Weather is the most important factor that determines the geographical distribution and periodic abundance of crop insect pests and parasites of animals. Weather controls the development rate, survival, fitness, and level of activity of individual insects; the phenology, distribution, size, and continuity of insect populations; migration and their establishment; and the initiation of insect outbreaks (Pedgley, 1990; Drake and Farrow, 1988). Weather influence can be immediate, cumulative, direct, indirect, time lagged, exported, or imported. Indirect effects arise through host quality and parasite populations. A time-lagged influence is one that occurs at a later stage as a consequence of both past and current weather. Imported/exported influences arise because insects are highly mobile, and outbreaks may be initiated by windborne migrations (Drake, 1994; Baker et al., 1990).
Among the weather elements, temperature, humidity, and wind play the major roles in insect life. Other elements of lesser importance are solar radiation and photoperiod. In interpreting the role of weather in an insect's life, we must remember that all weather elements are interrelated, so the role played by any individual element is not simple to understand and explain.
Each species of insect has a range of temperature within which it can survive. This range is referred as the tolerable zone (Atkins, 1978). Within the tolerable zone, there are different optimal temperature ranges for a variety of vital functions. Exposure to a temperature toward the upper or lower limit of the tolerable zone will usually result in death if it persists for a long enough time. At extremes of the tolerable zone, death will occur after a short duration of exposure.
The actual temperatures that limit the tolerable zone vary from species to species, but extremes exist that apply to all insects. Most insects have an up per temperature tolerance between 40 and 50°C, and no known insect can survive temperatures in excess of 63°C (Gerozisis and Hadlington, 1995). Some insects can adapt physiologically to survive several months of hot, dry weather in a dormant state called summer diapause. The absolute minimum temperature tolerated by any insect is not well defined but is almost certainly below -30°C. Some insects cannot survive for long if the temperature drops below the lower threshold for development. Other species can become dormant at low temperatures. Activity and development cease but begin again as soon as the temperature exceeds the activity or developmental thresholds. Yet others usually overwinter in a particular stage that is physiologically adapted and therefore can survive periods of extreme cold. Several insects inhabiting temperate or even arctic regions are able to survive by the process of supercooling, in which tissues are able to withstand the freezing of their fluid for extended periods without damage. The mean supercooling point for larvae of elm bark beetle Scolytus laevis (Coleoptera: Scolytidae) reached as low as -29°C in midwinter (Hansen and Somme, 1994).
The duration of the time of exposure to extremes of temperature is also important to the survival of insects. The pecan aphid, Monelliopsis pecanis (Hemiptera: Aphididae), could survive fairly well even at temperatures near freezing if exposed for only one hour, but many deaths occurred if they were forced to spend five hours at these low temperatures. At very high temperatures, survival is reduced even after a one-hour exposure (Kaakeh and Dutcher, 1993).
Insects are able to function faster and more efficiently at higher temperatures. They can feed, develop, reproduce, and disperse when the climate is warm, though they may live for a shorter time (Drake, 1994). Higher temperatures are not always favorable to insects, usually reducing their life span. All other factors remaining normal, insects live for shorter times at higher temperatures. An example is that of the parasitic wasp Meteorus trachynotus (Hymenoptera: Braconidae). Variation of temperature from 15 to 30°C reduces its adult life span from 40 days to a mere 10 days or so (Thireau and Regniere, 1995).
Low temperature is an advantage under certain conditions. For example, there are lower energy demands at low temperatures (Hunter, 1993). If resources such as food are in short supply, insects can survive longer without starving. Under extremely low temperatures some insects can remain in suspended animation until warmer conditions return.
In temperate regions, where the seasonal variation in temperature is often large, development starts slowly in early spring, progresses more rapidly as the season advances, and perhaps slows or is suspended in the heat of midsummer. Temperature is less limiting in the tropics and subtropics, but arrested development is commonly used to survive a dry season. Survival through unfavorable seasons is usually possible only in particular developmental stages.
Temperature also impacts insect activity. As the temperature rises, insects move more rapidly. Short-distance flight activity in relation to temperature has been documented for a number of insect species in Australia (Drake, 1994). Using radar observations, it has been established that daytime flights of Chortoicetes terminifera adults occur mainly when the sun is not obscured. The locusts often become concentrated in rising air in the walls of a warm convection funnel (Reid, Wardhaugh, and Roffey, 1979). Adults of Lucilia cuprina are most active at temperatures between 16 and 27°C and move further when the temperature is high. Flight and general activity of Musca vetustissima are limited by temperatures below 10 to 12°C (Hughes, 1981). Sterile Bactroceratryoni, mass-reared in warm insectaries, are uncompetitive with wild flies (and ineffective as control agents) unless preconditioned to field temperatures before release (Fay and Meats, 1987). Field-cage observations indicate that Eudocima salaminia ceases feeding at temperatures below 20°C, and laboratory studies have shown that at this temperature the flight capacity of Epiphyas postvittana is at its maximum (Gu and Danthanarayana, 1992). Females of the latter species fly longest at a relative humidity of 60 percent. Egg laying by the jarrah leaf miner, a pest of the native eucalypt forests of southwestern Australia, is most intense at 15 to 20°C (Mazanec, 1989).
