grasshoppers and locusts in the cooler regions of North America and Europe are favored by warm, dry summer conditions, whereas in warmer areas they appear to require spring and summer moisture. Locusts avoid thermal extremes by taking refuge in appropriate sheltering sites, loss of water by flying during favorable climatic conditions, and cannibalism (El Bashir, 1996).

Cotton Bollworms

The effect of weather conditions on various species of bollworms has been investigated in many cotton-growing regions of the world in laboratory as well as in field. The combined effect of weather factors, maximum temperature, minimum temperature, rainfall, and relative humidity on the population density of cotton bollworms is very high (El Sadaany et al., 1999).

Laboratory studies have demonstrated the temperature-dependent development of larvae and pupae of pink bollworm, Pectinophora gossypiella (Saund). No eggs hatched at less than 10° or more than 37.5°C. Mortality of larvae and pupae also increased at terperatures greater than 37.5°C. The development rate of all stages of the pest increased with temperature. Development of larvae was successful at all temperatures between 15° and 35°C. Larval period and adult longevity decreased as relative humidity increased (Wu, Chen, and Li, 1993; Gergis et al., 1990).

The most important weather factor for the abundance of cotton bollworm is the amount and distribution of rainfall. The incidence of cotton bollworms (Lepidoptera) in southern China is significantly affected by July and August rainfall. Populations of the pests were sparse when total precipitation was greater than 500 mm and dense when total precipitation was less than 400 mm, showing a very significant negative correlation. Continuous rain produced more severe damaging effects on the pupal stage (Li et al., 1996).

In the cotton fields of northwestern India, significant relationships are observed between the buildup of Heliothis spp. and pink bollworms (Pec-tinophora gossypiella) and mean air temperature and relative humidity. The optimum temperature and humidity range for the buildup of Heliothis during the growing season is observed to be 20 to 24°C and 46 to 60 percent, respectively. In the case of P. gossypiella it was 22 to 23°C and 52 to 72 percent, respectively (Bishnoi et al., 1996). In central India, the optimal conditions for emergence was observed to be 26.7 to 31.4°C maximum temperature and 62.2 to 77.7 percent relative humidity. Minimum temperature showed a significant correlation only with the emergence of P. gossypiella (Gupta, Gupta, and Shrivastava, 1996).

Fruit Fly

Queensland fruit fly Bactrocera tryoni (Froggatt) (QFF) is one of Australia's most costly horticultural pests, with major potential impacts that have local, regional, and policy dimensions. Its range extends from northern Queensland to eastern Victoria, and populations occur in many inland towns of Queensland and NSW as far west as Broken Hill. The distribution of fruit fly (including QFF) is primarily determined by climate (Bateman, 1972; Yonow and Sutherst, 1998).

The abundance of fruit fly is greater in regions where the daily maximum temperature does not exceed 38°C during summer. Immature adults are unlikely to survive when the maximum temperature exceeds 40°C for four continuous days. However, adults are mobile and can seek cooler habitats in the field and survive very hot weather periods when the temperatures suggest that they should not (Meats, 1981). Conversely, the lowest monthly mean of 2°C is only marginally favorable for winter survival.

Moisture appears to be the primary determinant of the number of fruit flies, and correlations between summer rainfall and fruit fly populations are significant. Populations reach extremely high numbers in wet years and decline in dry years. Other factors being favorable, a relatively high population would survive with a mean monthly rainfall of 48 mm and more. Mavi and Dominiak (2000) observed a highly significant correlation between the availability of moisture, as measured by summer rainfall, and the peak numbers achieved each year. Rainfall in excess of 170 mm in November, January, and February resulted in high fruit fly populations, and less than 170 mm resulted in low populations. Meats (1981) reported that more than 48 mm rainfall per month in summer resulted in at least three generations.

Studies conducted in southwestern NSW confirm (Mavi and Dominiak, 2000) that infestation has been severe in years when mild winters were fol lowed by humid summers. Infestation has been almost negligible in years when severe winters were followed by comparatively less humid summers.

The vulnerability of Australian horticulture to the QFF under climate change was studied by Sutherst, Collyer, and Yonow (2000). The study revealed that climatic warming to the extent of 1°C reduces the severity of cold season stress in southern parts of Australia and thus increases the suitability of southern states for both population growth and survival over the winter period. Under this scenario, damage costs will increase to the extent that horticulture may become hardly economical.

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