**7. Climate impact on vectors**

Grassland diseases are highly affected by the availability of the mode of transmission in the environment. The knowledge on entomology is required to understand the point of interference in the life cycle in order to have successful management strategies.

#### **7.1 Aphids**

Aphids belong to a superfamily of Aphidoidea, which belongs to the Hemipteran sternorrchyna with whiteflies, jumping plant lice, scale insects and mealybugs. This superfamily is then separated into two sub groups of primitive "aphids" and a group of new world aphids [19].

The studies on aphids' ability to transmit viruses have become more complex as they are capable of switching between sexual and parthenogenetic reproduction. Aphids are a vector of Ribonucleic Acid (RNA) grass pathogens and such viruses include barley and cereal yellow dwarf viruses. The severity of grassland diseases are dependent on the areas located. For instance, in California, United States of America (USA), the pathogen-mediated invasion in grasslands is the result of competition between native and exotic plants where aphids have higher fecundity on exotic plants compared to that of the natives [20]. This factor has potentially led to the increase in pathogens transmission rates throughout the community. The life cycle has then been studied closely to understand and in hopes to discover a point of interference which would break the reproduction rate.

A single host plant species is often observed to be utilised throughout the year. In response to the decreasing daylight, sexual morphs are produced in the fall. Genetically recombinant eggs that are reproduced by the male and his oviparae4 mate would overwinter on the host plant and often experience a high mortality rate. Fundatrix5 that emerges from the eggs in spring will proceed to reproduce to live births parthenogenetically. These nymphs would be viviparae6 and will continue the lifecycle in summer. The parthenogenetic mode of reproduction has ensured a rapid population build-up by ensuring that there are eggs available on the plant year round. The rapid increase in aphids population in a single host plant could quickly lead up to overcrowding. This would allow the future offspring of the aphids would be switched to those with wings to have efficient dispersal of feeding opportunity and ensures the genetic survival.

Aphids (49.4%) are transmitters for the majority of the mosaic virus and leafhoppers (6.7%) are transmitters for yellow-type viruses. There are many other insects of interest such as whiteflies (32.3%), thrips (4.0%), mealybugs, plant hoppers (4.5%), grasshoppers, scales and beetles. Aphids are sap-sucking insects and have piercing, sucking mouthparts. They strive generally well in regions with cold winters. The use of their mouthparts include a needle-like stylet that assist aphids to have access and feed on the contents of plant cells. The insect's feeding habits will weaken the plant and cause metabolic imbalance. In addition, aphids secrete honeydew. This is an ideal medium for a variety of fungi to populate. As such, sunlight would be blocked out as the fungi populate, building a barrier for the plant to photosynthesize. During the process of feeding, their stylet has created a point of entry for the pathogens to enter the system of the plant host. The plant, if infected with secondary infection, would then be infected and display disease symptoms.

Aphids have been covered in a relatively large proportion in this chapter. This insect has eggs that are cold-hardy to survive winter. The efficacy to have population build-up is only possible when the temperature is optimum. Every species of aphids have different optimal temperatures. However, the minimum range is said to be at 4 degree Celsius. *Acrythosiphon pisum* is an aphid that reproduction is dependent on the temperature. The overall increase in global temperature by 2 degree Celsius would have an approximation of generations increased from 18 to 23 generations per year (based on a study in the United Kingdom). However, the generation time of a female differs between species and this could potentially be shorten by the decrease in temperature due to global warming. France has a mean temperature of 10 degree Celsius (in the north) and 15 degree Celsius (in the south)

<sup>4</sup> Oviparity refers to female that produce eggs, not live young.

<sup>5</sup> Fundatrix is a viviparous parthenogenetic winged or wingless female aphid produced on the primary host plant from an overwintering fertilised egg.

<sup>6</sup> Viviparity meant that the female bringing forth live young which have developed inside the body of the parent.

which place the aphids in suboptimal temperature conditions. However, this is an alarming increase for entomologists to study aphids further. Apart from the temperature being favourable to the rate of reproduction. The temperature increase is also favouring the mobility of the aphids. The winged aphids have a threshold of 13 to 16 degree Celsius and an upper threshold of approximately 31 degree Celsius [18].

### **7.2 Soil dwelling organisms**

Apart from aphids that are attacking above ground, there are also vectors attacking below ground. Larvae of *Cerapteryx graminis*, a moth from the *Lepidoptera*, are soil-dwelling and the larvae can cause *Charaeas graminis* on grasses. Larvae of *Tholera decimalis* Poda (from *Lepidoptera*) are soil-dwelling and can cause disease to a plant by feeding on its roots. Other larvae that are soil-dwelling can cause large amounts of damage to the plant as the larvae mainly feed on the roots of the host plant and some adults may continue to dwell in the same plant causing more harm. Larvae feeding at the root system may invite secondary infection causing more complications. Nematodes and soil-dwelling borers are also a vector for infection in plants as they could create entry for bacteria to cause further complications to plant health.

