POTENTIAL EFFECTS OF CLIMATE CHANGE ON INSECT PEST DYNAMICS

Sikha Deka1*, Sharmistha Barthakur1 and Renu Pandey2

 

1National Research Centre on Plant Biotechnology, Pusa Campus, New Delhi 110012

2Division of Plant Physiology,

Indian Agricultural Research Institute, New Delhi 110012

 

INTRODUCTION

Climate change is the most important, and the most complex, global environmental issue to-date.  Effects of green house gases and climatic changes are already evident from the rising climatic temperature, recurrent droughts, erratic rains, flooding and submergence etc. Global climate is expected to  warm 1.4 to 5.8oC over the century with the maximum increase at Northern Latitude (Meehl, 2007). Such changes may have serious impacts on global crop productivity and agricultural production leading to famine and starvation.  A recent study predicts that crop harvest will decline by more than 30% in Indian subcontinent by 2050 (Rao, 1999).

 

Climatic factors like temperature and  precipitation in particular, have a very strong influence on the development, reproduction and survival of insect  pests and pathogens. Researchers found that the numbers of leaf eating insects are likely to surge as a result of rising levels of  CO2, at a time when crop production will have to be boosted to feed an extra three billion people living at the end of 21st century  (Connor, 2008). It is predicted that some extreme events will increase in frequency as a result of a change in natural climate  variability (McCarthy et al., 2001). Such changes in climatic conditions could profoundly affect  the population dynamics and the status of insect pests of the crops (Woiwod, 1997).

*sikhadeka@yahoo.com

 

 

 

These effects could either be direct, through  the influence of weather on the insects’ physiology and  behaviour (Samways, 2005, Parmesan, 2007 and Merrill et al., 2008), or may be mediated by host plants, competitors or natural enemies (Harrington et al., 2001 and  Bale et  al.,  2002).  Climate change  related factors like rise in temperature, changes in precipitation  patterns, milder and shorter winters, rise of sea levels and increased incidence of extreme weather events can directly influence insects by affecting their rate of development, reproduction, distribution, migration and adaptation. In addition,  indirect effects can occur through the influence of climate on the insect’s host plants, natural enemies and interspecific interactions with other insects. The impacts include changes in phenology, distribution and community composition of ecosystem that  finally leads to extinction of species (Walther et  al., 2002). As  insects represent  huge numbers of taxa and individuals, with their short generation times, high mobility and high reproductive rates, they will respond more quickly to climate  changes than long- lived organisms, like higher plants and mammals (Menéndez, 2007). Infact, insects may be the first predictors of climate change. Milder  and shorter winters will result in early start by the pest under warm weather condition breeding (Bale et al., 2002). Insects of medical importance, such as mosquitoes should have more impact of climate change (Hopp and Foley, 2001 and Epstein, 2001). Other changes include expanded pest ranges, disruption of synchrony between pests and natural enemies and increased frequency of pest outbreaks and upheavals (Parmesan, 2007  and Van Asch and Visser, 2007).

 

Some researchers have  been scoring the fossil  record of leaves that fell off  trees about 55 millions years ago.  At that time, the planet was undergoing  a period of warming. It has been suggested that the 5oC rise in global temperatures caused by a tripling of CO2 levels during the palaeocene-eocene thermal maximum (PETM) period sent  insect numbers soaring and left an indelible impression on the fossilized leaves preserved since that time. It was found that as the temperature rose, the leaves looked more nibbled (Hopkin, 2008). Detailed investigations  were done by analyzing various studies carried out across the globe on impact of increasing atmospheric temperature and CO2 on crop pests population and crop- pest interaction.

RISING TEMPERATURE AND PEST POPULATION

A key factor regulating the life history pattern of insect pest is temperature. Because insects are poikilothermic (cold-blooded) organisms, the temperature  of their bodies is approximately the same as that of the environment. Therefore, the developmental rates of their life stages are strongly dependent  on temperature. Almost all the insects will be affected to some degrees by changes in temperature and there may be multiple effects upon insect life histories. Laboratory and modeling experiments support the notion that the biology of agricultural pests are likely to respond to increased temperatures (Fye and McAda, 1972, Cammell and Knight, 1991 and Fleming & Volney, 1995).

