Effects and Impact of Climate Change on Insects [Effect of Temperature and CO2] on Insects!
Introduction to Climate Change:
The earth’s climate has always been subject to natural variations with shifts between cold and warm. However, this is the first instance when man appears to accelerate the change enabling warming to take place quickly. The things that normally happen during geological time are happening during the span of human life, thus leaving limited time for species to adapt and adjust, and avoid extinction.
Since different species adapt to climate change in different ways, the natural cycles of interdependent species tend to fall out of synchrony. Habitats are changing along with the timing of annual processes like flowering, leaf coverage, migration and birth.
Uncoupling of certain associations and establishment of new ones are in the offing, as species respond to changing climatic conditions. Loss of genetic variability in the newly established host ranges tends to influence the ability of insect populations to adapt to changing climatic conditions involving new food plants, competitors and natural enemies.
The key factors in changing earth’s climate are the greenhouse gases (mainly CO2) released by industry, agriculture, automobiles and coal fired electrical generation plants. The concentration of CO2 in the atmosphere helps to determine the earth’s surface temperature, and CO2 and temperature have risen sharply during the 20th century.
The global temperature had risen by only 4°C over the last 18,000 years. However, it has increased by about 0.5-1.0°C since the last century, when weather records started being maintained on the basis of data collected from about 2,000 weather stations located around the world.
Similarly, the concentration of CO2 has risen from about 280 ppm since the early 19th century to about 360 ppm by the end of 20th century. The current estimates of changes in climate indicate an increase in global mean annual temperatures of 1°C by 2025 and 3°C by the end of 21st century. If the present emission trend continues, the level of CO2 in atmosphere could reach up to 500 ppm by 2050 and 700 ppm in 2100.
Such changes in the climate would drastically affect insects in terms of geographical distribution, over wintering, population growth, number of generations per annum, length of growing season, crop-pest synchronization, intraspecific interaction, dispersal, migration and availability of host plants and refugia. The losses due to insect damage are likely to increase as a result of changes in crop diversity and increased incidence of insect pests due to global warming.
Geographical Distribution of Insects:
Climate change will have major effect on geographical distribution of many insect pests, and low temperatures are often more important than high temperatures in determining geographical distribution of insect pests. Increasing temperatures may result in a greater ability to overwinter in insect species limited by low temperatures at higher latitudes, extending their geographical range and sudden outbreaks of insect pests can wipe out certain crop species, and also encourage the invasion by exotic species.
Spatial shifts in distribution of crops under changing climatic conditions will also influence the distribution of insect pests in a geographical region. Some plant species may be unable to follow the climate change, resulting in extinction of species that are specific to particular hosts.
However, whether or not an insect pest would move with a crop into a new habitat will depend on other environmental conditions such as the presence of overwintering sites, soil type, and moisture, e.g., populations of the corn earworm, Helicoverpa zea (Boddie) in the North America might move to higher latitudes/altitudes, leading to greater damage in maize and other crops. For all the insect species, higher temperatures, below the species upper threshold limit, will result in faster development and rapid increase in pest populations as the time to reproductive maturity will be reduced considerably.
In addition to the direct effects of temperature changes on development rates, improvement in food quality due to abiotic stress may result in dramatic increases in growth of some insect species, while the growth of certain insect pests may be adversely affected. Pest outbreaks are more likely to occur with stressed plants as a result of weakening of plants’ defensive system, and thus, increasing the level of susceptibility to insect pests.
Global warming will lead to early infestation by H. zea in North America, and Helicoverpa armigera (Hubner) in North India, resulting in increased crop loss. Rising temperatures are likely to result in availability of new niches for insect pests. Temperature has a strong influence on the viability and incubation period of H. armigera eggs.
Egg incubation period can be predicted based on day degrees required for egg hatching, which decreases with an increase in temperature from 10 to 27 °C, and egg age from 0 to 3 days. An increase of 3°C in mean daily temperature would cause the carrot fly, Delia radicum (Linnaeus) to become active a month earlier than at present, and temperature increases of 5 to 10°C would result in completion of four generations each year, necessitating adoption of new pest control strategies.