These soil-dwelling organisms will be impacted by the decrease in moisture in soil (regions where desertification occurs). Increase in flooding will also be a concern to these organisms as they may not survive if the soil moisture increases too drastically. Their living conditions are also affected by the temperature. The adaptation to the changing climate is similar to that of other insects living aboveground.

### **7.3 Soil bacteria**

Bacteria can be transmitted naturally through exudation out of the host plant and when contact is made between injured plants, they can infect the plant through the wound site. Insects that come in contact with the exudates that infected host plants produce, they can also transmit to other plants as secondary infection. Insects are often attracted to the sugars in the bacterial exudates. During the process of consumption, the mouthparts of the insects will then carry the strains of bacteria. Upon travelling and feeding on other plant, they will create an entry for these bacteria they carry and the plant will now be infected.

Bacteria being microscopic organisms will be sensitive to the change in environment conditions. Depending on the types of bacteria, some may strive in the areas of higher temperature like the *Xanthomonas spp.* are at advantage but not for *Puccinia spp*. The virulence of the bacteria have been studied to have been affected by the change in temperature. *Agrobacterium* strains have their virulence gene amplified as temperature increases but they will have a loss of phosphorylation activity [21]. *Pseudomonas* have increased production of phytotoxins as there is an increase in temperature to maintain its virulence. Therefore, the change in climate will affect the physiological functions of the bacteria differently and ensure the continuum of bacteria in the environment. The effect of bacterial virulence of some effectors may become apparent under specific environment conditions such as humidity.

#### **7.4 Soil fungus**

Soil fungus has different roles in the soil which then serves different ecosystem services. They are also bioindicators of soil health. However, as mentioned earlier, crown rust is a genus of fungus (*Puccinia spp.*). Some of the *Puccinia spp.* are considered as parasites of plants. The presence of harmful plant pathogens indicates poor soil quality. The factors that cause changing soil fungal biodiversity are mainly due to the management practices, chemical fertilisation, application of herbicides and fungicides, biochemical amendments of the soil, soil degradation, soil contaminants and soil properties such as salinity and drought conditions.

Global warming can influence the host plant associations through alteration of interactions between plant and mycorrhizal fungi. This group of fungi have the role of having direct influence on individual plant function and the indirect impact processes such as plant dispersal and community interactions. However, to mediate and to survive, they have ways to mediate the current changes of climate. Such methods involve varying in hyphal exploration type liked to root density [5, 22]. Climate change does not seem all that bad when the essential fungus in the soil required are still able to survive.

The change in climate has created new environmental pressures that results in novel fungus diseases. The effect of climate change on the emergence and reemergence of fungal pathogens have raised concerns on food security, human and animal health, and wildlife extinction due to the report worldwide. There are new virulent fungal lineages with adaptations emerging and they have been suggested to have evolved alongside with the increased pressure of climate change. One such fungus is the *Puccinia striiformis*, commonly known as rust fungus (same genus to the current crown rust pathogen). Stripe rust has affected wheat crops worldwide. There were records that indicate the preference for cooler regions but has recently invaded to warmer regions. The ability to disperse to warmer regions has allowed the emergence of three novel strains. These strains have been described as being more aggressive with increased thermotolerant [23]. The spread of novel strains have been hypothesised for having the ability to replace older strains and expanding the spread of disease. Through microsatellite genotyping and virulence phenotyping on the novel strains, it has been demonstrated that the evolution can potentially be ongoing alongside with the change in climate.

Another fungal concern would be the emergence of *Fusarium* head blight in wheat and other cereal crops [14]. The infection can reduce crop yield and quality. Thus, threatening food security. Outbreaks have been reported specifically with years that experience warmer and humid weather. The economic loss during outbreaks could be up to 75 percent [4]. The shift in temperature due to climate change have allowed the fungi to be more aggressive and able to expand the spread of territory. The change of favourable weather has been observed in two species of the *Fusarium* genus. *Fusarium graminearum* and *Fusarium culmorum* are two of the species that display prominent change towards the shift in temperature over the regions. They have very contrasting weather preferences. Thus, these fungus can expand through larger areas that do not adapt. The increase in environmental stress due to the change in climate have also evidently shown some of the species to react by producing more mycotoxins. As such this has a rising concern not only to food security but also human and animal health.

Apart from the changes that the fungi have evolved to ensure the survival of its kind, the spread of spores have then been extensive through the rising disastrous events. Frequent flooding and strong winds causing dust storms are two such extensive transmission methods. Soil-borne fungal pathogens have been speculated to have increased frequency or range due to climate change [6]. They are found out of their normal range and at times can be challenging to have a first diagnosis [24].