 

With every degree rise  in global temperature,  the life cycle of insect  will be shorter. The quicker  the life cycle, the higher will  be the population of pests. In temperate regions, most  insects have their growth period during the warmer part of the year because of which, species whose niche space is defined by climatic regime, will  respond more predictably to climate change while those in which the niche is limited by other abiotic or biotic factors, will be less predictable  (Bale et al., 2002). In the first case, the general prediction is that if global temperatures increase,  the species will shift their geographical ranges closer to the poles or to higher elevations and increase their population size (Sutherst, 2000, Harrington et al., 2001, Bale et al., 2002 and Samways, 2005).

 

The increase in temperature associated with climatic change, would impact crop pest insect populations in several complex ways like (a) extension of geographical range (b) increased over-wintering (c) changes in  population growth rate (d) increased number of generations (e) extension of development season (f) changes in crop pest synchrony (g) changes in interspecific interactions (h) increased risks of invasions by migrant pests  and (i) introduction of alternative hosts and over-wintering hosts. But all these effects of temperature on insects largely overwhelm the effects of other environmental factors (Bale et  al.,  2002).

 

With the increase in  mean temperature by ~0.6ºC  over the past century and projected increases in the future (IPCC, 2001),  climate change has been shown to have effects on ecosystems worldwide (Walther et al., 2002).  Temperature  increases already have caused changes in species diversity and distribution. It will  alter the distribution of many species in different taxa (Hickling et  al.,  2005).  It has been recognized that global warming affects the individual species and communities in the form of  range shifts and extinctions (Walther et  al.,  2002,  Root et al., 2003 and  Battisti, 2004).  Depending on the development  strategy of an insect species, temperature can exert different effects (Bale et al., 2002). Temperature can impact insect physiology and development directly or indirectly through the physiology or existence of hosts. Some insects take several years to complete one life cycle. These insects (cicadas, arctic moths) will tend to moderate temperature variability over the course of their life history.  Some crop pests are “stop and go” developers in relation to temperature, so they develop more rapidly during periods with suitable temperatures. It has been estimated that with a 2oC temperature increase, insects might experience one to five additional life cycles per season (Yamamura and Kiritani, 1998). Warming could decrease the occurrence of   severe cold events, which could in turn expand the over-wintering area for insect pests (Patterson et al., 1999). In-season effects of warming  include the potential for increased levels of feeding and growth, including the possibility of additional generations in a given year (Cannon, 1998).

 

Migratory insects  may arrive earlier  or the area in which  they are able to over-winter may be expanded. Natural enemy and host insect populations may respond differently to changes in temperature. Parasitism could be reduced if host populations emerge and pass through vulnerable life stages before parasitoids  emerge. Hosts may pass though vulnerable life stages more quickly at higher temperatures, reducing the window of opportunity for parasitism. Temperature may change gender ratios of some pest species such as thrips (Lewis, 1997) potentially affecting reproduction rates. However, insects that  spend important parts of their life history in the soil may be more gradually affected by temperature changes than those that are above ground. This is because soil provides an insulating medium that will tend to buffer temperature changes more than the air (Bale et  al., 2002). Lower winter mortality of insects due to  warmer winter temperatures could be important in increasing insect populations (Harrington et al., 2001).  At higher temperatures, aphids have been shown to be less responsive  to the aphid alarm pheromone they release when under attack by insect predators  and parasitoids, resulting in the potential for greater predation (Awmack et al., 1997). Increases in mean temperatures particularly,  milder winters and longer summers, are highly favorable to increased aphid populations and are thought to have caused extensions to the geographical range of many insect  pests, leading to increased range and severity of infestations. There is also the prospect of new pests, which may become much more important as a result of increased  temperature due to global warming.

