The insect life cycle results from a phenomenon of development known as metamorphosis. Through this biological process insects undergo biological and physical changes throughout their life. These changes represent different stages of their life cycle. Abrupt changes in the morphological characteristic and the feeding patterns can be observed during these developmental stages of the insects. It includes four life stages: egg, larva, pupa, and adult (or imago). Not all insects will go through all four stages. It depends on the type of metamorphosis their insect group follows. Metamorphosis can be classified as complete and incomplete metamorphosis.
Figure 1.: Complete and incomplete metamorphosis (Source: https://upload.wikimedia.org/wikipedia/commons/a/a0/Holometabolous_vs._Hemimetabolous.svg)
Complete metamorphosis also known as holometaboly and holometabolous insects undergo complete metamorphosis.
Holometaboly has four life stages: egg, larva, pupa, and adult (or imago). Holometabolous insects have a life cycle where the egg hatches into a larva, then develops into an inactive pupa stage and after that develops into a complete adult. The larva does not resemble the adult insect. Their habitat and oral organs may also differ.
This type of metamorphosis is shown by following insect groups such as Coleoptera (beetles), Lepidoptera (moths, butterflies and skippers), Hymenoptera (sawflies, wasps, ants and bees), Diptera (flies).
Incomplete metamorphosis also called hemimetaboly and hemimetabolous insects undergo incomplete or partial metamorphosis.
Incomplete metamorphosis has only three stages: egg, nymph, and adult (or imago). After hatching, the nymphs grow in size and become the adult individual with the last molt. The nymph and the imago similar in appearance, but the nymph is much smaller size. Their lifestyle is also similar. The mouthpart and the number of limbs are identical to those of the imago. The wings and the external genitalia are missing.
Insect groups Hemiptera (scales, whitefly, aphid), Orthoptera (grasshoppers, cricket), Mantodea (praying mantids), Blattodea (cockroaches), Dermaptera (earwigs) and Odonata (dragonflies) are shown hemimetaboly.
The fertilized adult females lay eggs in a location that offers some protection and food for the newly hatched larvas.
The immature form of hemimetabolous insects.
Figure 2.: Nymphs hatching from eggs (Source: https://phys.org/news/2019-07-creation-big-tool-ideas-function.html)
The immature form of Holometabolous insects.
Figure 3.: Different types of insect larva (Insect larva can be categorized based on the number of their limbs.): (a) Caterpillar, (b) Semilooper, (c) Looper, (d) Campodeiform, (e) Scarabaeiform, (f) Eucephalous larva, (g) Hemicephalous larva, (h) Acephalous larva (Source: https://www.intechopen.com/chapters/69159)
This is the form in between the last larval instar and the adult. In this stage, the insect does not feed or move around. The pupa may be covered by a protective covering such as a cocoon created for a moth or a chrysalis for a butterfly.
Figure 4.: Different types of insect pupa (Insect pupa types can be defined morphologically.): (a) Decticous pupa (b) Adecticious pupa, (c) Exarate Adecticous pupa, (d) Obtect Adecticous pupa (Source: https://www.intechopen.com/chapters/69159)
The complete adult. Most adult insects have wings (excepting where secondarily lost) and functioning genitals. There is no intense growing stage after pupa stage. Some adult insects do not feed at all or have different mouth types (for example piercing-sucking, sponging, siphoning, and chewing), and focus entirely on mating and reproduction.
Figure 5.: Codling moth (Cydia pomonella) adult (Source: https://upload.wikimedia.org/wikipedia/commons/a/a2/LEPI_Tortricidae_Cydia_pomonella.png)
The fertilized adult females lay eggs in a hidden and protected place, sometimes in critical plant tissues. In terms of plant protection eggs are important in protection against mites. There are some active substance that inhibits the development of the mite eggs. Also, there are many opportunity in biological control. There are many predators (lady beetles, praying mantis, green lacewings or true bugs) or parasitic wasps (for example Trichgramma sp.) that can destroy moth and butterfly eggs.
