The cotton bollworm, Helicoverpa armigera Hbn. 1805 (Lepidoptera: Noctuidae) is an original Mediterranean fauna element (Anderson et al., 2016). It is a typically facultative migrant species, which wandered every 16-17 years only to northern areas in former years (Feng et al., 2004). Surprisingly, a significant escalation of its range was observed in the last decade of the 20th century. After this outbreak, It established stable colonies in many countries worldwide (Pedgley, 1985; Keszthelyi et al., 2013; Kriticos et al., 2015). It has appeared in the Australian (Fitt et al., 1995) and the American (Czepak et al., 2013) continents, too, where it is gradually gaining ground in many agrohabitats, displacing the native corn earworm (Helicoverpa zea Boddie, Lepidoptera: Noctuidae) (Bentivenha et al., 2016).

The species develops in various generations as a function of its distribution range. Hungary has two to three generations and overwinters as a pupa (Keszthelyi et al., 2013). The pest is highly polyphagous. It has more than 300 host plants. It can damage maize, sunflower, pepper, tomato, bean and many other arable, horticultural and ornamental crops (Firempong and Zalucki, 1989; Liu et al., 2004). A common characteristic of damage in different host crops is that the larvae specifically damage the generative organs. Thus, first of all, it masticates the ears and tassels of maize; on sunflowers, it bores into the blooms; on pepper and tomato, it damages the berries (Virić Gašparić et al., 2022). Due to its damage, the wounds may be secondarily colonised by various pathogens, which can further reduce the quantity and quality of the crop (Darvas et al., 2011; Dimitrov et al., 2013).

Therefore, the insect can be found on a wide range of host plants with different organic matter compositions and a wide variety of colours, where it is intensively feeding. As a canopy-dwelling herbivorous organism, it has a fundamental interest in matching the colour of its direct environment, its host plant, to reduce the likelihood of becoming prey. Colour changes often occur in lepidopterous larvae, which are based on a physiological colour change due to the migration of pigments (Raabe, 1982). Many factors can influence the development of these colour variants (Lehtonen et al., 2009), representing environmental effects between different populations and various selection forces (Raabe, 1982; Mallet and Joron, 1999). In most cases, environmental factors such as light, temperature and diet often have a decisive effect on this manifested colour composition (Raabe, 1982). It is a well-known biological fact that the larval colour of H. armigera is polymorphic (Deepa and Srivastava, 2010). Its larvae can be green, brown, beige, yellow, or any shade. In H. armigera, different body colours appear during the middle larvae stadiums (L₃, L₄) and become pronounced until the fully-developed larval stage. Ramos and Morallo-Rejesus (1976) showed that the source mainly determines the colour of H. armigera larvae.

The formation of different coloured larvae is explained by the experimental results of Yamasaki et al. (2009). Their results-based laboratory experiments pointed out that H. armigera larvae fed on different plant organs differ in colour. Thus, the larvae feeding on leaves are green, in contrast to those developing on a diet of flowers and fruits. Their results suggested that the colouration of the larvae was mainly determined by the part of the plant on which the larvae were reared. Chakravarty et al. (2020) analysed the larval colours of H. armigera populations originating from different locations and showed that the dominant colours differed significantly in their place of origin. It is hypothesised that diet-induced body colour polymorphism in H. armigera may play a role in larvae avoiding visual predation (Yamasaki et al., 2009).

Objective informatics values based on colour image analysis (such as RGB, HUE) have been known since 1989 (Chang and Reid, 1996). They are becoming increasingly popular as methodological methods based on colour comparison. RGB is represented digitally by 256 values each, representing more than 16 million possible colour combinations. Their objective identification allows for comparing certain physiological phenomena through the parameterisation of individual colours (McColl and Martin, 1989)). RGB and HSV models were used in the current study for several reasons. RGB directly corresponds to modern display hardware, and it is easily interpretable and implementable in analysis pipelines. Moreover, it is widely used in colour composition analyses. HSV can be easily derived from an RGB colour cube by projecting it onto a plane with R+G+B=1. It can also be statistically advantageous, as the position of the examined colour on the colour spectrum is expressed by a single variable (Hue) (Schwarz et al., 1987). Their application in entomological research has also provided solutions to several problems that were previously unresolved or difficult to understand. Colours and colour patterns have been used to study a wide range of ecological and practical plant conservation topics, including colour changes caused by pests on food plants (Clement et al., 2015), to assess phenotypic peculiarities accompanying insect development (Lacotte et al., 2023), to provide data for prognostic tools (Hassan et al., 2014), or to understand different mechanisms of crypsis (Sherratt et al., 2007).