 

Insect distribution

Environmental factors influenced by global climate change determine the distribution ranges of organisms. It plays a major role in defining the distribution limits of a species. It is predicted that the distribution of most  insect species will shift towards the poles and to higher elevations with predicted temperature increase due to climate change and temperate regions will bear the main burden of these shifts. With changes in climate, these limits are shifting as species expand into higher latitudes and altitudes and disappear from areas that have become climatically unsuitable (Parmesan,  2006 and Menéndez, 2007). Climate change will alter the distribution of many species in different taxa (Hickling et al., 2005) especially, ectothermic animals are expected to shift their distribution ranges northwards in the next hundred years or so (Vanhanen et al., 2007). It has been recognized  that global warming affects the  individual species and communities   in a form of range shifts and extinctions (Walther et al., 2002, Root et al., 2003, Battisti, 2004 and Battisti et al., 2005). The increasing  winter temperatures have been proposed to be the key factor affecting range shifts  in insects by reducing winter mortality (Battisti et al., 2005). As a result of  temperature increase, the ranges of species could expand poleward and in the mountainous areas also,  an upward elevation takes place because the number of insect species is inversely related to latitude and elevation from the sea  level (Hickling et  al.,  2005). According to a study conducted on 1100 insect species,  climate changes due to global warming may cause 15-37% of those  species to extinct by 2050 (Thomas et al., 2004 and Hance et al., 2007).

 

It is now clear that poleward and upward shifts of species ranges have occurred across many taxonomic groups and in a large diversity of geographical locations during the 20th century. According to one survey of about 1600  species, about 940 of them showed the effects of climate change. For instance, in Europe, 35 species of butterflies have already shifted their ranges 35-240 km northward. In California, 70% of 23 butterfly species now  start their first flight about 24 days earlier than they used to do 31 years ago (Parmesan and Yohe, 2003 and Parmesan, 2007). Parmesan and Yohe (2003) reported that more than 1700 Northern Hemisphere species have exhibited significant  range shifts averaging 6.1 km per decade towards the poles (or 6.1 m per decade upward). At the same time effects of defoliators, wood borers and bark beetles could become more detrimental due to prolonged growing season leading to  multivoltinism, absence of extreme temperatures in winter that diminish population levels and possible shifts to novel host plants (Battisti et al., 2006 and Stastny et al., 2006).

Climate change will alter the ranges and abundances of insects and therefore have profound impacts on agriculture by the movement of existing crop  pests into new areas and potentially, by raising currently disregarded insect species to pest status. The range and abundance of insect changes under  global warming would not necessarily be derived from the physiology of individual insect species. Species may increase in abundance at physiologically non-optimum temperatures raising the concern that rare species currently not regarded as pests may become economically important with global warming (Jenkinson et al., 1996).

 

Insect phenology

Phenology is the timing of biological events such as seasonal activities of plants and animals like flowering or breeding. These events are dictated by photoperiod, temperature or other stimuli. Many insects which  are in synchronization with such events are affected if global warming initiates changes in phenology. It is one of the easiest impacts of climate change to monitor and is by far the most documented in this regard  for a wide range of organisms (Root et al., 2003). With increased temperatures, it is expected that insects will pass through their larval stages faster and  become adults earlier. Therefore, expected responses in insects could include an advance in the timing of  larval and adult emergence and an increase in the length of the flight period (Menéndez, 2007). Members of the order Lepidoptera are the best examples of  such phenological changes. Changes in butterfly phenology have been reported in UK, where 26 of 35 species have advanced their first appearance (Roy and Sparks, 2000).  First appearance for 17 species in Spain has advanced by 1-7 weeks in just 15 years (Stefanescu et al., 2003). Seventy percent of 23 butterfly species  in California, USA have shown an advancement of first flight date of approximately eight days per decade (Forister and Shapiro, 2003). Early adult emergence and an early  arrival of migratory species have also been reported for aphids in the UK (Harrington et  al., 2007). Gordo and Sanz (2005) investigated climate impacts on four Mediterranean insect species viz. butterfly,  bee, fly and  beetle and indicated  that all species exhibited changes in their first appearance date over the last 50 years, which was correlated with increases in spring temperature.

