Ready for winter

Only a couple of weeks ago, 7 spot ladybirds were mating. Today we found this little cluster, with a harlequin trying to merge in, nestled on a wall, ready for winter. More BugBlog ladybird posts here.

Mating 7 spots on 9/10/11.

Poisonous bug babies cluster together

These newborn green shieldbugs, Palomena prasina, look most unlike adults. They have a bright black-red warning colouration in their first instar, for a few days they cluster tightly together on top of their egg shells. After moulting into their second instar they change colour to green and black and they disperse away from their siblings. All shieldbugs, including nymphs have stink glands between the first and second pair of legs. If handled roughly they can release repellent chemicals and they are often brightly marked. But why do these bugs cluster together?
 The contrasting black and yellow or black and red patterns of Cinnabar moths, ladybirds, Burnet moths, bumblebees and wasps are visual signals to potential predators indicating that the animal is distasteful, poisonous or dangerous in some way. The predator, having had a nasty encounter with the aposematic organism learns to avoid it. But how does does aposematism evolve in the first place? You would think that the first aposematic individual either dies or is injured in the process, so it cannot be though natural selection, right? One possibility, first suggested by Sir Ronald Fisher in 1958 is aggregation of related aposematic organisms:

For, although with the adult insect the effect of increased distastefulness upon the actions of the predator will be merely to make that individual predator avoid all members of the persecuted species, and so, unless the individual attacked possibly survives, to confer no advantage upon its genotype, with gregarious larvae the effect will certainly be to give the increased protection especially to one particular group of larvae, probably brothers and sisters of the individual attacked. The selective potency of the avoidance of brothers will of course be only half as great as if the individual itself were protected; against this is to be set the fact that it applies to the whole of a possibly numerous brood

Fisher's hypothesis of kin selected aposematism has been questioned recently. Although aposematic organisms tend to be gregarious, phylogenetic analysis suggest that aposematism evolved before gregariousness, so aposematism makes gregariousness easier to evolve, and not the other way round.
 Birgitta Sillen-Tullberg carried out some elegant experiments showing that aposematism can give direct benefits to the individual, and that kin selection is unnecessary. She presented hand reared Great Tits (Parus major) with two colour forms of the same bug species (Lygaeus equestris), one grey and black (cryptic), and the other - the common form-  red and black (aposematic). A group of tits was presented with cryptic prey and another group with aposematic prey in 11 trials per bird. Great Tits learned to avoid both cryptic and aposematic prey - remember both are equally distasteful - but attacked cryptic prey more readily from the first trial.

In addition, when attacked, aposematic prey survived more, indicating that the tits were more wary when attacking it.
Being grouped, though, can confer further advantages. Gabriella Gamberale and Birgitta Tullberg carried out experiments testing the effect of grouped versus solitary prey - bugs, Spilostethus pandurus - on learning avoidance by predators - chicks in their experiments. Chicks learn to avoid aposematic shieldbugs in fewer predation attempts, and were less likely to attack twice they are aggregated than if the prey is solitary. They concluded that gregarious aposematic prey are a more effective signal for the chicks to learn, the reasons why this could be are still unclear.

References
Fisher, Ronald A. (1958). The Genetical Theory of Natural Selection Dover Publications, Inc. Other: 0-486-60466-7
Sillen-Tullberg, B. (1985). Higher survival of an aposematic than of a cryptic form of a distasteful bug. Oecologia, 67 (3), 411-415 DOI: 10.1007/BF00384948
Gamberale, Gabriella, & Tullberg, Birgitta S. (1996). Evidence for a more effective signal in aggregated aposematic prey. Animal Behaviour, 52 (3), 597-601 DOI: 10.1006/anbe.1996.0200

Six-spot burnets: chemical weapons as nuptial gifts

Six-Spot Burnets, Zygaena filipendulae, are spectacular day flying moths. The contrast between their colour and the green meadows where they live makes them very obvious. Their body and forewing background are black, with a metallic green-blue sheen. Their forewings have six crimson-red spots. In addition they are large and heavy, and females like to perch conspicuously atop flower heads. You cannot miss a sitting burnet, but a flying one is even harder to miss: when they fly, a slow, buzzing, heavy flight, their crimson rear wings with a narrow black border become visible. As you could predict, this bright, black-with red spots contrasting colouration is a warning sign. Burnet moths are toxic, when they are injured, they release cyanide, a highly poisonous chemical. Cyanide compounds are found at some level in every life stage from egg to adult. The larvae sequester and store cyanogenic compounds from their food plants - Bird's Foot Trefoil, Lotus corniculatus - and all life stages are able to synthesize these chemicals themselves. However, production of the chemicals is costly, as larvae grow much more slowly when reared on varieties of Bird's Foot Trefoil lacking cyanogenic chemicals, as they have to synthesize them all themselves. There is a sudden loss of cyanogenic compounds from the last larval stage to the adult, and there also appears to be large variation in the amount of toxic chemicals in the adults. This could partly be explained by volatile hydrogen cyanide emissions by larvae - possibly as an antipredator strategy. Also, males convert some of their cyanogenic compounds into a pheromone; upon approaching a female, they release it. Females are able to determine how much pheromone a male is producing, the larger the quantity, the more likely the female is to accept him. But things get even more interesting. Mika Zagrobelny and co-workers, from the University of Copenhagen, collected Six-Spot Burnet larvae, pupa and adult from a local fallow field and made detailed measurements of the levels of cyanogenic compounds in the tissues of the different life stages, and also their toxic emissions. They found that females, as the larvae, emit hydrogen cyanide. Males are attracted to these chemical plumes, which form part of a pheromone cocktail produced by the female.

