Lacewing (Insects)

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Ladybugs

Ladybugs are one of the most familiar groups of insects. These beetles have received attention in both pure and applied areas of biological research. In some senses they are typical insects, having regular life cycles comprising egg, larval, pupal, and adult stages. However, close scrutiny of the behavior and habits of ladybugs has revealed a variety of fascinating evolutionary and ecological features, including color pattern polymorphism, extreme promiscuity, cannibalism, sexually transmitted diseases, and biased sex ratios, some of which seem to be contrary to theoretical expectation. Here the basic biology of ladybugs and some of these conundrums are considered. In addition, the roles of ladybugs as biocontrol agents are briefly discussed.
As a group, the ladybugs are the most popular of beetles. The bright colors of many species and their reputation of being beneficial, because many species eat plant pests, are at the root of this popularity. In many parts of the world ladybugs are named after religious figures and are revered, often being considered harbingers of good fortune. Indeed, the common English language name for this family of beetles derives from the Virgin Mary. Ladybirds are “Our Lady’s birds.”


DESCRIPTION

Ladybugs are beetles of the family Coccinellidae. This family consists of over 5000 described species of small- to medium-sized, oval, oblong oval, or hemispherical beetles. The dorsal surface is convex and the ventral surface is flat. The forewings, or elytra, are strong and are often brightly colored, sporting two or more strongly contrasting colors in a bold pattern. Not all species are red with black spots. Almost every color of the rainbow is found as the predominant color of some species of ladybug. These ground colors are usually allied to a second color, which differs starkly from the first, particularly with respect to tone. Thus, ladybugs may be red and black, or yellow and black, or black and white, or dark blue and orange, and so on. Sometimes the spots are replaced with stripes or a checkered pattern. The elytra cover the membranous flight wings, which are folded away when the beetle is not in flight.

THE LADYBUG LIFE CYCLE

Ladybugs through the Year

The life cycle of ladybugs has four stages: egg, larva, pupa, and adult. The length and timing of the different stages varies greatly with geographic region. Mating usually occurs when food is available, and eggs are laid in the vicinity of larval food. In contrast to many other insects, the two feeding stages (larvae and adults) usually have the same diet. In regions with winter and summer seasons, reproduction usually occurs in late spring and early summer. In some climates, reproduction can continue throughout the summer, with several generations being produced. However, in places with hot summers, some ladybugs have a dormant period (or estivation) in the hottest months, sometimes having a second period of reproduction in the fall. The winter is generally unfavorable for ladybug reproduction, and ladybugs usually pass the winter as dormant adults. In wet/ dry seasonal climates, particularly in the tropics, many ladybugs are dormant through the dry season, beginning to reproduce at the start of the wet season when food becomes more readily available.
The rate of development of ladybugs, like that of other insects, depends largely on ambient temperature. In a species such as Adalia bipunctata in a temperate climate, the egg stage lasts from about 4 to 8 days; larvae feed for about 3 weeks. When they stop feeding, they form a humped prepupa and shed the final larval skin about 24 h later to produce a pupa that is attached to the substrate at its posterior. The pupal stage lasts 7-10 days. When the adult emerges, the elytra are pale yellow and unpatterned. Hemolymph is pumped into the elytra and flight wings to expand them, and the color patterns develop over the next day or two. Adult ladybugs live for up to a year.
The eggs of many species of ladybugs are bright yellow and are laid upright in batches (Fig. 1) in the vicinity of food. Newly hatched
Egg clutch of Anatis ocellata.
FIGURE 1 Egg clutch of Anatis ocellata.
larvae habitually eat any remaining eggs in their clutch and then disperse to find food. For many species, this food comprises small sap-sucking insects such as aphids or coccids. However, some species feed on fungi, while others are true vegetarians, eating the foliage of plants. The larvae (Fig. 2) are usually elongate, and the ratio of leg length to body length is variable, being correlated to diet. The pupa (Fig. 3) is usually formed on the host plant. Both larvae and pupae may be brightly colored and patterned.
Larva of Harmonia axyridis.
FIGURE 2 Larva of Harmonia axyridis.
Pupa of Halyzia sedecimguttata.
FIGURE 3 Pupa of Halyzia sedecimguttata.

Generalist and Specialist Ladybugs

Broadly, different species of ladybugs can be split into generalists and specialists on the basis of their dietary array and the range of habitats that they live in. Most of the commonest species feed on a variety of aphid species and move from one host plant to another as aphid colonies wax and wane. However, some species have a specialized diet and so are confined to specific habitats where their food occurs. This is true of some of the aphid feeders, as well as for many of the species that feed on other diets, such as coccids, mildews, the leaves of plants or the pollen, and nectar of flowers, as their principal food. Many of these species have evolved precise adaptations to their diets and habitats.