An increase of 2°C will reduce the generation turnover of the bird cherry aphid, Rhopalosiphum padi (Linnaeus) by varying levels, depending on the changes in mean temperature. An increase of 1° to 3°C in temperature will cause northward shifts in the potential distribution of the European corn borer, Ostrinia nubilalis (Hubner) up to 1,220 km, with an additional generation in nearly all regions where it is currently known to occur.
Overwintering of insect pests will increase as a result of climate change, producing larger spring populations as a base for a build-up in numbers in the following season. These may be vulnerable to parasitoids and predators if the latter also overwinter more readily. Diamondback moth, Plutella xylostella (Linnaeus) overwintered in Alberta in 1994, and if overwintering becomes common, the status of this insect as a pest in North America will increase dramatically.
Many insects such as Helicoverpa spp. are migratory and, therefore, may be well adapted to exploit new opportunities by moving rapidly into new areas as a result of climate change. There may also be increased dispersal of airborne insect species in response to atmospheric disturbances.
Conditions are more favourable for the proliferation of insect pests in warmer climates. Longer growing seasons will enable insects such as grasshoppers to complete a greater number of reproductive cycles during the spring, summer, and autumn. Warmer winter temperatures may also allow larvae to winter-over in areas where they are now limited by cold, thus causing greater infestation during the following crop season. Altered wind patterns may change the spread of both wind-borne pests and of the bacteria and fungi that are the agents of crop diseases. Crop-pest interactions may shift as the timing of developmental stages in both hosts and pests are altered.
Some of the insect pests which are likely to assume serious proportions due to the changing climate and cropping patterns include American bollworm, Helicoverpa armigera (Hubner); tobacco caterpillar, Spodoptera litura (Fabricius); cotton whitefly, Bemisia tabaci (Gennadius); brown planthopper, Nilaparvata lugens (Stal); gall midge, Orseolia oryzae (Wood-Mason); pink stem borer, Sesamia inferens (Walker); wheat aphid, Sitobion miscanthi (Takahashi); pyrilla, Pyrilla perpusilla (Walker); sugarcane woolly aphid, Ceratovacuna lanigera Zehntner; serpentine leaf miner, Liriomyza trifolii (Burgess); diamondback moth, Plutella xylostella (Linnaeus); rhinoceros beetle, Oryctes rhinoceros (Linnaeus) and tea mosquito bug, Helopeltis antonii Signoret. The possible increases in pest infestations may bring about greater use of chemical pesticides to control them, a situation that will require the further development and application of integrated pest management techniques.
3. Effect of Temperature on Insects:
The rate at which most insect pests develop is dependent on temperature as insects are poikilothermic. Thus, temperature is probably the single most important environmental factor influencing insect behaviour, distribution, development, survival and reproduction and every species has a particular ‘threshold temperature’ above which development can occur. It has been estimated that with a 2°C temperature increase, insects might experience 1-5 additional life cycles per season. This effect may be most noticeable in insects with short life cycles such as aphids and diamondback moth.
It was predicted that Chilo suppressalis (Walker), which is presently confined to the easternmost part of Hokkaido Island, would occur throughout Hokkaido and produce two generations a year after 2°C warming. The outbreaks of C. suppressalis are taking place in association with cool and wet summers or in July, because of the high survival rate of the first generation larvae.
The expansion of its distribution ranges has usually occurred though increased density mediated by increased reproduction. As long as C. suppressalis remains at the current low population density under the conditions of global warming, the northward range expansion of this species would scarcely be expected.
Very low and very high temperatures will be critical in determining the geographical distribution of H. armigera. Climate change will possibly shift the northern boundary of H. armigera by 500-700 km, while the southern edge of ranges may have little effect as this pest is currently distributed throughout Australia and Pacific islands, but with little chance to invade the Antarctica.
The abrupt climate change during cropping season or in the preceding period triggers the outbreak of H. armigera. The off season rains during May-June in north India promote the growth of alternate hosts. It also helps in the carryover and population increase during off seasons. In Andhra Pradesh, H. armigera outbreaks have been recorded in years experiencing un-seasonal rains/hurricanes during November-December.
Studies suggest that temperature increases may extend the geographical range of some insect pests currently limited by temperature. The European corn borer is a major pest of grain maize in many parts of the world. It is multivoltine (multigenerational) and, depending on climatic conditions, can produce up to four generations per year.
