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As young larvae of the two-spotted 

 ladybird disperse from their egg batch to 

 seek aphids, they leave behind a jumble of 

 egg debris. The yellowish remains are 

 unhatched eggs that have been 

 cannibalized. 



Michael Majerus 



terium may help insure the continued sur- 

 vival of its relatives, all geneticaUy identi- 

 cal to itself. 



Ladybirds lay clutches of about fifteen 

 eggs, which hatch over a period of two to 

 three hours. Any eggs that have not 

 hatched within this time are eaten by sib- 

 hngs that emerged earUer; 5 to 10 percent 

 of all ladybirds may die this way. The bac- 

 terium's actions reduce this cannibahsm, 

 at least on female embryos. Male embryos 

 killed by the bacterium no longer pose any 

 threat as cannibals and instead serve as 

 food for their female sibhngs. They may 

 even provide some protection for late- 

 hatching females, which are less Ukely to 

 be cannibalized when there are so many 

 perfectly nutritious dead male eggs lying 

 around for the taking. And anything that 

 increases the survival of females — which, 

 unUke males, can transmit cytoplasmic 



material — is, of course, also good for the 

 bacterium. 



The death of male ladybird embryos 

 may have other, even more potent effects 

 on female survival, however. Sibling egg 

 cannibalism is common in ladybirds, 

 probably because getting a meal early on 

 in life greatly increases the likeUhood that 

 a ladybird larva wiU survive. The larvae 

 are small (no more than 2 mm long) and 

 bom with scant energy reserves; without 

 food, they will not five much more than a 

 day. Newly hatched larvae feed on aphids, 

 which they search out primarily by touch. 

 If they do happen to bump into an aphid, 

 their chances of capturing it are poor. The 

 aphids are two to three times their size and 

 have several defense mechanisms: they 

 may kick the larvae away, run away them- 

 selves, or drop off the host plant to avoid 

 capture. Many larvae die without obtain- 

 ing their first aphid meal. 



Cannibalism boosts energy reserves at 

 this vulnerable stage, and any larva that 

 gets a highly nutritious egg meal is more 

 likely to last long enough to catch that 

 first, crucial aphid. Having lots of dead 

 male embryos around is an additional ad- 

 vantage; in such clutches, every female 

 has, on average, at least one dead brother 

 to feed on. Again, the dead males' sisters 

 are not the only ones to benefit; the set-up 



is also advantageous to the bacterium hv- 

 ing in them, for with the death of the 

 males, the survival and propagation of the 

 bacterium depends totdly on that of the 

 female ladybirds. 



The bacterium thus seems to have 

 worked matters out quite nicely: by killing 

 males, the sex through which it cannot be 

 inherited, it enhances the survival of fe- 

 males, the sex through which it can. But 

 why has the ladybird beede not died out 

 for lack of males? After all, while the fe- 

 male beeties may not need many males to 

 reproduce, they cannot do without them 

 entirely. 



As it turns out, uninfected individuals 

 are being produced all the time, for a cou- 

 ple of reasons. For one, the bacterium is 

 not perfectly transmitted from generation 

 to generation: 10 percent of daughters are 

 free of the infection. For another, a bac- 

 terium residing within a female inevitably 

 uses some of its host food reserves for its 

 own metabolism, thus reducing her fecun- 

 dity and longevity. This, too, slows down 

 the rate of bacterial transmission. Bad for 

 the bacterium in the short term, this imper- 

 fect ttansmission is necessary for its sur- 

 vival over the long term. A completely 

 successful bacterium, like a predator that 

 wipes out all of its prey, would be doomed 

 to follow its host to extinction. 



34 Natural History 6/94 



