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Ludwigia octovalvis (and Eumorpha fasciata)

(Ludwigia commemorates 18^th Century German botanist Christian Ludwig.  Octovalvis indicates the fruit opens along 8 lines.)

Onagraceae (and Sphingidae) 

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The approximately 30 species of primrosewillows (Ludwigia) in Florida could keep you busy awhile.   Most but not all have bright yellow flowers, but some have no petals at all.  They range from the large invasive exotic Peruvian primrosewillow rising way taller than a human from roadside ditches  to itsy bitsy mud creepers. 

Today’s species is large and showy, sunny yellow and often in large “decorative” stands in wet habitats.  Bees love it.

Mexican primrosewillow differs most conspicuously from the Peruvian PRW by having a long  narrow nearly round fruit as opposed to a stubby four-sided fruit.  Mexican PRW is hardly “Mexican,” as it ranges seemingly natively across most of the Southeastern United States, and has become one of the tropical world’s most widespread weeds, most odiously in rice fields.

It and other ludwigias have traditional uses in medicine, not worth listing here.  The interesting thing is that this species and its kin produce linoleic acid, a fatty acid essential in human nutrition and the subject of a considerable literature in that connection.    Who knows what potential lies there.

Ludwigias are preferred, if not exclusive, host plants for the caterpillars of the banded sphinx (hawk) moth, a pollinator of the crinum lilies (and probably also similar Hymenocallis spider lilies) with which the primrosewillow shares marshy habitats.    The caterpillars are astounding in at least two ways, their enormous size (about like your index finger) and their mixed coloration scheme.    Even nibbling together on a single plant there can be camouflage green ones, showy yellow ones, and eye-catching psychedelic siblings. Wow! How? and Why? This is a situation where research and speculation swirl into one. 

Just this summer biologists CL Francois and G Davidowitz studied a related sphinx caterpillar color mix and found the differences to be controlled by a very simple genetic system, perhaps (mainly) just one gene.   This sort of suggests that the colors of the caterpillars more or less to depend on a genetic roll of the dice…sort of like, “will my next grandchild be a boy or a girl?”  (It is a girl.)   Just like a family can have boys and girls in one house, a caterpillar family can have all those different color types on one plant (as well as boys and girls). The researchers suspect that the built-in automatic color mixes give the diverse caterpillars an advantage in a diverse world. Don’t put all your eggs in one color basket.  In any place and time some may become bird food but others may be better hidden, or more conspicuously toxic.   Interesting in this connection that some are showy and others are camouflaged.  

That all is no doubt true, but it gets more complicated.  Ecologist Linda Fink, formerly of UF, has looked into the banded sphinx moth in addition to related species and found that all the color variants appear mixed on any given host plant species, and (here is the surprise), the ratios of the different colors in the mix depend on the plant species.