San Francisco State University
Geography 316: Biogeography
The Biogeography of the Convergent Ladybird Beetle (Hippodamia convergens)
by Ian Signer, student in Geography 316, Fall 1999
Winter aggregation of H. convergens
Species: Hippodamia convergens
Fly away home,
Your house is on fire
And your children are gone;
All except one
And thats little Ann
And she has crept under
The warming pan.
The ladybird beetle holds a warm place in European art and folklore
rarely reserved for insects. As an enemy of aphids, bringer of money or good
fortune, and favored insect of the Virgin Mary, people have shown a fondness for ladybugs
for many hundreds of years. The traditional poem above is still repeated today, but
has murky origins in European folklore. Some claim that it originated in medieval Europe,
where the burning of hop vines in Fall would cause an exodus of ladybugs and burn their
helpless larvae. (Cluasen 1961). Others say it may have its roots in the
Egyptian scarab, which was associated with rebirth and the fiery orange of the sun.
With their round shape, bold patterns and bright colors, ladybugs still find favor with many people who loathe most other insects. But these beetles are more than just a pretty visitor to the garden. They have a complex biology that has made them well adapted to survive in a broad diversity of environments throughout the world. This paper focuses on the twelve-spotted ladybug, Hippodamia convergens , and the unique adaptations that it has developed to survive in the Mediterranean climate of California.
The adult Hippodamia convergens is a small, oval beetle
1/4 3/8 (6-8mm) long with orange-red elytra (hardened wing-covers) that
sport 6 black spots each. Where the elytra meet the thorax, there is a small 13th
spot. The pronotum (a plate covering the thorax) is black with a white border
and two small white stripes that give the impression of false eyes. The small head
is mostly black, with a patch of white near the mandibles. Like all beetles in the
family Coccinellidae, adult twelve-spotted ladybugs have 3 tarsal segments (the most
distal leg segments) on each leg and short, clubbed antennae.
Hippodamia convergens eggs are creamy yellow and elipsoid, laid in a dense cluster. The larvae are soft-bodied with grey and orange segments and six stumpy legs. The pupae are rounded and mottled in orange and black, often having the last shed larval skin adhering to the edges.
Eggs Larva Adult
Ia. VARIATION AMONG INDIVIDUALS
Timberlake (1919) argues that this species is relatively constant, and that the variations observed by both lay people and other entomologists are, in fact separate species. He notes that very rarely some of the spots may be joined, but, is not as variable as the other members of the Hippodamia genus (which he distinguishes by the structure of the aedeagus).
I would like to find a more modern reference regarding this topic, because I feel that breakthroughs in genetic research will result in interesting revisions to the (already constantly in the process of revision) classification of this and other insect species.
Hippodamia convergens lives throughout California in a wide variety of habitats. This insect can be found in urban gardens, agricultural fields, or pristine mountain meadows. In order to breed, Hippodamia convergens needs to be in a place where there are abundant aphids, but the adults are highly adaptable and can be found eating a wide variety of other foodstuffs. This beetle has been found from the salt marshes of the San Francisco bay to the high peaks of the Sierra. (Edwards: 1957)
III. GROWTH AND REPRODUCTION
Ladybugs, like all beetles, undergo a complete metamorphosis in four stages: egg, larva, pupa, adult. In California there is generally one generation a year. These beetles spend the majority of their lives as adults, much of it in a state of limited activity called estivo-hibernation (in summer, fall and winter). (Hagen:1962)
When female ladybirds arrive in the aphid filled valleys after hibernation, they begin laying eggs. Females lay 200-300 eggs in upright clusters of about 15-30 eggs each. (Interestingly, females who lay eggs before winter die during hibernation or fail to lay eggs the following spring). Eggs hatch after 5-6 days.
Once the larvae begin feeding, they grow quickly. Hippodamia convergens goes through 4 larval instars (shed exoskeletons) before pupation. The average duration of each instar is as follows:
|Instar I||Instar II||Instar III||Instar IV|
To shed each skin, the ladybug larvae attaches the skin to a
substrate by the posterior end and crawls out head-first. At the completion of the
last larval instar, the beetle fixes itself to a branch or leaf and sheds its last skin
(which remains attached) to reveal a pupa underneath.
