Saturday, June 4, 2011
Dueling Fabricii: Two Views on Complex Lifecycles
Johan ChristianFabricius. The aphid’s name first appeared in his Systema entomologiæ (1775) and the beetle originally in his Systema eleutheratorum (1801) as Crioceris vittata.
Fabricius’ ‘System of the Beetles’ (available at Google Books) used Eleutherata for the order of beetles that we now call Coleoptera. I’m not sure why his named didn’t stick – the rules of nomenclature do not apply above the level of superfamily – but he was unlucky with his Crioceris vittata too – the species name was preoccupied and its replacement has wandered through several genera before settling in Phyllotreta. However, Fabricius was able to propose a replacement name in 1803, and so the correct name for the Striped Flea Beetle is Phyllotreta striolata (Fabricius, 1803) (not 1801, as in some references). (The parentheses around the author’s name indicate that the species originally resided in another genus.)
Thanks to some interesting archeological work excavating a buried privy in Boston, we know that the Striped Flea Beetle has been falling into outhouses in North America at least since 1775 or so (Bain & LeSage 1998). Why are there so many in my backyard? Well, my guess is two-fold: polyphagy and monoculture. The Striped Flea Beetle feeds on a variety of plants in the mustard family (Brassicaceae aka Cruciferae) and so can take advantage of many weedy mustards and the numerous backyard crucifers: radish, turnip, cabbage and similar crops. Also, the Striped Flea Beetle is a major pest in canola in the cooler parts of Alberta (another flea beetle, Phyllotreta cruciferae (Goeze, 1777), is more important further south) (see Dosdall & Mason 2010). Yellow canola fields surround Edmonton in the summer. All of this adds up to a monstrous number of flea beetles.
Adult flea beetles overwinter in the soil and emerge ravenous in the spring just as the first radish seedlings are germinating. The tiny radish seedlings – just a pair of cotyledons – don’t last long. In contrast, a horseradish that came with the HBG produces large leaves quickly from its perennial taproot and can tolerate a massive amount of damage from its host specialist flea beetle. If I can get the radishes past the cotyledon stage, then I have a hope, but I've failed too many times. I’m trialing turnips this year – planted late and so far not showing much of the typical shot-hole damage. Flea beetles have complex metamorphosis: their grubs live in the soil and feed on roots of the same plants during the summer. The grubs are not considered major pests and for crops like radish and turnip that are soon harvested, they would be trivial. So, if you can beat the adults, you can grow radishes. We will see about turnips.
Strangely, the Black Cherry Aphid also likes crucifers. You would think that a cherry aphid would stick to cherries, but newly emerged cherry leaves and buds is only their spring food. Although aphids have gradual metamorphosis – their young look more or less like the adults as you can see from the heading picture – they often have a complex life cycle with various winged and apterous forms on two or more host plants. A small dictionary of jargony names been coined for these forms and hosts.
I’m not a specialist in these bugs, but I’m guessing that the fat black aphids are the fundatrix forms. If so, each developed from a fertilized egg that overwintered on the cherry and are now producing live young parthenogenetically (a long word for without sex). A fundatrix is not going anywhere, nor are her immediate offspring, so they don’t waste any energy producing wings (or males). During the summer, however, winged female aphids develop and disperse away from cherry and on to weedy crucifers or cleavers where they again crank out young without stopping to fool around. In late summer, winged males and females are produced, get it on, and eventually the females leave fertilized eggs on the cherry to spend the winter. This seems to me a complicated way to get from one year to the next, but perhaps all the natural enemies of aphids, the lady bird beetles, lacewings, hoverfly larvae, midge larvae, parasitoid wasps, hunting wasps, gardeners, and assorted diseases, make it worthwhile.
For more information on flea beetles and crucifers see:
Bain, A & LeSage, L (1998) A late seventeenth century occurrence of Phyllotreta striolata (Coleoptera : Chrysomelidae) in North America. Canadian Entomologist 130, 715-719.
Dosdall, Lloyd M. & Mason, Peter G. (2010) Key Pests and Parasitoids of Oilseed Rape or Canola in North America and the Importance of Parasitoids in Integrated Management. pp. 167-213 in Biocontrol-Based Integrated Management of Oilseed Rape Pests.
And for an interesting recent paper on the complex interactions between landscape factors, parasites and predators, ants, and Black Cherry Aphid see:
Stutz, S & Entling, MH (2011) Effects of the landscape context on aphid-ant-predator interactions on cherry trees. Biological Control 57, 37–43.