Chemical defence in beetles and moths

Insects take to various defence strategies against predation. These strategies include homochromy (i.e. resembling the background in colour and marking pattern, very common among adults and developing stages of insects), camouflage (i.e. having a close resemblance with some objects in the surroundings, e.g. the dry leaf butterfly of the Orient, Kallima, and various stick insects or Phyllies resembling dried up twigs or leaves), aposematism (warning coloration for defence), mimetism, mechanical devices (e.g. spines on the body of the leaf-beetles belonging to the subfamily Hispinae, springing mechanism in the third legs in members of another leaf-beetle subfamily, Alticinae), reflex bleeding (i.e. coming out of blood through ruptured skin at certain places in the body, e.g. the leaf beetles of the genera Timarcha, Oreina and Galeruca and others, like Meloe, show reflex bleeding, on being disturbed, at the tibiofemoral joints of their legs or near the mouth and chemical defence (i.e. through presence of certain toxic chemicals in their body).

Meloe and other blister beetles, like the Paederus, secrete cantharidin which is a toxic and dangerous substance. One of us (PJ), while collecting meloids near a dam in Sudan, got many beetles trapped under his shirt. He was covered with blisters and had a high fever during the whole night. People know that cantharidin has the reputation to be an aphrodisiac. It is remarkably toxic, and 100 milligrams are lethal to humans. There are many criminal cases of poisoning due to meloids and meloid powder given to humans. Thomas Eisner, in his recent book (2003), reports that French legionnaires in Algeria, in 1893, were poisoned by frogs that they had eaten, because the frogs had fed on local meloids, which are very abundant there after the rains. As is well known in England, the French people eat frogs and snails; that is why they are sometimes called "Froggies". So no wonder, why those legionnaires in Algeria found and ate the unfortunate amphibians. In North Africa, the larval stage of these meloids feed on nymphs of grasshoppers. Meloe, at the larval stage (the larvae of meloids are called triungulins), is parasitic on bees. Meloe is now relatively rare now in Europe or United States, but PJ remembers that once on the Tchiaberimu, hilly area situated on the west side of Lake Edward, he saw many of them on Galium and grasses all over the mountains. But there Timarcha, another beetle, was not present on Galium. This other beetle, being apterous and slow moving, did not reach Central Africa. It could not even cross the Sahara and cannot be found in Hoggar mountains, for instance. But Meloe, thanks to the triungulins, which climb from flowers to the body of bees, had their aerial transportation, and the Sahara was not an obstacle to their migration.

Bombardier beetle stores in its abdominal glands hydroquinones and hydrogen peroxide. In the reaction chamber, with enzymes, the two substances interact and this leads to an explosion. The resulting spray reaches the boiling point and vaporizes. All entomologists have at least once experienced the heat of the vapour on their skin. One young Danish entomologist lost the use of one eye that way. His cornea was burnt by the quinones. It is like what some millipedes do in the tropics; they can blind you if the spray reaches the eyes.

Leaf beetles (Family Chrysomelidae) live as adults and larvae well exposed on the plant body, as they are mostly leaf feeders. Hence they are readily available to preying insects. Quite naturally they have taken to several different ways of protection against predators, including the interesting chemical defence.

Chemical defence in leaf beetles has been extensively studied by Pasteels and his coworkers (Pasteels ei al, 1988, 1889, 1992, 1994). So far only three subfamilies of the huge family Chrysomelidae, namely Chrysomeli-nae, Criocerinae and Galerucinae have been concentrated upon.