Hippodamia convergens remains in the pupal stage for approximately 8 days. The total time from egg to adult is relatively rapid, 33.2 days.(Balduff 1935)
Shortly after emerging from the pupa (within1-3 days), mating takes place. The male fixes himself on top of the female and internally fertilizes her using his aedeagus (the insect equivalent of a penis). Female ladybugs tend to mate with many males, although this is not necessary for fertilization of the eggs. If there are abundant aphids, she can begin laying fertile eggs 5 10 days after fertilization. But if conditions are not favorable, she may wait for up to 9 months before laying eggs.(Balduff 1935)
I have personally seen lots of mating going on among aggregations of Hippodamia convergens in the Sierra Nevada in early spring before the beetles return to the Central Valley. I have not found information regarding this phenomena, and am curious as to whether the males that mate in the early spring may have a higher chance of fertilizing the eggs than the ones who mated with the same females before hibernation.
Despite an extensive search through primary sources, I have still not read a definitive description of the range of Hippodamia convergens. This species is the most common ladybug in North America (Essig 1926), and is found throughout the continent (Hagen 1962)
If exposed to temperatures below freezing, these ladybugs quickly die (Latta 1928) To survive the winter, adult beetles hibernate in protected areas, insulated by leaf litter or underneath snow. Hippodamia convergens can make long distance migrations, and have been collected from salt marshes along San Francisco Bay to the peaks of the Sierra. (Edwards 1957) Wind currents could carry these beetles just about anywhere, but they can only reproduce where there are large numbers of aphids. This has had a profound effect on their biology and distribution in California.
IV a. Ladybird Migration in California (The information in this and the following section is largely taken from Hagen 1962)
In California this insect has a unique life history which is
directly influenced by our unique climate, topography and ecology. Historically,
aphids were most prolific in spring, when the grasses of the Great Central Valley were
lush from the winter rains. In order to produce eggs, adult ladybugs need to
eat aphids, which then constitute the only food for their larvae. So, from February
to March, adult ladybugs born the previous year would breed and lay eggs in the Central
Valley. By May, most adult ladybugs from last year are dead, and new adults are
beginning to emerge.
When these new adults emerge from their pupae, the Central Valley is baking hot and dry, and (except in areas with extensive irrigated agriculture) there are no longer large numbers of aphids. During the morning calms of May and June, beetles take flight as the temperature rises over 63?F (17?C). (Hagen 1962) At this time of year, there are mid-morning westerly winds that blow the beetles towards the high Sierra. (See Fig. 1)
Because these beetles can not operate at temperatures below freezing, when they reach a certain altitude, they stop flying and free-fall until they are warm enough to resume. So, ladybugs exhibit oscillations in their flight which may be as much as 1000 vertical feet.
Fig 1 ( Ian Signer adapted from Hagen 1962)
The flights in May and June take adult H. convergens into the
high Sierra (6000-8000ft.), where they form small but widespread aggregations.
During this time, the beetles eat pollen and nectar to put on fat which they will use
As summer moves into fall, the ladybugs slowly migrate down the mountains following river valleys. With the onset of the first rains in October and November, the beetles begin to gather in the lower elevations of the Sierra (2000-5000ft.) in high numbers. These first rains bring them out of the leaf litter, and on warm days, they will form congregations in sunny riverside areas that increase in number until the onset of cold temperatures in winter.
When the Sierra begins to warm in February and March, the beetles begin their flight back to the valley. Ladybugs have an oscillating flight pattern similar to the one that brought them to the mountains, but this time they are blown by northeasterly wind currents which result from developing high pressure areas over the central Rockies. They continue flying until cool night temperatures force them to land. Those individuals that leave earlier in the morning, therefore, travel farther from their hibernation sites than those that leave later in the day.(Fig. 2)
Fig 2 ( Ian Signer - adapted from Hagen 1962)
The adults that land in the central valley generally arrive when aphids are most abundant, and conditions are best for them to reproduce.
