Warning Colors and Chemical Defense
The bright, eye-catching color patterns of most ladybugs are their first line of defense against many predators. The bold markings of one bright color set on a background of another contrasting color provide a memorable image that warns potential predators that ladybugs have hidden defenses, being foul smelling and evil tasting.
The chemical defenses of ladybugs involve a range of chemicals: alkaloids, histamines, cardiac glucosides, quinolenes, and pyrazines, some of which are synthesized by the beetles while others are sequestered from food.
Anyone who has picked up a ladybug a little roughly will have noticed that they often secrete a yellow fluid. This
FIGURE 3 Pupa of H. axyridis.
FIGURE 1 Egg clutch of Harmonia axyridis laid among aphids.
FIGURE 3 Pupa of H. axyridis.
behavior, called reflex-bleeding, is part of their defense. The fluid is filtered hemolymph and is exuded through pores in the leg joint (Fig. 4), whence it runs along grooves to form small droplets at the edge of the pronotum and elytra. This "reflex blood" contains a cocktail of volatile chemicals that have a strong and acrid scent to deter naïve predators.
Many species of ladybug share the same basic color combinations, red with black spots or yellow with black spots being the most common. From an evolutionary perspective, the similarities between many species are beneficial to all. The reasoning is simply that the more chemically defended species share the same color pattern, the smaller the number of individuals of each species likely to be harmed by naïve predators as the latter learn to associate a particular color pattern combination with unpalatability. This type of resemblance, involving a complex of species that resemble one another and are all unpalatable, is known as Mullerian mimicry.
Most of the generalist ladybugs have fairly simple patterns of just two strikingly different colors. However, some of the habitat specialists have more complex coloration. For example, some of the reed-bed specialists, such as Anisosticta novemdecimpunctata, have the ability to change color during their adult life. Through the fall and winter these ladybugs are beige with black spots and are well camouflaged between the old browning reed leaves where they overwinter. In spring, when the ladybugs move to new green reeds to feed and reproduce, their elytra become flushed with red pigment, thus giving the ladybugs a warning pattern.
Many ladybug species have variable color patterns, and, in many cases, distinct color forms occur together as genetic polymorphisms. Thus, for example, in many parts of Asia, several different forms of Harmonia axyridis, can be found. The different forms are controlled genetically, with the inheritance of most depending on differences in just one gene. The existence of these genetic polymorphisms is surprising because the theory of warning coloration leads to
the expectation that all members of the species will look the same. Considerable research time has been expended on this evolutionary conundrum, particularly in A. bipunctata, which has some forms that are mainly red with black spots and other forms that are black with red spots (Fig. 5). The factors implicated in the evolution and maintenance of the forms of this species include different levels of activity (black surfaces warm up more rapidly than red ones), sexual selection by female choice (some females have a genetic preference to mate with black males), and different levels of unpalatability to different predators. A fully convincing explanation for these polymorphisms has yet to be found.
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