Major meteorological factors affecting migration are the vertical profiles of temperature, wind velocity, and the presence of convergence. Temperature may determine the time of flight, height of flight, and thus the speed and direction of the transporting wind, as well as flight duration. Chortoicetes terminifera adults initiate their long-distance nocturnal flights only if the temperature at dusk (the time of takeoff) exceeds 20°C and rain has fallen on them at certain stages during their development (Hunter, 1993; Hunter, McCulloch, and Wright, 1981). The flight duration and dispersal distance of E. postvittana adults is much greater when the larvae are reared at high (25-28°C) rather than low (15°C) temperatures (Gu and Danthana-rayana, 1992). Flight duration may sometimes be determined by the migrants' supplies of physiological fuel, which could depend on food abundance and quality (and thus in turn on weather conditions) at the development stage. Direct influences leading to the termination of migration include falling temperatures below the threshold for flight, the onset or cessation of thermal convection, and an encounter with rainfall that can cause flying insects to be washed down.
Temperature also affects behavior of insects. Insects may remain totally inactive at both high and low temperatures or move actively along a temperature gradient until a preferred zone is encountered. The influence of tem perature on dispersal, mating, and reproduction is of great importance. If conditions are adverse for dispersal, local populations neither increase because of influx, nor decline because of exodus. If temperatures are not suitable for mating for several days, some adults may die without leaving offspring and others may become less fertile due to age.
The moisture content in the habitat of an insect directly determines whether or not an individual survives. Moisture also has indirect effects on insect populations through its influence on plant growth. All forms of environmental moisture (atmospheric humidity, rain, snow, hail, dew, soil moisture, and surface water) influence the water balance of insects.
The humidity in an insect's habitat may have some indirect effects. Some parasites do not search for hosts or oviposit in them if the relative humidity is either low or high. The susceptibility of insects to fungal, bacterial, and viral diseases also changes with environmental moisture. Moist conditions seem to facilitate the spread of some insect pathogens and may also affect their survival and virulence.
Rainfall can act as a direct cause of mortality. Insect eggs and small larvae can be permanently washed from their host plants by heavy rain. Rain may also saturate the soil and drown insects that are unable to escape. Many insects cease feeding during periods of precipitation and may seek refuges in which to pass a rainy period. Small parasitoids have difficulty moving around in wet conditions. A prolonged rainy spell, particularly when the temperature is suitable for development, may lengthen the time required to complete development or cause mortality by starvation.
Heavy or excessive rain can cause high mortality, either directly through knockdown, saturation, or flooding, or by providing conditions favorable for disease. Heavy rain washes aphids off of their host plants, and both beetles and bugs may be killed by violent thunderstorms.
Insect abundance varies with seasonal variations in rainfall. Some species are more abundant in the dry season, whereas others proliferate only during the rainy season. Lack of rain can cause desiccation and death of insects. In Australia, the onset of hot, dry conditions in summer reduces populations of the aphid vectors of a variety of plant-virus diseases (Drake, 1994). In the eucalyptus forests of northern Australia, the sap feeder (Hemip-tera: Psylloidea) is much more common in the late dry season than at any other time of year, whereas defoliator grasshoppers (Orthoptera) were most abundant during the rainy season. It is suggested that sap feeders receive nutrients from sap produced by the regrowth of trees in response to fires which sweep through these forests in the late dry season. The defoliating grasshoppers, on the other hand, benefit from the relatively luxuriant production of new leaves during the rains (Fensham, 1994).
Rain may not have an influence at the time it falls but may promote insect performance some months later. This phenomenon is well illustrated by the seasonal outbreaks of African armyworm, Spodoptera exempta (Lepidop-tera: Noctuidae) which can reach enormous numbers to become serious pests of cereals and pasture. In Kenya, the number of outbreaks was negatively correlated with rainfall in the six to eight months preceding the start of the armyworm season. The high correlation between rain and later armyworm outbreaks has been used to construct a prediction model for Kenya, providing an accurate forecast of the likelihood of armyworm outbreaks (Haggis, 1996).