IVb. Human Influenced Changes in Ladybird Migration
Irrigated agriculture in California has created food that can sustain aphids throughout the long, dry summer in certain locations. H. convergens has reacted to this change in these areas, where it now breeds throughout the year.
I have not read any current studies to this effect, but theorize that these year-round populations of H. convergens may have a life-history that is similar to those in the Eastern U.S., which hibernate in small aggregations near spring and summer feeding grounds. Because they are feeding though the end of summer, I do not know to what extent they would be able migrate using the wind currents that help lift ladybugs into the Sierra during the spring.
In addition to changing the abundance of aphids, people have also impacted these beetles by collecting them for commercial use as biological control agents. Today, harvesters in the Sierra Nevada can sell ladybird beetles for $20 a gallon (there are about 75,000 individuals per gallon), and experienced bug harvesters may make $1000 a day during the season! (Hubbell 1993) As biological control becomes more popular, more people may go into the mountains to collect ladybugs for a growing number of consumers. According to Kenneth Hagen at UC Berkeley (1954) extensive harvesting from easily accessible over-wintering sites could create areas in the Central Valley where ladybugs fail to return, thus increasing farmers need to use pesticides.
I believe that the removal of extensive numbers of 12-spotted ladybird beetles from their hibernating areas would have many effects that relate not only to their distribution, but to other aspects of their biogeography and that of the organisms that interact with them.
Unanswered questions remain to be studied. These include: Has the export of Californian Hippodamia convergens had a significant effect on other species of ladybirds in the United States? Have diseased ladybirds from over-wintering beds significantly affected populations by infecting healthy individuals? What effect does the removal of ladybugs have on the plants and animals that may be associated with them in the Sierras?
Map of Distribution (in California)
** Due to technical difficulties, the pink in the above map did not print, and is
represented as white areas.
The eggs of H. convergens are laid in a group, and the first larvae to hatch generally begin by eating the remaining eggs. This may reduce competition for aphids that are on the plant, as well as providing energy for larvae that must actively wander in search of large number of aphids to sustain their growth. When the larvae encounter aphids, they generally bite a hole in the body and suck out the contents. They then pump the liquid back into the body and suck it out several times to effectively mix the innards of their victim with digestive juices. The fourth-instar larvae consume about 50 aphids per day. (Clausen:1940).
Adult Eating Aphid
Adult 12-spotted Ladybugs, prefer to eat aphids, and consume, on
average, 22 per day. (Balduf: 1935) But, as with most aphid-feeding insects, these
insects have a diverse diet so they can survive when aphids are scarce. Among their
preferred foods are honeydew, nectar, and pollen. They may also eat flower petals
and other soft plant parts. (Hagen: 1960) However, in order to lay eggs,a
Hippodamia convergens female must consume aphids. If aphids are not the main element
of her diet, she will reabsorb her eggs and none will be laid. (Haug: 1938)
Because aphids are a seasonally available source of food in California, the 12-spotted ladybug has evolved to reproduce when aphids are most abundant. During the summer and fall, most ladybugs are in an adult stage feeding on non-aphid food. They hibernate during the winter.
Beetles are among the most successful creatures in the Earths history. The first beetles appeared in the Permian period, about 225 million years ago. Today, they represent the largest group of animals on the planet, and contain over 370,000 species.
The character that separates the beetles (Order Coleoptera) from other insects is the presence of shield-like hardened fore-wings called elytra. Lower Permian fossils of beetle-like insects called Protocoleopterans, have leathery wing covers with veins that clearly tie them to insects in the suborder Megaloptera (Order Neuroptera) which includes modern dobsonflies. Protocoleoptera had short legs, short antennae low down on the sides of the head and non-projecting coxae (middle leg segments). Crowson (1981) states these may have been adaptations to living under loose bark, a lifestyle for which beetles would have been well suited. Coincidentally, the modern beetles which most resemble Permian fossils (Cupes, Priacma, and Micromalthus) all have wood-boring larvae. However, there are not yet fossil records of insect borings in Permian woods, so Crowson theorizes that the Protocoleopterans may have fed on fungus.