Using degree-day (thermal) requirements, the potential distribution of the European corn borer in Europe has been mapped under present (1951-80) temperatures. With a 1°C increase in temperature a northward shift in distribution of between 165 and 500 km is indicated for all generations. In addition to favourable climatic conditions, the distribution of any pest is dependent on the availability of a host plant.
The potential limit of grain maize cultivation is also likely to shift northwards with increasing temperatures providing suitable conditions for the European corn borer. This example serves to highlight the need to examine crop-pest interactions in any climate impact assessment.
Under a warmer climate at mid-latitudes, there would be an increase in the overwintering range and population density of a number of important agricultural pests, such as the potato leafhopper which is a serious pest of soybeans and other crops in the USA.
Assuming planting dates did not change, warmer temperatures would lead to invasions of pest earlier in the growing season (i.e. at more susceptible stages of plant development) and probably lead to greater damage to crops. In the US Corn Belt, increased damage to soybeans is also expected due to earlier infestation by the corn earworm, which could result in serious economic losses.
Temperature increases already have caused changes in species diversity and distribution. For example, the mountain pine beetle, a major forest pest in the United States and Canada, has extended its range northward by approximately 300 kilometers with the temperature increase of approximately 1.9°C. Temperature has shown profound effect on some biological parameters of insects.
4. Effect of CO2 on Insects:
Insect host plant interactions will change in response to the effects of CO2 on nutritional quality and secondary metabolites of the host plants. Increased levels of CO2 will enhance plant growth, but may also increase the damage caused by some phytophagous insects. In the enriched CO2 atmosphere expected in the next century, many species of herbivorous insects will confront less nutritious host plants that may induce both lengthened larval developmental times and greater mortality.
A gradual continual rise in atmospheric CO2 will affect pest species directly (CO2 fertilization effects) and indirectly (via interactions with other environmental variables). Generally, elevated CO2 levels induce increased consumption rates in insect herbivores. The CO2 effects on insects are usually indirect in terms of insect damage that results from changes in the host crop.
Among the probable effects of elevated atmospheric CO2 are changes in plant nitrogen balance that will adversely affect host plant quality for many herbivorous insects. Plants grown in elevated CO2 typically have lower N content per unit of plant tissue.
This results in higher C/N ratio. Insects that feed on plants with lower N per unit of plant tissue generally respond by increasing consumption, but may still suffer longer developmental times and higher mortality. In atmospheres experimentally enriched with CO2, the nutritional quality of leaves declined substantially due to dilution of nitrogen by 10 to 30 per cent.
Increased CO2 may also cause a slight decrease in nitrogen-based defenses (e.g. alkaloids) and a slight increase in carbon-based defenses (e.g., tannins). Acidification of water bodies by carbonic acid (due to high CO2) will also affect the floral and faunal diversity. Lower foliar nitrogen content due to CO2 causes an increase in food consumption by the herbivores up to 40 per cent, while unusually severe drought increases the damage by insect species such as spotted stem borer, Chilo partellus (Swinhoe) in sorghum.
Cotton is a good example of a plant with phenolics that can reduce insect feeding. Elevated carbon dioxide levels allow many plant species to greatly increase their carbon-based defenses. For example, potatoes and plums are plants containing defensive compounds based on nitrogen.
Larval growth and development of Spodoptera litura (Fabricius) on groundnut plants subjected to elevated (550-750 ppm CO2) and reduced (350 ppm in chamber and open conditions) levels of CO2 revealed that the larvae consumed significantly higher quantity of foliage under elevated CO2 than under reduced and ambient conditions.
An increase of nearly two days in larval duration was observed with elevated CO2 level. Rising carbon dioxide will increase the carbon-to-nitrogen balance in plants, which in turn will affect insect feeding, concentrations of defensive chemicals in plants, compensation responses by plants to insect herbivory, and competition between pest species.
5. Pest Management Strategies:
Host plant resistance, microbial pesticides, natural enemies, and synthetic chemicals are some of the potential options for integrated pest management. However, the relative efficacy of many of these pest control measures is likely to change as a result of influence of global warming on extension of geographical range of insect pests, increased over-wintering and rapid population growth, changes in insect – host plant interactions, increased risk of invasion by migrant pests, impact on arthropod diversity and extinction of species, changes in synchrony between insect pests and their crop hosts, introduction of alternative hosts as green bridges, and reduced effectiveness of crop protection technologies.