In light of this scientific background, our studies, based on field and retrospective image processing, aimed to confirm the colour differences in fully-developed larvae feeding on different host plants and plant organs. Besides, we aimed to elucidate the host-plant-related colour polymorphism of H. armigera larvae using digital colour analysis methods. We wanted to determine the dominant colours in their positions in the visible light range. Finally, we wanted to see if the colour of the larvae was related to the colour of the damaged organs of the host plant. The anticipated outcomes may provide crucial insights into the crypsis of this significant herbivore. They may also illuminate the physiological processes underlying the success of a pest with exceptional adaptability, and contribute to the discovery of the distinctive ecological characteristics underpinning invasive species’ spread.

Our study has confirmed that H. armigera can successfully adapt to the colour of its host plant. The colour polymorphism of the larvae of this species is a known fact, the triggers and causes of which are reported in detail in Yamasaki et al. (2009) and Chakravarty et al. (2020). Based on the results of their laboratory studies, they point out that the colour of the larvae is related to the colour composition of the host plant currently consumed, which is mainly a function of the incorporation of carotenoid content of the plant (Ramos and Morallo-Rejesus, 1976),

As a diurnal herbivore, its adaptation of H. armigera to its environment’s colour is crucial for its survival. This phenomenon is known as crypsis (Endler, 1981) and has been identified as a survival mechanism in several prey-predator relationships. H. armigera has successfully used crypsis to ensure successful larval survival. In their study, Rodríguez-Morales et al. (2021) showed that the visual perception of a predator depends on the spectral sensitivity of the prey’s eye, spatial understanding and temporal resolution. In addition, abiotic factors, such as wind or obstacles, can increase the sensitivity of visual perception (Thery and Gomez, 2010).

The colour variation of larvae developing on various plants is different. Development on the maize ear effectively forces the larva into a narrow spectrum of colours to achieve perfect crypsis. In contrast, a broader range of colours appears on larvae on leaves with varying shades of green or wind-pollinated blooms. The significant difference in colour composition of larvae developing on the inflorescence of wind pollinated blooms should be noted compared to larvae developing on other hosts. This segregation may initially lead to the emergence of new ecotypes as an initial step of adaptation to ecological segregation (Lowry, 2012). Later, as certain host plants become dominant, it may lead to genetic fixation of individuals, i.e. the emergence of new biotypes.

A study by De Moraes and Mescher (2004) showed that the colour variability in larvae of some butterfly species depends on the colour of the food plant consumed and the colour of the leaves and plant organs consumed by the female laying her eggs. Thus, genetic and epigenetic origin is also a determining factor. Several genes responsible for larval colour have been identified in moths. For example, the juvenile hormone is responsible for the manifest larval colouration of Bombyx mori L (Futahashi et al., 2008; Zhao et al., 2012) and some butterflies (Shirataki et al., 2010), as well as for the abrupt changes in colour patterns of these larvae (Futahashi and Fujiwara, 2008). In addition, melanin synthesis is under ecdysteroid regulation in larvae of the butterfly species Papilio xuthus L. (Futahashi and Fujiwara, 2007) and Manduca sexta L (Hiruma and Riddiford, 2009). Overall, in these cases, despite hormonal regulation, larval body colour is genetically determined without being influenced by environmental factors.

In addition to genetic factors, environmental factors are crucial in determining larval colouration in some species (Hazel, 2002). In the case of Battus philenor L., larval body colour is determined by prevailing temperature, probably as a consequence of adaptation to high temperature environments (Nice and Fordyce, 2006).

Our studies raise the question of the analysis from the perspective of different predator species, which has been analysed in other prey-predator relationships by Bennett and Cuthill (1994) as well as Rodríguez-Morales et al. (2021) among others. The question arises whether H. armigera can successfully use crypsis against predators that sense different light ranges (such as various spider, bird, and wasp species). The invasive characteristic of this species follows that when it conquers new areas, it can encounter not only food plants but also predators that were previously unknown to it. The successful application of crypsis prior to these species may even be a critical factor in colonising a new area. The demonstrated colour polymorphism of H. armigera larvae could be a key moment in the invasive insect’s conquest of new agricultural areas. This sophisticated ecological adaptive tool, crypsis, supports the chances of avoiding becoming a potential prey in new habitats and thus establishing a stable population as soon as possible. This phenomenon, in turn, points towards an increase in agricultural damage through the success of this important herbivore species.

Colour analysis and image processing may become an essential tool in future entomological studies, as Byers (2006) has successfully demonstrated that modern high-resolution cameras can detect a sufficient number of colours and isolate them with high reliability even under natural conditions. However, scientific results on the comparative colour analysis of herbivores and their host plants are still relatively scarce in the literature. By applying these approaches, several unresolved insect phenomena could be identified and analysed.

The original contributions presented in the study are included in the article/supplementary materials. Further inquiries can be directed to the corresponding author.

The manuscript presents research on animals that do not require ethical approval for their study.

SK: Conceptualization, Formal Analysis, Investigation, Visualization, Writing – original draft. SO-G: Data curation, Investigation, Writing – review & editing. FP-F: Investigation, Writing – review & editing. KS-T: Data curation, Formal Analysis, Investigation, Writing – review & editing. AB: Conceptualization, Supervision, Writing – review & editing.