The insect that we are introducing today is not really from North Carolina. Not even close. The Coffee Berry Borer, Hypothenemus hampei, is a minute (1 mm) bark beetle (Coleoptera: Curculionidae: Scolytinae) which originated in the highlands of central Africa. It would probably remain one of the many inconspicuous and inconsequential little beetles in the tropics, if not for the lucky choice of its host: seeds of several Coffea.
The Coffee Berry Borer, Hypothenemus hampei, on a fresh coffee bean. © USDA
You can imagine that evolution smiled at the beetle when the joy of coffee was discovered by humans, and the beetle’s host became planted on a massive scale worldwide. The Coffee Berry Borer invaded essentially every coffee-growing region in the world, and by destroying coffee beans before the farmers get to them, it’s causing up to $500,000,000 damage annually.
The Coffee Berry Borer is undoubtedly a bad news for the world’s coffee farmers and coffee lovers, but for a biologist, it is a delightfully bizarre creature. For one, its sexual life is entirely a family affair. The beetle is so small, that a whole family develops in a single coffee bean. The beetle mother creates a small cave (“gallery”) in the bean and lays eggs. Most of the eggs are fertilized and diploid, just like in any properly reproducing animal. But one egg is haploid. From this egg, the mother somehow eliminated the father’s genetic contribution. We don’t know how that happens, and it’s possible that it is facilitated by the symbiotic bacteria Wollbachia, which are ubiquitous in the reproductive tract of many insects. This one haploid egg produces a male larva, while all the other diploid eggs produce females. When mature, the male is dwarfed, flightless, barely walking. Its only goal in life is to fertilize all the females in the gallery, his sisters. The male then usually dies in the bean, and it’s the sisters, much stronger and with wings, that fly out into the world to find their own coffee beans. This reproductive system is called arrhenotokous parthenogenesis, and it differs from the haplo-diploidy that we see in ants, bees and some other bark beetles. In these, the male eggs remain haploid because the never got fertilized, not because the father’s genome is eliminated.
Notice that despite its classification as a “bark beetle”, the Coffee Berry Borer actually lives in seeds. This is a good example of the tremendous diversity of life strategies evolved in the weevils subfamily Scolytinae, the bark beetles. While many of the 6000 species do live in bark and phloem of trees, hundreds of others are seed specialists, or live inside herbs, and nearly half are fungus farmers, the so called ambrosia beetles.
It is a good habit of any scolytine species to be accompanied by fungal symbionts. So is the Coffee Berry Borer frequently associated with a species of Fusarium, and several yeasts. It is not clear if these are mutualistic symbionts, supplementing the beetle’s nutrition, or just commensals, taking advantage of the beetle transport between beans. It was hypothesized that the fungi might help the beetle mitigate the threat of caffeine, but this was recently found not true – the beetle does not live in decaf beans.
There is another good reason why the insect of the week is the Coffee Berry Borer. On April 15th, the seminar speaker in the NCSU Biology Department is the world’s guru on the Coffee Berry Borer’s biology and management, Dr. Fernando Vega. Fernando works with the USDA in Washington, DC, and discovered most of what we know about the fascinating beetle. Please join us on Thursday, April 14th at 3:30 in the main lecture room in the David Clark Labs to learn more about “The Coffee Berry Borer: a threat to our morning brew.”