Host Plant Resistance:
Host plant resistance to insects is one of the most environment friendly components of pest management. However, climate change may alter the interactions between the insect pests and their host plants. Global warming may also change the flowering times in temperate regions, leading to ecological consequences such as introduction of new insect pests, and attaining of pest status by non-pest insects.
However, many plant species in tropical regions have the capability to withstand the phenological changes as a result of climate change. Global warming may result in breakdown of resistance to certain insect pests. Sorghum varieties exhibiting resistance to sorghum midge, Stenodiplosis sorghicola (Coquillett), in India become susceptible to this pest under high humidity and moderate temperatures near the Equator in Kenya.
There will be increased impact on insect pests which benefit from reduced host defenses as a result of the stress caused by the lack of adaptation to sub-optimal climatic conditions. Chemical composition of some plant species changes in direct response to biotic and abiotic stresses; as a result, their tissues become less suitable for growth and survival of insect pests.
However, problems with new insect pests will occur if climatic changes favour the introduction of insect susceptible cultivars or crops. The introduction of new crops and cultivars to take advantage of the new environmental conditions is one of the adaptive methods suggested as a possible response to climate change.
Insect-host plant interactions will change in response to the effect of CO2 on nutritional quality and secondary metabolites of the host plants. Plants, grown under low-nutrient conditions, do have higher concentrations of carbon-based allelochemicals than plants grown under high-nutrient conditions.
Host plants growing under enriched- CO2 environments exhibited significantly larger biomass (+38.4%), increased C/N ratio (+26.57%), and decreased nitrogen concentration (-16.4%), as well as increased concentrations of tannins (+29.9%) and other phenolics. In contrast to the C/N balance hypothesis, plants grown in elevated (700 ppm) CO2 conditions had similar, or lower, concentrations of carbon-based allelochemicals than plants grown in ambient (350 ppm) CO2 conditions.
Larvae fed with foliage grown in elevated CO2 with low N fertilization consumed significantly more plant material than insects fed with foliage grown in ambient CO2; but, again, no differences were observed with high N fertilization and the insects fed on low N plants had significantly higher mortality in elevated CO2.
The production of the nitrogen-based toxin was affected by an interaction between CO2 and N; elevated CO2 decreased N allocation but the reduction was largely alleviated by the addition of nitrogen, thus indicating that future expected elevated CO2 concentrations by climate change, alter plant allocation to defensive compounds and have enough impact on plant-herbivore interactions.
Increases of carbon-defensive compounds by elevated CO2 or low N availability or both, adversely affected growth and survival of Spodoptera exigua (Hubner) in Bt cotton. It was observed that feeding guild, in which some species have shown increases in population density in elevated CO2, are the phloem feeders.
It is likely that climate change will not minimize the outbreaks; on the contrary, it might benefit some pests, which might increase the consumption of pesticide. Chewing insects have shown no change or reduction in abundance, though relative abundance may be greatly affected since compensatory feeding is common in these groups.
Densities of leaf miner species on host species were lower in every year in elevated CO2 than they were in ambient CO2. The results showed that elevated CO2 significantly decreased herbivore abundance (-21.6%), increased relative consumption rates (+165%), development time (+3.87%) and total consumption (+9.2%), and significantly decreased relative growth rate (-8.3%), conversion efficiency (-19.9%) and pupal weight (-5.03%). No significant differences were observed among herbivore guilds.
To the contrary, thrips population size was not significantly affected by CO2, but laminar area scraped by thrips feeding was 90 per cent greater at elevated than at ambient CO2. Because of increased growth, however, undamaged leaf area was approximately 15 per cent greater at elevated than at ambient CO2. Endophytes, which play an important role in conferring tolerance to both abiotic and biotic stresses in grasses, may also undergo a change in response to disturbance in the soil due to climate change.
Transgenic Crops:
Environmental factors such as soil moisture, soil fertility, and temperature have strong influence on the expression of B. thuringiensis (Bt) toxin proteins deployed in transgenic plants. Cotton bollworm, Heliothis virescens (Fabricius) destroyed Bt-transgenic cottons due to high temperatures in Texas, USA.