LADYBUG COLOR PATTERNS

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 lady-bugs 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 pyra-zines, 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 behavior, called reflex bleeding, is part of their defense. The fluid is filtered hemol-ymph 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 pro-notum and elytra. This “reflex blood” contains a cocktail of volatile chemicals that have a strong and acrid scent to deter naive 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
Anatis ocellata reflex bleeding.
FIGURE 4 Anatis ocellata reflex bleeding.
smaller the number of individuals of each species likely to be harmed by naive 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. The conifer specialist, Harmonia quadripunctata, manages this trick of changing from a camouflage to a warning color pattern in another way. When active on pine needles, its streaked pink to red ground color with black spots is obviously a warning pattern. However, when resting, this ladybug habitually sits on the reddish pine buds where its color pattern acts very effectively as camouflage.

Polymorphism

Some ladybug species have variable color patterns, with distinct color forms occurring 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.
 Melanic and nonmelanic forms of Adalia bipunctata.
FIGURE 5 Melanic and nonmelanic forms of Adalia bipunctata.

REPRODUCTIVE BIOLOGY

Promiscuity

The reproductive biology of ladybugs raises several evolutionary problems. Both male and female ladybugs are highly promiscuous. Theoretically, females that produce large and energetically expensive germ cells should mate only often enough to ensure high fertilization rates. This theoretical limitation reflects the energetic and temporal costs of copulation and the possibility of contracting sexually transmitted diseases. Yet females of A. bipunctata mate about 10 times as often as they need to fertilize all their eggs. Research suggests that this is an evolved response to high levels of genetic incompatibility between the eggs and sperm of many females and males. Females can store sperm from several males in their spermathecae. By mating promiscuously, females increase the probability that some of the sperm they are carrying are genetically compatible with their eggs.

Female Mate Choice

The study of the reproductive biology of ladybugs has an important place in evolutionary biology, for one of Darwin’s mechanisms of evolution was first demonstrated on a ladybug. In addition to natural selection, Darwin argued that some characteristics of some organisms were the result of sexual selection through either male competition or female choice of mates. That females may have a genetically controlled preference to mate with males of a particular genetic type was first demonstrated in A. bipunctata. over a hundred years after the theory was first proposed. In brief, it was shown that some females carry a single gene that is expressed as a preference to mate with melanic rather than nonmelanic males, irrespective of their own color. Subsequently, mating preferences were shown to exist in other species of ladybug.

Apparent Waste of Sperm

Male ladybugs also present some interesting problems. In a single copulation, for example, a male A. bipunctata can transfer to a female up to three sperm packages, or spermatophores. The sper-matheca of a female can store about 18,000 sperm. An average sper-matophore contains about 14,000 sperm. Therefore, a male that transfers three spermatophores passes more than twice the number of sperm a female can contain. This apparent waste is difficult to comprehend. Possibly by transferring an excess of sperm, the male is indulging in a coarse type of sperm competition, in which sperm in the female’s spermatheca from previous matings are flushed out.

Consequences of Promiscuity: Sexually Transmitted Diseases

Not only do both sexes of many species mate many times, but the duration of each copulation is considerable, lasting several hours in many species. This promiscuity has had one obvious consequence: some species of ladybug are infected by sexually transmitted diseases. Such diseases are generally rare in invertebrates, yet both sexually transmitted mites and fungi infect ladybugs. The mite Coccipolipus hippodamiae, which appears to specialize on ladybugs, lives under the elytra, with its mouthparts embedded into the elytra, from which it sucks hemolymph. Mite larvae emerging from eggs produced by the adult females move to the posterior end of their host before crossing onto a new host when the ladybug copulates. Female ladybirds are rendered effectively sterile about 3 weeks after becoming infected with this mite: infected females continue to lay eggs, but the eggs fail to hatch. A sexually transmitted fungus (in the Laboulbeniales) also occurs but does not have any strong adverse effects on the ladybugs.

CANNIBALISM

Ladybugs indulge in cannibalism. Both adults and larvae will resort to eating conspecifics and sometimes other species, particularly when other food is scarce. The most vulnerable individuals are those that either are immobile (eggs, ecdyzing larvae, prepupae, pupae) or have a soft exoskeleton (recently ecdyzed larvae, newly formed pupae, newly emerged adults). Aphidophagous species tend to be more prone to cannibalism than those with other diets, largely because of the ephemeral nature of their prey and because they are more prone to large fluctuations in population density.