Average total cyanogenic compounds content in virgin and mated Z. filipendulae adults as well as in discarded males (which females would not mate with). Error bars are standard deviation. (figure modified from Zagrobelny et al. 2007).

The researchers then compared the cyanogenic compounds of virgin males and females, as well as mated males and females they paired up in the laboratory (see figure above). Virgin males and females had roughly similar levels of cyanogenic compounds. In contrast, after mating, females had larger levels, whereas males had lower levels. This indicates that during mating, males transfer some of these chemicals to the female, likely with the sperm. The levels of cyanogenic compounds in rejected males (males in an experimental pair that the females refused to mate with) were lower than average levels in virgin males, which suggests that females will mate preferentially with those males better loaded with chemical weapons. Why would the female benefit from acquiring more cyanide compounds. Possibly because the more she puts into eggs, the better defended they will be from predators, so this nuptial gift might be seen as a form of paternal behaviour. Alternatively, the female might gain through using this nuptial gift to produce more pheromone, attract further males and increase the vigour of her offspring.

A Six-Spot Burnet on Bird's Foot Trefoil

References
Zagrobelny M, Bak S, Olsen CE, & Møller BL (2007). Intimate roles for cyanogenic glucosides in the life cycle of Zygaena filipendulae (Lepidoptera, Zygaenidae). Insect biochemistry and molecular biology, 37 (11), 1189-97 PMID: 17916505

Why do ladybirds overwinter in groups?

You have probably come across ladybirds clustered under leaves or bark during winter. To spend the winter, seven spot ladybirds - otherwise solitary creatures -   they seem to actively seek each other. I took the photo above a few minutes ago in my garden. I counted 16 ladybirds - most were 7-spots, with two Harlequins - on the shady side of an agave killed by this winters' harsh frosts. Before I go on to explain this communal overwintering behaviour I have to explain why ladybirds are so colourful. Ladybirds are aposematic, a term describing an antipredator adaptation by which organisms have evolved bright, contrasting colours (think on the yellow and black stripes in cinnabar moth caterpillar or wasps) to warn predators of dangerous behaviour (stinging) or distastefulness. Ladybirds belong to this later group. Their beautiful glossy red and yellow elithra with black spots is the first line in a complex defence system, a warning signal to predators, probably birds, of their foul taste. Their bodies contain a bitter tasting alkaloid. If the predator ignores the warning signal and attacks the ladybird - or if you handle them a bit roughly - they release a yellow liquid from their leg joints, with high concentrations of the alkaloid, coccinelline, in what is called "reflex bleeding".
  Given that ladybirds can secrete up to almost a quarter of their body weight during the reflex bleeding, it is an energetically demanding defence mechanism. During the winter, when ladybirds do not feed and need to save precious resources, they do not reflex bleed, although they still taste bitter. It is because of this that they cluster together: as other distasteful prey that lack a mechanism to let know of their foul taste to a potential predator, clustering allows individuals with warning coloration in a group - even if they are unrelated - to benefit from just one of them being injured or killed by a predator, as the predator is unlikely to attack further ones in the cluster.

Reference
Holloway, G., Jong, P., Brakefield, P. & Vos, H. (1991). Chemical defence in ladybird beetles (Coccinellidae). I. Distribution of coccinelline and individual variation in defence in 7-spot ladybirds (Coccinella septempunctata) Chemoecology, 2 (1), 7-14 DOI: 10.1007/BF01240660

Cinnabar moths and caterpillars

During a walk in my local wildlife garden we noticed the first Cinnabar Moth (Tyria jacobaeae) caterpillars of the year, and an adult also flew by. The caterpillars clustered at the flower heads of Ragwort (Senecio jacobaea), which they prefer as food, and had defoliated the plants quite a bit. Many had already left in search for greener pastures and were around the ground or wandering over other plants. Cinnabar Moth caterpillars have a strong tendency to cluster together, an antipredator behaviour.

This behaviour , however, gradually dissapears as the larvae grow so that fully grown fifth instar larvae are actually agressive to each other and tend to be found spaced out in the plants.

Adults emerge in late spring from overwintering pupae and after mating, female lay eggs in the basal leaves of ragworts. Larvae hatch in about two weeks and their development takes about one month, after this, they pupate on the ground and remain in diapause until the following spring.

This moth is often active by day and therefore easy to spot due to its contrasting coloration, but they are most active at dawn and dusk. When they fly, their pink hindwings are quite apparent and they can be taken for some exotic bright pink butterfly. The bright colours of the moth and particularly, the striking yellow and black stripy pattern of the larvae are a warning sign of their distastefulness to vertebrate predators, a phenomenon called aposematism. The larvae ingests and stores in its tissues toxic alkaloids from the foodplant and the adult also synthesizes additional toxins itself. Surprisingly, many invertebrate predators feed on eggs and caterpillars of this moth.
Given that Ragwort is a toxic plant for horses and has been introduced in several countries where it has become an invasive weed, a lot of effort has been directed to study the ecology of the Cinnabar Moth as a way of controlling Ragwort.

More information
J.P. Dempster (1982). The Ecology of the Cinnabar Moth, Tyria jacobaeae L. (Lepidoptera: Arctiidae) Advances in Ecological Research, 12, 1-36 DOI: 10.1016/S0065-2504(08)60076-8