The toxic chemicals, present in the body of leaf beetles, are released to work against predators in several different ways. One way is through externally opening glands, which are glandular pockets of the epidermis. Such glands are called exocrine glands. These glands are located on the pronotum (i.e. the dorsal shield of the first segment of the thorax) and on the hard forewings or elytra. Pasteels et al. (1988) have described the discharge from these glands thus: "After disturbance, the secretion oozes out from the gland pores and accumulates in the marginal grooves of elytra and pronotum as well as in more or less defined pronotal and elytral depressions, constrictions and concavities. These contours certainly help to retain the secretions on the insect". Toxins in the blood of the insect may also be released by reflex bleeding. In many insects, as in Pimeia (Family Tenebrionidae), there is discharge of fluid contents of the gut either through the mouth or through the anus when they are disturbed. The discharged fluid is likely to contain the toxins present in their plant food, but a definite demonstration of this remains to be done. It often contains quinones, and, therefore it is toxic and repellent. In some places in Morocco, Pimelia and Timarcha species show some kind of Mullerian mimicry. They are quite similar in appearance and both are toxic: Timarcha by reflex bleeding and Pimelia by regurgitation. Both are totally apterous. Pimelia, being omnivorous, is more adaptable than Timarcha and has a much wider distribution in Africa and Asia. Timarcha, a chrysomelid, needs its host-plant, not always available, and, therefore it could not cross deserts like Pimelia

In New Guinea, the Polyconoceras millipedes squirt their very toxic quinones covering more than one meter. The quinones burn skin, and eyes, and can be very toxic. One of us (PJ), around Lae, in the east of New Guinea, got the secretions over his body. He came back with a beekeeper equipment and all the plastic was badly burnt. The skin after receiving the spray was disintegrating rapidly. Blind dogs are found in the area, and it is said that criminals among the natives used the extract to poison their enemies. The papuans are terribly afraid of them. Similar cases of quinone projections are known in tropical America among several millipedes, but they are rare. Generally they ooze quinones, on their diplosegments, as in Africa, but do not project it. Centipedes bite, but all millipedes produce secretions, cyanide, proteins, as varied as the group to which they belong. This account of millipede toxins is a little deviation from the topic of this chapter, but it may be accepted, as it illustrates what arthropods may do for their protection.

Larvae of some tortoise beetles (leaf beetle subfamily Cassidinae) carry a fecal mass at the end of their abdomen. Besides the repulsive nature of the fecal discharge, secretions of some exocrine glands may be added to it to improve its defensive value. The discharge may also contain some toxic compounds present in their leafy food.

Still another possibility is that toxins accumulate in blood, and affect the predator, when the latter attacks and wounds the leaf beetle. The attacked insect may get killed, but after this agonising experience the predator will avoid attacking kins of the insect killed. This situation has been referred to as "kin selection" by Pasteels. Ferguson and Metcalf (1985) have observed that a preying mantid does not attack galerucine beetles, which have been fed in cucurbitacin rich diet, but it readily attacks galerucines reared on cucurbitacin free diet. (Cucurbitacins are toxic compounds in plants of the Family Cucurbitaceae.)

Leaf beetles get a variety of compounds with their plant food. Some of these chemicals are toxic or repulsive to predators, and are stored in the insect body and used as such in defence. Other compounds may provide material for de novo synthesis of defensive compounds in the insect body.

Defensive compounds identified in insect body include:

(1) Nitropropanoic acid and isoxazolinone glucosides.

(2) Cardenolides.

(3) Polyoxygenated steroid glycosides.

(4) Pyrrolizidine alkaloids.

(5) Amino acid derivatives.

(6) Anthraquinones.

(7) Cucurbitacins.

(List from Pasteels et al, 1994.)