Rain also plays a role in altering a host's susceptibility to windborne insects and disease vectors. Rain-drenched or moisture-stressed crops and stock may be particularly vulnerable to insects or the pathogens they carry (Risch, 1987). Sheep are especially susceptible to strike by L. cuprina during periods of frequent rainfall, which increase the incidence of predisposing factors such as fleece rot and nematode infestation (Wardhaugh and Morton 1990; Waller, Mahon, and Wardhaugh, 1993). Epizootics of Aka-bane disease, which causes calving losses in cattle, occur when winds carry the vector (Culicoides brevitarsis) outside its normal range. In the region where the vector overwinters, the disease is endemic, and cows usually develop immunity before becoming pregnant (Murray, 1987). Jarrah trees stressed by low rainfall during the previous winter are particularly susceptible to attack by Perthida glyphopa (Mazanec, 1989).
Wind is an important factor of the environment of insects, and it influences insect populations in a number of ways. It is a vital component of broad weather patterns, giving rise to fronts and convergence zones. Low pressure systems and anticyclones in temperate regions determine migration trajectories of insects, while trade winds and monsoons determine the trajectories in tropical and subtropical areas. Wind causes insect displacement and therefore affects population changes by influencing the numbers moving into or out of an area. It can carry them considerable distances away to new habitats and regions. Many insects and pathogens appear to undertake enormous migrations covering hundreds if not thousands of kilometers on occasions. They perform this feat by exploiting the wind as an external source of energy (Shields and Testa, 1999; Byrne, 1999).
In Australia, warm northerly or northwesterly winds emanating from the Intertropical Convergence Zone have introduced Japanese encephalitis virus. Outbreaks of Akabane disease and bluetongue infection have been linked to long-range windborne dispersal (Mackenzie, Lindsay, and Daniel, 2000). These winds bring C. terminifera, Nysius vinitor, various noctuid moths, and Musca vetustissima into the cropping regions of southeastern Australia during spring and summer, often in large numbers. In southwestern Australia, similar movements of insect populations occur along with the northeasterly winds. Movements also occur in other directions, and these may be important in reestablishing populations in the inland regions from which the major invasions originate. The spread or reestablishment of disease infection by windborne migration of pathogens and insect vectors has been recorded for crops and cattle (Drake, 1994; Limpert, Godet, and Müller, 1999; Aylor, 1999).
In southeast and east Asian countries, most migrations of rice insect pest populations are determined by the direction and extent of wind (Rutter, Mills, and Rosenberg, 1998). Out of nearly 2,600 trajectories drawn upwind from 15 catching sites, only 5 percent of the trajectories failed to locate a possible source, and over 90 percent were completed in 40 hours or less. Nearly 80 percent of the trajectories were constructed in the prevailing winter monsoon and trade winds, resulting in a southward displacement of insects toward overwintering areas. Tropical cyclones in autumn produced trajectories that differed in both direction and extent from those in the prevailing winds, supporting the suggestion that the contraction of the distribution areas of rice pests at that time of year is the product of a series of movements in different directions. The results suggest migrations continue throughout the year in the tropics and subtropics and indicate this may be one way the capacity for long-distance migration is maintained in some rice pest populations.
In atmospheric convergence zones, some insects rise until they reach their flight ceiling and subsequently land to feed and reproduce. Normally, the same convergence zones are the harbingers of rain. The semiarid tropical regions of the world are particularly affected by such weather patterns. Rain associated with these systems results in luxuriant growth of vegetation on which large densities of new-generation insect larvae feed. Pests such as the desert locust, Schistocerca gregaria (Orthoptera: Acrididae), and the armyworm, Spodoptera exempta (Lepidoptera: Noctuidae), are two classic examples of this phenomenon that is particularly noted in Africa.
A risk for insects who rely on wind to aid in dispersal is the chance of going too far from their destination. Wind velocity to a certain extent helps movement and host finding. Strong wind could kill the insects by carrying individuals to unsuitable areas, completely out of their habitat range. There fore, some species will fly only in winds of a certain velocity. Most insects will not take flight when the speed of the wind exceeds their normal flight speed, simply because they will lose control over the direction of movement.
Some insects, especially aphids, rely on daytime thermals (convective currents) to carry them aloft. However, many insects ascend under their own power at dusk and migrate above the nocturnal boundary layer in the fast-moving, stable airflows found at altitudes of 100 to 2,000 m (Drake and Farrow, 1988).
Like adult insects, larvae can also be carried considerable distances, especially if they are attempting to escape because of limited food supply. In Europe and North America, female adult gypsy moths, Lymantria dispar (Lepidoptera: Lymantriidae), are wingless (except in the Asian strain), and the insect population disperses mainly as first instar larvae which use "ballooning" as a means of colonizing new areas (Diss et al., 1996).
An important property of wind is its ability to convey chemical messages to insects from point sources. These messages can include information about the distance and location of a specialized food plant or of a suitable and receptive mate. A chemical plume formed in the shape of a tongue in a laminar, wind-driven system provides an unbroken guide to the location of the source. Insects can then fly along a concentration gradient of a chemical signal to precisely locate their prize.
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