### **8. Future perspective**

There might be results that the resistant plant type is achieving ideal suppression of damage done. However, all living things have the ability to change and adapt to

#### *Earth's Energy Budget Impact on Grassland Diseases DOI: http://dx.doi.org/10.5772/intechopen.99971*

the environment they live in. Plants that as antibiosis may achieve ideal results when planted in Region A. However, when planted in Region B and C, the result may vary. Assuming that the soil conditions in all three regions are the same. However, abiotic factors cannot be controlled and that may be the factor that causes the difference in results. Therefore, grassland management has to be very specific to a particular location and changes that occur through the years can be used to study closely to have a more effective management plan.

Disease in grassland have been affected by the Earth's energy imbalance due to the change in living environment and transmission mode. The change in climate have affected the population of the vectors. Having vectors in the environment is essential for transmission mode in the ecosystem. The reduced availability vectors in the population will ideally have a decreased in the extensiveness of the spread of disease. However, if the disease, in particular consideration to viroids, where to mutate, the mode of transmission could change to either, air-borne or even have a longer dormancy capability to ensure sustainability of its existence.

#### **8.1 Innovative management**

The different kinds of vectors will require different methods of surviving as the climate changes. The increase in certain greenhouse gases in the atmosphere makes it complex to understand the change that the vectors are going through. However, there are a few significant points that could be brought across in this chapter. Vectors such as insects have expanded their distribution to regions where it will be more habitable to them. This can be supported by the pink bollworm (*Pectinophora gossypiella*), an infamous cotton pest that has expanded towards the central of California and away from the South. Other vectors such as the Olive fly (*Bactrocera oleae*) have demonstrated migration behaviour. They would travel southwards during winter to experience summer in other areas. However, this would increase competition of insects for the availability of food for the population. This will potentially lead the insects to have a change in diet if available and adaptable.

Apart from migration behaviour, vectors that have remained have increased in overwintering survival. They will produce eggs that are more hardy to withstand the change in temperature and environmental damages. Insects have also adapted to migrate as mentioned earlier. Vectors that are freeze-tolerant have physiological adaptation to be diapause7 . They could be obligate or facultative. Regardless of which they are, the insects will be hormonally mediated to a state of having low metabolic activity. This will suppress development, suspend activities and increase resistance to adverse environmental factors change. Insects will also display aestivation or hibernation. The ability for the insect to synchronise with the changing environment will be the most ideal situation where the expansion and the spread of diseases are still highly plausible.

The change in ambient temperature have accelerated reproduction rates. This has caused an increase in population size. As such, it can lead to the number of species having dynamic equilibrium. To understand the phenological shifts caused by climate variability, it has been measured with growing degree days (GDD). The GDD will then aid in determining the minimum and maximum temperature threshold. Insects of multivoltine, such as aphids, are at the advantage of the rising temperature. Increase in 2 degree Celsius in temperature could have an estimate of additional five generations. Other insects demonstrated having earlier flight as the ambient temperature increases.

<sup>7</sup> Diapause is an adaptive trait that plays an important function in the seasonal regulation of insect life cycles and is influenced by environmental factors.

The change in climate has also brought about the change in precipitation patterns. High rainfall will have insects such as aphids being washed off and will decrease the opportunity of the insect or pathogens overwintering. However, the insects can migrate further up the soil horizons or deep down. Soil-dwelling wireworms have adapted to the change in precipitation pattern by populating on the upper soil horizon and migrate as they grow as an adult.

#### **8.2 Potential limitations**

The cruciality of understanding vectors in plant diseases in relation to that of climate change is complex and underestimated. The importance of vectors' movement and traceability have yet to be identified clearly as they are showing signs of evolution with rapid reproduction rate. As such, plant diseases can be said to have spread as rapidly as the vectors expand their area of infection. Plant diseases cause secondary infections which are mainly facilitated by insects to allow entry to a pathogen by creating a wound site on the plant organ regardless of it being above or underground. Pathogens that are vectored by insects can also overcome survival to adverse environment factors through the maintenance of over-seasoning in the body of the insect.

There are different management methods innovated in order to suppress the damage incurred. Having a plant that is resistant to specific insects or pathogens is an innovative way or management. For example, antibiosis in host plant resistance is a primary mechanism that works against aphids [17]. The process occurs at the utilisation phase of the interaction between the plant and the insect. It is the result of action of plant-biochemicals in the biological processes of herbivorous insects. Antibiosis would then be expressed in terms of larval mortality, decreased larval and pupal weights, prolonged larval and pupal development, reduced fecundity, prolonged generation time and overall effect on insect survival and development.