 

Parmesan and Yohe (2003) estimated that more than half (59%) of 1598 species investigated exhibited measurable changes in their phenologies and/or distributions over the past 20 to 140 years.  They also estimated a mean advancement of spring events by 2.3 days/decade based on the quantitative analyses of phenological responses for these species. In a similar quantitative study, Root et al. (2003) estimated an advancement of 5.1 days per decade. Degree-  day or phenology based models are often used to predict the emergence  of insects like cabbage maggot, onion maggot, European corn borer, Colorado  potato beetle etc. and their potential to damage crops. Increased temperatures  will accelerate the development of these types of insects resulting in more generations  and possibly more crop damage per year.

 

ELEVATED CO2 AND PEST POPULATION

One of the most studied aspects of  climate change is the effect of increasing concentrations of CO2 on plants. Plants consist primarily of carbon and elevated CO2 levels allow them to grow more rapidly because they can assimilate carbon more quickly. Greenhouse growers have known this  for decades and add CO2 to encourage plant growth. Similarly, because CO 2 increases the photosynthetic rates of most crop plants, scientists initially thought that increasing CO2 would be a solution for the world’s food supply (LaMarche et al., 1984). In addition to enhanced growth, many  crop plants become more drought-tolerant due to CO2 enrichment. This  is because the openings in the leaves (stomata) that let CO2 in also let water vapor out and if there is high CO2 concentration in the vicinity of leaf  then the stomata need not open as much. It was suggested that under conditions of elevated CO2, plants will produce better yields even when conditions are harsh (LaMarche et al.,  1984). Unfortunately, this optimistic predictions have not proven accurate. One reason for this is  that insects also eat more when plants are grown under elevated levels of CO2 to compensate  their low nutritional quality. A rise in CO2 generally increases the carbon to nitrogen ratio of plant  tissues thereby reducing the nutritional quality for protein limited insects diluting the nitrogen content of the tissues (Coviella et  al., 1999).  The expected reactions from herbivores to the increase in carbon to nitrogen  ratio are compensatory feeding, concentrations of defensive chemicals in plants  and competition between pest species. Insects may accelerate their food intake to  compensate for reduced leaf nitrogen content (Holton et al., 2003), although this is not always  the case (Knepp et al., 2005). However, the response of plants  to increased CO2 varies among species.

 

Increased carbon to nitrogen ratios in plant tissue may slow  insect development and increase the length of life stages vulnerable to attack   by parasitoids. Phytophagous insects may also develop adaptations to overcome higher carbon to nitrogen ratios, for example the pine sawfly Neodiprion lecontei,  showed  an increase in the efficiency of nitrogen utilization when  reared on plants treated with high CO2 concentration (Williams et al., 1994). However, other insect species seem unable to compensate the lower nutritional quality of the plants by  increasing the efficiency of nutrient utilization (Brooks and Whitekar, 1999). The experiments of Lindroth et al. (1993) on three species of saturnid moths showed that the performance of caterpillars is  only marginally affected when the nitrogen content of the leaves is reduced by 23% and the carbon to nitrogen ratio  increased by 13-28%.

RISING TEMPERATURE AND CARBON DIOXIDE ON HOST-INSECT INTERACTION

The capacity of an  herbivore insect to  complete its development  depends on the adaptation to both, the environmental conditions and the host plant. The changed temperature, which promotes the expansion of insect’s range, may  also involve a new association between an herbivore and its host. This has been shown by the pine processionary moth attacking the mountain pine (Pinus mugo) in the Southern Alps. The large outbreaks observed in the expansion  areas on the new hosts may be explained either by the high susceptibility of the hosts  or by the inability of natural enemies to locate the moth larvae on an unusual hosts or environment (Stastny et al., 2006).

