Invasive Insects in India

Invasive alien species (IAS) is an exotic species that becomes established in natural or seminatural ecosystems or habitats. It is an agent of change and threatens native biological diversity. Invasive insects in India have been a major threat to household commodities, human health, agricultural produce and environment. India is highly diversified in its weather and climate, which supports the establishment of various introduced insects from other parts of the world. Furthermore, globalisation has facilitated numerous introductions of invasive insect pests. There have been 23 invasive alien insects reported from India to date. This chapter exclusively deals with the characteristics of insects that make them invasive, the common pathways of entry of the invasive insects, the steps through which the introduced insect has established itself in the foreign land and finally the ways to manage them.


Introduction
India is the seventh largest country in the world with a total area expanse of 3.2 million square kilometres (appox.) [1]. It measures 3214 km from north to south and 2933 km from east to west. India is the largest peninsula, whose three parts are covered with water, viz. the Arabian Sea in the west, the Bay of Bengal in the east and the Indian Ocean at the south. India, being such a huge landscape, bears various climatic conditions, soils and vegetations all around the country. This makes India one of the richest diversities in the world. The country has 15 agro-climatic zones comprising of mountains, plateaus, deserts, river, lakes, oceans and grasslands [2]. Apart from that 21.54% of the total area is covered by dense forests [3]. Hence, the complex topography, diversified climate and vegetation make India one of the favourite venues for alien species invasion. Furthermore, India being one of the fastest-growing economies has contributed an export of $330.07 billion in 2014 to as many as 190 countries in the world. On the other hand, the import statistics reveal an importation of $462.9 billion from around 140 countries around the world. This excessive trade among nations has made India an opportunistic target for the entry of the alien species [4].
Biological invasion can be well regarded as a biological pollution which causes maximum losses to the biodiversity [5]. Invasive species are threats to agricultural biodiversity as well as human and animal health. These species are non-native or exotic species which have great power of dispersal and adaptation. They are introduced unintentionally into a new area where they get a favourable climate increase in number and establish. Furthermore, the new area will be devoid of its natural enemy which unleashes the invasive species' growth without any limitation. According to the International Union for Conservation of Nature (IUCN), an invasive alien species (IAS) is an exotic species which becomes established in natural or seminatural ecosystems or habitats, is an agent of change and threatens native biological diversity. India has harboured a total of 173 invasive species including 47 invasive species of agricultural ecosystem, out of which 23 are insects [6].

Pathways of invasion
Pathways are the predicted routes helping the invasive species in transit to new environments. There is a long history regarding the classification of these pathways. The most common pathways include the sea, land or air. There have been several examples of entry of various species through these pathways into India. For example, cashew which is considered as one of the most important cash crops of India was introduced in the sixteenth century by the early Portuguese purposefully realising its importance, through their sea voyages [7]. Today, due to globalisation, the frequency of invasion and its consequences has increased exponentially. Some of the species were knowingly transported to a new ecosystem, while some are a matter of ignorance.
i. Introduced as contaminants. The trade of logs is considered to be one of the most profitable businesses. It provides a huge foreign exchange, but it comes with the consequence of introduction of alien species also. Being stored in godowns for a long period without any treatment makes these logs store house of many insects and pathogens. The importation of these logs also transmits their residing alien species [8].
There has been an importation of various food products and seeds to India from foreign countries. In the past when there was a lenient legal restriction for importation of food products, an admixture of contaminants in the food lot have been proven to be detrimental to the importing country. For example, the importation of food grains from the USA under the US PL 480 scheme (a food assistance programme of the US government to India) also brought Parthenium hysterophorus along with it [9].
ii. Living industry pathway. Living industry implies the trade of living organisms including landscaping or horticultural crops, agriculture, aquaculture/aquarium, pet trade and live seafood trade. There is every possibility that these living entities escort their respective pests while being exported. For example, Apple woolly aphids have been predicted to enter India through the importation of apple rootstock from England [10].
iii. Transportation related. India being rich in its cultural heritage attracts numerous tourists every year. The foreign tourist arrival rate has increased from 2.54 million per annum in 2001 to 10.56 million per annum in 2018 [11]. The increase in number of tourists has increased the chances of invasion by the following ways: DOI: http://dx.doi.org /10.5772/intechopen.91986 • Accidental introduction of insects or other organisms [12] • Unintentional transit of alien species from one place to another • Cargo shipments by air, land and water [13,14] • Food trade [15,16] • Infested fruits or vegetables carried by tourists [17] There has been enhanced domestic conveyance with enriched networks of railways, highways, airways and harbours which further facilitate the easy spread of the introduced insects all over the country mostly by unintentional transit along with passengers.