Upper Permian deposits show a diversity of primitive beetles which have elytra which cover the abdomen and show more regular venation. By the Triassic, the characters of modern suborders of beetles begin to emerge. In the Triassic, the development of other organisms significantly shaped the course of beetle evolution. New animals appeared, such as lizards and small mammals. The first cone-bearing plants sprouted, with the first indications of resin-soaked wood. As time progressed, more new organisms arose to influence beetle evolution.
In the Jurassic, conifers proliferated, along with cycad-like gymnosperms. Insectivores continued to proliferate, and Crowson proposes that it was during this time that various chemical, visual and behavioral defenses began to develop, such as the tendency of many beetles to drop off a disturbed plant (this behavior is practiced by H. convergens ). During this time, there was a tremendous adaptive radiation of beetles.
Many cycad-like plants of the Jurassic had bisexual cones surrounded by petal-like bracts resembling flowers. These cones did not produce nectar, but produced abundant pollen. The presence of large amounts of pollen and a protected inner ovary has led some paleobotanists to suggest that beetles were the pollinators of these plants. (Delvoryas :1968), The first beetles belonging to the superfamily Cucujoidea appeared during the Jurassic. The Cucujoidea share characteristics that allow them to survive in dry conditions. (Lawrence & Newton: 1982) These include valves which close the larval spiracles and a water-efficient excretory system.
The first identifiable Coccinellidae in the fossil record date back to the Cretaceous. Crowson suggests that warning coloration, such as the bold patterns sported by modern ladybugs, developed during this time as birds became more important predators.
I have not been able to find a specific record of the development of the genus Hippodamia. Perhaps the red and black warning coloration exhibited by ladybugs developed during the Late Jurassic and Cretaceous, when continents were still fairly close to one another. Wide-ranging species such as Hippodamia convergens and Harmonia axyridis (the Asian Ladybug) can fly long distances, so I theorize that they did not speciate until the continents became significantly separated after the end of the Cretaceous.
VII. SPECIAL ADAPTATIONS
Ladybugs are among the most bold and active beetles. Throughout the world, many species share a similar pattern.
The dainty shell upon backe
Of crimson strewed with spots of blacke.
- Drayton (1630)
The 12-spotted ladybird, like its relatives in other parts of the
world, is poisonous. When disturbed, both the larvae and adults can force blood out
through joints and other weak areas in their exoskeleton, an adaptation called
reflex bleeding. Their blood contains toxic alkaloids, and has a
distinct odor that deters predators. (Berenbaum: 1995)
In order that predators, particularly birds, recognize it as a poisonous species before taking a bite, ladybugs have developed bright warning coloration. Those birds that do taste these beetles are not likely to quickly forget their distinctive pattern.
Despite these chemicals (or perhaps because of them) people in pre-Industrial Europe used ladybugs as a cure for measles and colic. They were also mashed and stuffed in cavities to cure toothaches (Berenbaum 1995)
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Barrenbaum, May R. 1995 Bugs in the System: Insects and their Impact on Human Affairs Menlo Park, CA. Addison-Wesley
Crowson, R.A. 1981 The Biology of the Coleoptera. London, Academic Press
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Delvoryas, T. 1968 Investigation of North American Cycadeoids. Paleontographica (A) 121: 122-123
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Palmer, M.A. 1914. Some notes on life history of lady-beetles Annals of the Entomological Society of America 7: 213 238
Powell, J. and Hogue, C.L. California Insects. Berkeley, CA. UC Press
Timberlake, P.H. 1919 Notes on the North American Species of Hippodamia (Coleoptera) Journal of the New York Entomological Society Vol. 37(June-Sept) :162-174
White, Richard E. 1983. Field Guide to the Beetles of North America
Boston, MA : Houghton-Mifflin.
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