Figure 6.: The Samurai wasp (Trissolcus japonicus) sets to work parasitizing the eggs of the brown marmorated stink bug (Halyomorpha halys) (Source: https://www.cabi.org/news-article/cabi-shares-expertise-in-fao-study-paper-exploring-use-of-microbial-and-invertebrate-biological-control-agents/)
True bugs (Heteroptera), scale insects (Coccoidea), and mites (Acarina) are important hemimetabolous insect groups from a plant protection point of view. The nymphs have piercing-sucking mouthparts. They feed on plants piercing the epidermis and sucking sap from cells. This causes colorful spotting on leaves, fruits, or twigs; leaf curling; deformed fruit, bud, or flower; or a general wilting, browning and dying of the entire plant. There is no biological control method against the nymphs because they are too big for predators or wasps. However, there are chemical control strategy against nymphs.
Figure 7.: Deformed fruit (Source: https://ostvigtree.com/diagnose-a-tree-problem/apple-fruits-deformed-fruit/)
Larva stage has chewing-type mouthpart. The chewing insects include all species of beetles (Coleoptera), grasshoppers (Orthoptera), moths, and butterflies (Lepidoptera) larva. They injure the crops by chewing off external plant parts, including the shoot system, generative parts and roots. Some insects can be identified based on their chewing pattern. In general, the protection against subterranean insects are more difficult than shoot system pests, but there are effective agrotechnical techniques against them.
It is essential to highlight that, for many pest species, it is critical to focus control measures on the larval stage rather than the adult form. For instance, in the case of the codling moth (Cydia pomonella), management strategies should prioritize the control of first-stage larva which chewing in developing fruit. Furthermore, the European grapevine moth (Lobesia botrana) is one of the most important pest of grape, as it damages the generative parts. The larva of the first generation target the inflorescence, while the second generation consumes the growing berries. The third generation poses the greatest risk, as it feeds on ripening berries. The protection against larva stage is effective, there are many methods. There are chemical control strategies and biological control strategies (for example Bacillus thuringiensis sp. and green lacewings) agains insects in larva stage. However, for certain pests, such as the navel orange worm (Amyelois transitella), preventive measures remain the most effective approach. Sanitation is the most important control strategy. Because fruit and nut mummies are the main source of the pests in the spring. The most effective control is to knock down the old fruits and nuts from trees or to pick up the dropped fruits and nuts and dispose of them before the overwintering larva pupate in late winter and mature into imagos.
Figure 8.: European grapevine moth (Lobesia bortana) larva damage (Sources: https://www.cropscience.bayer.eg/en-eg/pests/pests/european-grapevine-moth.html)
The insects are in an inactive stage and the pupa sometimes be covered by an extra protective covering. The pupa is in a location that offers some protection and is hidden. Farmers can protect their fields with chemical control strategy and agrotechnical methods (against subterranean insects). There aren’t any direct biological control strategies against pupa stage but there are natural enemies that parasite pupa.
Figure 10.: Navel orangeworm (Amyelois transitella) pupa in pistachio (Source: https://www.kentonline.co.uk/whitstable/news/horror-as-man-finds-worms-inside-sainsburys-pistachios-256934/)
Some adult insects do not feed at all (Lepidoptera) or have different mouth types (for example piercing-sucking, sponging, siphoning, and chewing) with which can cause different injury on plant. Adult insects may damage plants by laying eggs in critical plant tissues and create an entrance point for a disease or another pest. They invade the crop by “marching in”, by the wind or by flying into the field. The farmers can use many ways of plant protection (agrotechnical, chemical or biological control). However, the most important aspects are the continuous monitoring and the good pest control timing.
In conclusion, pests are one of the most important limiting factors in agriculture. A minor insect damage can lower the crop’s health and the crop’s value. The farmers need to quickly recognize the appearance of any life stage of pests to react to this with effective strategy. Since every stage affects crop health and plant protection strategy, therefore the knowledge of the insect life cycle is essential in Integrate Pest Management (IPM). Fortunately, there are many method to make recognition more effective, for example monitoring and forecasting. By taking a soil sample, the quantity and species composition of subterranean insect larva or pupa can be determined. The determining of peak swarming season is essential to establish the pest control timing. The peak swarm season can be deduced by calculating the Degree Days (DD). Nevertheless monitoring with insect traps is also an effective method to determining the peak swarm season. There are innovative opportunities also, such as AI-driven monitoring systems by scoutlabs.