MALE-KILLING BACTERIA AND LADYBUG SEX RATIOS

The population sex ratio of the majority of sexually reproducing organisms is close to 1:1; selection will normally promote the production of the rarer sex, so that the stable strategy is for sex ratio equality. Female-biased sex ratios were first recorded in the ladybug A. bipunctata in the 1940s. Some females were found to produce only female offspring. The trait was inherited maternally. Subsequent research has shown that male embryos die while in the egg as a result of the action of bacteria such as Wolbachia. These male-killing bacteria live in the cytoplasm of cells and are transmitted from infected mothers to their eggs. Although the bacteria in male eggs die when they kill their host, they benefit clonally identical copies of themselves in their host’s siblings, which consume the dead male eggs. The additional resources gained by these neonate female larvae increase their fitness and hence that of the bacteria that they carry. Bacteria of five different clades cause male-killing in ladybugs.

PEST CONTROL

The benefits of allowing ladybugs to eat plant pests have long been recognized. Their importance in controlling aphids on hops in England was noted as early as 1815. For over a hundred years, many attempts have been made to use ladybugs as biological control agents of plant pests such as aphids and coccids. The first reported attempt, and still one of the most successful, was the introduction into California of an Australian ladybug, Rodolia cardinalis, to control the cottony cushion scale, Icerya purchasi. in 1888/1889. This project, costing just $1500, saw an almost immediate return because the orange crop in California increased threefold in 1890. It was the startling economic success of this project that began the biological control “explosion” that occurred through the first half of the 20th century, until the development of cheap and effective synthetic insecticides.
Not all attempts to use ladybugs in biological control have been as successful as that involving R. cardinalis, and in general, the successes reported have involved ladybugs that have been used to control scale insects (Coccidae) and mealybugs (Pseudococcus spp.). Ladybugs introduced to control aphids on a large scale have been less efficient, largely because aphid populations increase much more rapidly than ladybug populations. This means that once aphid populations on a crop have reached sufficient density to attract ladybugs in numbers, the aphid population is already causing damage.
Despite this shortcoming, ladybugs are widely used on a smaller scale to reduce aphid populations. However, these biological pest control agents can also have a negative effect. In the last quarter of the 20th century, the Asian ladybug, H. axyridis, has been introduced into many parts of the world (e.g., U.S.A., Continental Europe). In the U.S.A., it became established in the 1980s, and increased prodigiously in both density and range. In many parts of North America H. axyridis is now the most common ladybug. In Europe H. axyridis became established in the late 1990s in north-west continental Europe (Belgium, Holland, France) and has since spread now being established in 13. Its spread continues.
Although there can be no doubt that the establishment of H. axyridis has had an impact on aphid populations in some crop systems, there is a growing body of evidence from both North America and Europe that this species is having negative effects in its introduced range. First, H. axyridis is causing declines in native species of aphid predators, including other ladybugs, through intraguild competition and intraguild predation (Fig. 6). This is a serious concern at a time when reductions in biodiversity have a high political profile. This ladybug threatens not only endemic species of ladybug in their introduced range, but also many species of aphid that do not feed on crop plants, as well as some other insects that are eaten as secondary foods by these ladybugs and many of the parasitic and pathogenic organisms that are hosted by these native insects.
In addition to these negative effects on biodiversity, H. axyridis also has negative anthropogenic effects. In late summer, as it feeds up for the winter, it takes juice from fruits. In particular, it aggregates on ripe grapes and is harvested with the crop. If the ladybugs are not separated from the crop before it is crushed, the distasteful alkaloids and pyrazines that these ladybugs contain will taint the vintage. A second problem that people have with H. axyridis is that in the fall these ladybugs often seek places to overwinter in houses. Some houses are particularly prone and are inundated by hundreds, thousands, or even tens of thousands of the ladybugs. The ladybugs release yellow reflex blood if disturbed, which has an strong acrid smell and also stains soft furnishings. Moreover, the ladybugs occasionally bite people, usually causing only a slight skin irritation and stinging sensation, but very occasionally causing a very severe allergic reaction. In the U.S.A., H. axyridis is now considered as a pest. The case of H. axyridis should serve as a warning that careful assessment of the ecological and anthropogenic impacts of biocontrol introductions should be made before future programs are approved.
Intraguild predation: H. axyridis larvae eating a Coccinella septempunctata larva.
FIGURE 6 Intraguild predation: H. axyridis larvae eating a Coccinella septempunctata larva.

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