Similarly, H. armigera and H. punctigera (Wallengren) destroyed the Bt-transgenic cotton in the second half of the growing season in Australia because of reduced production of Bt toxins. Cry1Ac levels in transgenic plants decrease with the plant age, resulting in greater susceptibility of the crop to insect pests during the later stages of crop growth.
Possible causes for the failure of insect control in transgenic crops may be due to inadequate production of the toxin protein, effect of environment on transgene expression, Bt-resistant insect populations, and development of resistance due to inadequate management. It is, therefore, important to understand the effects of climate change on the efficacy of transgenic plants for pest management.
Natural Enemies:
Climate change can have diverse effects on natural enemies of pest species. The fitness of natural enemies can be altered in response to changes in herbivore quality and size induced by temperature and CO2 effects on plants. The susceptibility of herbivores to predation and parasitism could be decreased through the production of additional plant foliage or altered timing of herbivore life cycles in response to plant phenological changes.
The effectiveness of natural enemies in controlling pests will decrease if pest distributions shift into regions outside the distribution of their natural enemies, although a new community of enemies might then provide some level of control. The abundance and activity of natural enemies will be altered through adaptive management strategies adopted by farmers to cope with climate change. These strategies may lead to a mismatch between pests and enemies in space and time, decreasing their effectiveness for bio control.
Relationships between insect pests and their natural enemies will change as a result of global warming, resulting in both increases and decreases in the status of individual pest species. Changes in temperature will also alter the timing of diurnal activity patterns of different groups of insects, and changes in interspecific interactions could also alter the effectiveness of natural enemies for pest management.
Quantifying the effect of climate change on the activity and effectiveness of natural enemies for pest management will be a major concern in future pest management programs. Oriental armyworm, Mythimna separata (Walker) populations increase during extended periods of drought (which is detrimental to the natural enemies), followed by heavy rainfall because of the adverse effects of drought on the activity and abundance of the natural enemies of this pest.
Aphid abundance increases with an increase in CO2 and temperature, however, the parasitism rates remain unchanged in elevated CO2. Temperatures up to 25°C will enhance the control of aphids by coccinellids. Temperature not only affects the rate of insect development, but also has a profound effect on fecundity and sex ratio of parasitoids.
Effects of predators can be encouraged or discouraged by temperature increases. For instance, below 11°C, the pea aphid reproduction rate exceeds the rate at which the lady bird beetle, Coccinella septempunctata Linnaeus, can consume it. Above 11°C, the situation is reversed. In contrast, natural enemies of the spruce budworm, Choristoneura fumiferana (Clemens), are less effective at higher temperatures.
The growth and development of insect herbivores varied with the nutritional quality of their diet (host plant) and the dietary differences showed varied effects on parasitoids. The population size of the insects significantly differed under elevated CO2 and in turn influencing the insect fecundity. Thus, any dietary differences that prolong developmental time, increase food consumption, and reduce growth by herbivores serve to increase the susceptibility of herbivores to natural enemies.
Increasing CO2 concentrations could alter the preference of lady beetle to aphid prey and enhance the biological control of aphids by lady beetle in cotton crop. This study provided the first empirical evidence that changes in prey reared on host plant grown at different levels of CO2 altered the feeding preferences of the predator. Some studies suggest that higher temperatures increase the probability that a host will kill its parasitoid.
For instance, parasitism of the caterpillar, Spodoptera littoralis (Boisduval), by the parasitoid, Microplitis rufiventris Kak, is less efficient at 27°C than at 20°C. The interactions between insect pests and their natural enemies need to be studied carefully to devise appropriate methods for using natural enemies in pest management.
Microbial Pesticides:
There will be an increase in variability in insect damage as a result of climate change. Higher temperatures will make dry seasons drier, and conversely, may increase the amount and intensity of rainfall, making wet seasons wetter than at present. Natural plant products, entomopathogenic viruses, fungi, bacteria, and nematodes, and synthetic pesticides are highly sensitive to the environment.
Although, microbial biopesticides are among the widely used eco-friendly methods of pest control, their efficiency is highly variable across microorganisms with the change in climatic factors (Table 32.1). The available information indicated that high relative humidity is associated with high mortality in host insects infected with fungal and viral pathogens, however, such effects on host infected with bacteria are variable, as environmental influence on host-pathogen interactions operates through biology of the pathogen, immune response of the host.
