**4. 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].