Cardenolides are present in the exocrine secretion of some chrysome-line beetles, e.g. Oreina and Chrysolina, but these compounds are lacking in their plant food. It has been demonstrated that they are synthesized de novo in the insect body from phytosteroids, present in food. Pasteels et al. (1992) have found that in Oreina cacaliae pyrrolizidine alkaloids in the form of N-oxides, present in the food, are retained and concentrated in the insect body as such, and are translocated to exocrine glands, but in another species of the same genus, Oreina gloriosa, synthesis of cardeno-lides occurs within its body from phytosteroids in the food. Timarcha has a red blood, the composition of which is poorly known. Its hemolymph is generally rich in anthraquinones. No bird or lizard feeds on it, and day living species (Timarcha s.str. and Timarchostoma) show an abundant reflex bleeding through mouth or legs. Kids in France and elsewhere play with them; they take them in their hand and say: "Give me your blood, and I'll give you my white wine". There are several nursery rhymes in Western Europe about bloody-nose beetles. Their only enemies are parasitoids (Hymenoptera and Diptera). They have also protozoans in their gut, but those gregarines are harmless and are only commensals. The nocturnal species (Metallotimarcha and Americanotimarcha), having practically no enemies, do not show any clear reflex bleeding. Timarcha blood is extremely toxic and a small dose can kill a dog. Timarcha remains toxic with anthraquinones, when feeding on Rubiaceae or on Plantaginaceae. Nobody has yet tested the toxicity of the nocturnal species feeding sometimes on Rubiaceae (Metallotimarcha), and also on Ericaceae. Being nocturnal, they don't need so much toxicity in their blood. The American species, feeding exclusively on Ericaceae and Rosaceae, do not show any reflex bleeding. Metallotimarcha however shows scanty bleeding on being disturbed.

Some Central American moths, like Utetheisa ornatrix, studied by Eisner (2003), emit, from thoracic glands, froth in response to a disturbance. Cells from their blood are present in the froth, as in Timarcha, which frequently has blood cells in the ejected blood. One of us (PJ), with colleagues from STRI, has witnessed another member of Arctiidae, Pericopinae, in Panama mountains, a Hypocrita sp. (John Heppner det.), which projects a long cylinder of solid paste instead of froth. PJ has named it as the tooth paste moth. The observation is still unpublished. This moth feeds mostly on Crotalaria, a rather toxic plant. J.-M. Maes, an entomologist in Nicaragua, has told me (PJ) that some Hypsidae, Chetone angulosa, which is a mimic of a Heliconiid and an Ithomiid, and Phaloesia socia and some others produce a smelly yellow froth from their thorax; it is an effective repellent against birds, lizards and some other predators. Bubbled mass producing is frequent among beautiful acridians in Africa, like Zonocerus elegans, feeding on Calotropis and other toxic milkweeds (Asclepiadaceae). Perhaps it will be interesting to mention here similar and strange phenomena among some other insects. Pyrgomorphidae locusts are mute, as they lack stridulatory or rubbing sound producing apparatus, but their evil-tasting and foul-smelling secretions, mixed with air, render them very poisonous. Often they are short-winged, but long-winged forms are also found. Aposematic yellow aphids are often seen with them on Caloiropis. Ejection of blood in reflex bleeding was named auiohaemorrhage and ejection of blood with air — haemaphrorrhea. The first term came from Hollande, a worker on Timarcha, and the second from Grasse, who studied Zonocerus.

Many beetles, belonging to the leaf beetle subfamily Galerucinae, feed on plants of Cucurbitaceae. They get with their food the compound cucurbit-acin-B. This toxic compound gets conjugated with some smaller molecules, and gets stored in the body of the beetle in a concentrated form. As has been pointed out earlier, the general predator mantid avoids attacking those galerucine beetles, which have fed on cucurbitacin containing diet.

Some insects, due to a small mutational change in their genic set, are able to attack even plants with toxins in their sap. Along with their plant food, the toxins enter their body. They not only tolerate the toxins, but also use them for their defence. This situation is well illustrated in a meticulous study by Labeyrie and Dobler (2004). They have worked on the genus Chrysochus (chrysomelid subfamily Eumolpinae). Two species of this genus, Chrysochus auraius and Chrysochus cobaliinus, feed on plants containing the toxic cardenolides. All other species of the genus feed on plants without cardenolides. The authors have analysed DNA of the cardenolides feeding and cardenolides rejecting species, and have found only one small difference between the two. They have noted that all the species of the latter category have at the position 122 the sequence for the amino acid, asparagine, but cardenolides feeding Chrysochus auraius and Chrysochus cobaliinus have at this position the sequence for another amino acid, histidine. Thus, just substitution of one amino acid with another not only removes sensitivity for cardenolides but also provides for defence preparedness.