Temperature, relative humidity, elevated atmospheric CO2, UV radiation, pH, rainfall, soil moisture, etc., are the most influential climatic factors determining the efficiency of microbial biopesticides. Therefore, there is greater need to understand their interactions with biopesticides identify robust and adaptive microbial strains, modify their beneficial traits as per the requirements using molecular techniques and devise appropriate formulations and strategies for their effective use in insect pest management under diverse array of environmental conditions.
Toxins from B. thuringiensis have been used as pest management tools for more than 50 years. The effect of these toxins depends on the quantity of Bt toxins ingested by susceptible insects. Food ingestion is affected by CO2 concentration; plants grown in elevated CO2 often have increased carbon/nitrogen ratios (C/N), resulting in greater leaf area consumption. Elevated CO2 would improve the efficacy of foliar applications of B. thuringiensis.
Botanical Pesticides:
Among the botanical pesticides, azadirachtin, an important limnoid of neem tree (Azadirachta indica A. Juss.) has been found to be quite promising. However, the azadirachtin content of neem tree is highly variable across genotypes, regions, habitats, fruit bearing season and climatic conditions.
The neem genotypes belonging to coastal, arid and semi- arid ecosystems synthesize higher azadirachtin A content than those from the sub-humid regions. The neem trees growing in Andhra Pradesh, Karnataka, and Tamil Nadu in India synthesize higher azadirachtin content as compared to those growing in Orissa, Delhi, and Rajasthan; whereas those growing in Maharashtra, Madhya Pradesh, Uttar Pradesh and Punjab produce intermediate azadirachtin.
The neem trees growing in hot sub-humid, hot arid, and hot semi-arid with cold winter regions synthesize lower azadirachtin content compared to those grown in hot semi-arid with mild winter region, indicating that moderate climatic conditions are favourable for azadirachtin synthesis as compared to extreme climatic condition.
Similarly, there are significant variations in azadirachtin content of neem seeds of Asian and African origin. The neem seeds of Nicaragua and Indonesia produced more azadirachtin followed by those from India, Myanmar and Mauritius. It has been found that the winter stress favours synthesis of azadirachtin B and F in the neem tree seeds.
Moreover, azadirachtin A is highly influenced by environmental factors compared to oil in the seeds and a significant and positive correlation was found for azadirachtin A and oil content with relative humidity, and significant and negative association with atmospheric temperature.
The neem genotypes bearing fruits in November produce 2-4 times more azadirachtin A than those producing fruits in July, suggesting that the neem tree genotypes bearing fruits in November should be identified and commercially deployed for producing neem based biopesticide formulations.
6. Future Outlook of Pest Management:
Climate change will have serious consequences on diversity and abundance of insect pests and extent of losses. Pest outbreaks might occur more frequently, particularly during extended periods of drought, followed by heavy rainfall.
Some of the components of pest management such as host plant resistance, microbial bio-pesticides and natural enemies will be rendered less effective as a result of increase in temperatures and UV radiation, and decrease in relative humidity. Adverse effects of climate change on the activity and effectiveness of natural enemies will be a major concern in future pest management programmes.
The immediate solutions include:
i. Careful monitoring of pest damage, including associated pathogen and insect populations. This would be the first step in a strategy to understand and deal with the effect of global climatic changes as they occur. The acquired knowledge would provide tools to enhance resistance/tolerance to biotic stresses, which would lead to improved IPM and sustained crop productivity.
ii. Development of prediction models for insect pest outbreaks and understand the influence of climatic change on species diversity and cropping patterns, and their influence on the abundance of insect pests and their natural enemies.
iii. Transgenic crops are likely to play a prominent role in future IPM programmes. Therefore, studies on the effect of climate change on the efficacy of transgenic crops should receive priority.
iv. Studies on the expression of resistance/virulence genes under different temperature regimes, and the identification and use of resistance genes that are stable under high temperature regimes, should be intensified.
v. The possible increased use of insecticides resulting from an increase in pest outbreaks will likely have negative environmental and economic impacts for agriculture. The best economic strategy for farmers to follow is to use integrated pest management practices to closely monitor insect occurrence. Keeping pest and crop management records over time will allow farmers to evaluate the economics and environmental impact of pest control and determine the feasibility of using certain pest management strategies or growing particular crops.