Steps in invasion
The process of invasion of an alien species follows certain sequential steps, viz. introduction, establishment, spread and naturalisation [18].
i. Introduction. In order to become a habitant of a new locality, beyond the natural ranges, the insect must have to first move or get itself moved from its current habitat. This movement of the insect is called as passive transport which is brought about by vectors. The most common vectors nowadays are humans or human consignments. Passive transports of these invasive species are very common and are difficult to control. Even after strong quarantine inspections, it is difficult to detect the cryptic early stages of the insect which might be transported through the tourists. The transportation of a single gravid female insect also has the ability to create havoc in the introduced environment since it is a prolific breeder and lacks its natural enemy. One of the most common examples of passive transportation of insect is the occurrence of timber beetles (Cerambycidae) in Antarctica which is predicted to be introduced through transported logs to one of the base camps [19].
Sometimes insects themselves have an inherent capacity of migration to long distances. Hence any deviations from the favourable condition make them migrate from that place. Fall armyworm, Spodoptera frugiperda, is one of the recent invasive insect pests in India. They are excellent fliers [20,21] and can fly almost 100 kilometres in certain hours; hence they are believed to have fled from Africa to the Indian subcontinent, but still the mode of arrival of the invasive pest is uncertain. Another example is the Monarch butterfly, Danaus plexippus (Nymphalidae), native to North America which has been found migrating to Australia in the early nineteenth century [22].
ii. Establishment. Short colonisation is very common, but the insect cannot be regarded as an invasive one unless it has established itself in the new environment which is possible only when the invaded insect overcomes the environmental barriers. Unlike local colonisation, establishment of an invasive species is dependent on the amount of propagule introduced. It is believed that the establishment is promoted by disturbances conducted at the receiving environment. Disturbance promotes establishment either because native species are poorly adapted to the frequency, intensity or timing of human-mediated disturbances or because IAS are often adapted to disturbance and thus predisposed for colonisation in such environments [23]. Insects are more prone to invasiveness due to increased resource availability and decreased biotic resistance.
Global warming is another cause of rapid invasiveness of the insects. Global warming has modified the resource availability and habitat suitability, thereby deteriorating biological regime of the native insects, hence favouring the establishment of the alien insects [24]. However some of the regions might be least affected by climate change as far as the invasiveness is concerned, whereas climate change always affects negatively the native species [25].
iii. Spread. Spreading is the process where the initially established species spreads to other areas. Spreading is guided by environmental factors such as weather conditions, microclimate and habitat quality [26]. The community context of spreading includes interspecific competition and predators also. The decision for spreading in an individual insect is also brought about either by its behavioural responses (aggressiveness or sociality of the insect) or morphological responses (wing development as in the case of aphids). Apart from that the established insect can also spread through human-mediated transportation. The latter conserves energy for the invasive insect since the human-mediated transports are quick but the success rate of the spread is highly dependent on the habitat permeability of the habitat to which the lien insect spread [27].

Management of invasive insects
The process of management of invasive insects (Figure 1) includes management at three different levels of invasion of pest: a. When the pest has not been introduced: Preventive measures are taken to avoid the entry of the invasive insect, viz. pest risk analysis (PRA), quarantine and monitoring. This is the best way in managing the invasive species.
b. When the species is introduced but is not spread to nearby areas: Postquarantine measures are taken in such cases such as rejection of the consignment from which the pest has introduced and eradication by means of fumigation of the consignment lot.
c. When the introduced insect has established itself: Various curative measures such as cultural, biological and chemical means of management are adopted.
The Indian government has framed certain laws to cope with the invasive species. The formulation of laws to prevent the entry of invasive species ages back to 1914 when the Destructive Insects and Pests Act 1914 was framed [45].   i. Monitoring. There is a total of 71 plant quarantine stations across major and minor ports (34 seaports, 12 airports, 14 land frontiers and 11 foreign post offices) in India [47] which deals exclusively on restricting the import of any foreign contaminants. One of the major approaches for managing the invasive insects is its early detection. But the problem lies with the exact identification of the insect (to species level). Identification of the species requires expertise in insect taxonomy which is a limiting constraint in India [48]. Hence the government of India has established molecular diagnostic facilities across the quarantine station for easy and rapid detection of invasive insects [49]. Apart from molecular techniques, monitoring is also done by the use of chemical pheromones (sex pheromones and attractants) or by physical with the help of yellow sticky traps, light traps, etc.
ii. Biological. The boom reproduction of invasive insects in a new environment is because of non-availability of their natural enemy and unlimited food supply [50]. Biological control is an ancient practice to control introduced pests, which deals with a timely introduction (classical biological control), augmentation (mass release of native or exotic natural enemies) and conservation (habitat management) of natural enemy (predators and parasitoids) from their (invasive insect's) native places in hope that they may reduce the invasive pest population to non-harming levels [51]. There are many examples of biological control which are listed above ( Table 1). Biological control is sufficient to control the alarming invasiveness if once the natural enemy is established, has long-term effects and is cost-effective too [52]. Sometimes the biological control comes with a consequence which should be pre-analysed before the introduction of the bioagent [53]. For example, Zygogramma bicolorata was introduced in India to manage Parthenium hysterophorus which ended up as a pest of sunflower [54]. There must be prior research to prevent such introduction of natural agents which may have a negative impact.
iii. Chemical control. Prevention is always better than cure. Hence, strict quarantine is the best solution for the management of invasive insects, that is, a thorough investigation of all kinds of imported goods and products in order to hamper the introduction of dangerous species. However, after the breaching for this barrier, the next prompt control measure is pesticides (chemicals). Pesticides are quick acting and are very efficient in reducing or eradicating the invasive insects. But prior knowledge about the insecticide regarding its mode of action, selectivity and residual effect is very much essential while applying an insecticide to manage the invasive insect.

Conclusion
Globalisation has enriched us beyond belief, but it too comes with a consequence. Increased connectivity due to globalisation has resulted in the introduction of numerous invasive insect pests to India which not only have devastated several agricultural crops but also have caused huge monetary loss. This situation further got worse with the advent of global warming and climate change which favoured the establishment and spread of the invasive species. Hence, management of the invasive insects is a challenge. Minimising the loss caused by invasive insects requires an "international management approach" with strict legislation laws and better cooperation among countries with respect to exchange of information regarding the invasive species and their natural enemies. Moreover, there should be a hierarchical setup