Eisner (2003) in his very interesting book, "For Love of Insects" has attempted to answer the question how insects keep themselves from suffering the effects of the compounds they deploy for their defence, and in the study of Labeyrie and Dobler there is a notable answer to this question.

Another mechanism used by many beetles feeding on latex plants, like Asclepiadaceae, Euphorbiaceae and others, having a white latex (containing cardiac glucosides in Calotropis and other milkweeds, and diterpenes in Euphorbiaceae) is to lightly cut those parts of leaf which contain veins. They slow down that way the latex flow. The mechanism is very efficient and is used, for instance, by many eumolpines and chrysomelines (Chrysomelidae) and some caterpillars. Platycorinus sp. (Chrysomelidae, Eumolpinae), for instance, feeding on Calotropis procera, an Asclepiad, avoids this way getting trapped in the elastic latex. In a similar way Chrysochus, Labidomera and many other leaf-beetles are able to live on their latex producing host plants ( Jolivet & Verma, 2002).

Spiders are as a rule carnivorous and predaceous. Many, as Nephila clavipes in Brazil, encircle tightly their preys with silk threads. They do it immediately after the prey is caught. Some of the caught insects are toxic. When the predator realizes the toxic property of the prey, at least some spiders, like the Nephila spp., are known to break the silken net and release the beetle or the moth. If it is a moth, it can fly away immediately. Unpalatable butterflies stay motionless when entangled and while the spiders release them. Remaining motionless in webs seems to be a prerequisite to allow recognition of their distastefulness and to escape from getting bitten by the spider. Warning coloration, however, does not produce spider's release response. Spiders reject a toxic prey, but do not spare their palatable mimics. Distastefulness is probably signalled to the Nephila by chemical clues. This behaviour was discovered independently by Joao Vasconcellos-Neto and Thomas Lewinsohn (1984) in Brazil and at some other locations by Thomas Eisner (2003), who also describes in his book how sometimes the spider's preys escape from the web by themselves. For this defensive value of insect toxicity see also Jolivet (1991). Let us also note that certain tipulids, mentioned by Etienne Rabaud, because of their so-called inefficient long legs, succeed to rest with impunity over some spider's webs. A web-building spider, Nephila edulis, in Australia, attracts prey, the sheep blowfly, Luciia cuprina, by storing decaying matter in its net. It incorporates into its web a band of decaying animal and plant matter and it replenishes the debris to maintain its efficacy for attracting prey (Bjorkman-Chiswell et al, 2004). See Chapter 11, "Insects and Tools".

Spiders seem very clever, but sometimes they are deceived by predators which are cleverer than they. On the evening that it will kill its orb-weaving spider host, the larva of the ichneumonid wasp Hymenoepimecis sp. induces the spider, Plesiometa argyra, in Central America, to build a unique cocoon web to serve as a durable support for the wasp larval and pupal cocoon (Eberhard, 2000). Many parasites manipulate their host's behaviour (Jolivet, 1998), but this case, described by Eberhard, is probably the most remarkable alteration in the host behaviour ever attributed to an insect parasitoid.

— Fig. 8.1. Colorado potato beetle (Leptinotarsa decemlineata) in a lateral view. Legs and other appendages not included. Areas, covered with fine dots, denote channels and other depressed areas, and bigger dots openings of exocrine glands (based on Pasteels et al, 1989).

cuticle- _

opening of the gland /

. integument

s epidermis

epidermal cells dilated with stored secretion

— Fig. 8.2. An exocrine gland in the tegument of the Colorado potato beetle (Leptinotarsa decemlineata) in a vertical section (after Pasteels et al., 1989).

— Fig. 8.3. Copulation in Nephila maculata (Argiopidae). The small male is placed near the genital opening of the female.

— Fig. 8.4. and Fig. 8.5. Phaloesia saucia (Pericopinae), a moth emitting its repellent frost in Nicaragua (photo J.-M. Maes).


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