 

 

 

The effects of a modified atmosphere  on herbivore insects could also involve the third trophic level, i.e., their parasitoids and predators. A delay in the developmental time of the  herbivores after exposure to high CO2, can increase the probability of parasitism and predation as well. Dury et  al. (1998)  showed that an increase in temperature  by 3°C might lead to the same effect  as that of an increase in CO2 (decreased  nitrogen and increase in condensed tannins)  on oak leaves. However, an increase in temperature may enhance the feeding of the herbivore and thus compensate for  the negative effects of a lower food quality. The effects of different levels of CO2, nitrogen and temperature  on the monoterpene production of Pseudotsuga menziesii was tested and it  was indicated that the synthesis  of these defense compounds were  more affected by variability in individual trees rather than by the treatments (Litvak et al., 2002).

 

The response of herbivore insects to increased  CO2 may also differ among the feeding guilds (Bezemer  and Jones, 1998). Defoliators are generally expected to increase leaf consumption by about 30%, but  leaf miners show a much lower rate. Phloem-sucking insects appear to take the greater advantage from increased  CO2, as they grow bigger in a shorter time. Elevated CO2 increased the susceptibility of soybean to invasive insects  by down-regulating the expression of genes related with hormonal defense, which down-regulate important anti- digestive defenses against beetles. Soybean respond to insect attack by producing defense compounds that inhibit digestive  enzymes (proteinases) in the gut of insects, thereby reducing their performance and crop damage. The production of these anti-digestive compounds are regulated in plants by the hormone jasmonic acid. However, elevated CO2 levels disrupt this  equilibrium in plant-insect interactions and benefit the herbivore.

 

PRECIPITATION AND PEST POPULATION

There are few scientific evidences on the effect  of precipitation on insect pest population and their growth. Some  insects are sensitive to precipitation and are killed or removed from crops by heavy rains. In  some northeastern US states, this consideration is important when choosing management options for onion thrips (Reiners and Petzoldt, 2005). However, some  insects that over-winter in soil, such as cranberry fruit worm and other cranberry insect pests, flooding the soil has been used as a control measure (Vincent et al., 2003). It is predicted that  more frequent and intense precipitation events  forecasted during climate may change have negative impacts on these insect pest population. Similar to temperature, precipitation changes can impact insect pest predators, parasites and diseases  resulting in a complex dynamic manner. Fungal pathogens of insects are favored by high humidity and their incidence would increase by climate changes that lengthen periods of high humidity and reduced by those  that result in drier conditions.

 

 

CONCLUSIONS

The greatest challenge facing humanity in the coming century will be the necessity to double our global food production to meet the booming increase in population by using less land area, water, soil nutrients, droughts from  global warming and increasing insect damage to crops as insect migration expands. Understanding how these rapid anthropogenic changes in climate and atmospheric chemistry will affect the ‘goods and services’ provided by native and agricultural ecosystems, is one of the greatest scientific  challenges of our time. Conversely, there are some indications that the interrelated effects of climate on plant and direct influence on natural enemies can make the overall effects difficult to predict and it is considered that not all climate change scenarios will be detrimental.

 

There are many interactions and it is extremely  difficult to predict the impact of climate change on insect pests in the future,  but we may expect an increase of certain primary pests as well as secondary pests  and invasive species. It has been assumed that global warming will increase the prevalence  of insect pests in many agro-ecosystems, but just to identify the problem is not enough,  we need to find some solutions. IPM methods provide enough flexibility by which we will be able to deal with many of the pests. But reducing the amount of global warming is desirable. Global warming is one of the problems caused by human activities and can also be minimized by human activities. By acting now, we can mitigate the problem and will not have to face the doomsday forecasts of melting icecaps, flooded seacoasts, and species extinctions.

 

In India, the government’s agenda includes three main strategies. First,  through international negotiations, gaining access to technology, funding and energy transfer to adapt to climate change. Secondly, through adaptation policies,  such as implementation of the national action plan, which has eight core national missions running through 2017. The third strategy is to conduct research on climate change through a network of institutions. In addition, the private sector may  also plays a major role through financing, development and deployment of technologies suitable for mitigating and adaptation to climate change.

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