Photo of the Week – July 13, 2018

You know how you can look at something for years and still not see every aspect of it?  I was walking through Lincoln Creek Prairie this week, stretching my legs after photographing for my square meter project, when I came upon a couple big patches of Illinois tick clover (Desmodium illinoense).  There weren’t any active flowers on the plants, and I was about ready to move on after just a quick glance when I spotted something white along one of the stems.  Upon a closer look, I could see it was a moth, and it seemed to be plastered up against the plant.

This white moth seemed to be stuck on the stem of this Illinois tick clover plant.

As I inspected the moth more closely, it was clearly dead, and appeared to be essentially glued to the stem.

Here is the same moth, photographed from a different angle. Here you can see the abdomen stuck to the stem, and the mess it apparently made as it struggled to escape.

Now, I’ve known that tick clover plants (and especially their seeds) can be sticky, but I always ascribed that to the tiny stiff (and sometimes hooked) hairs covering them.  I sure wouldn’t have thought those hairs could catch and hold an insect.  However, as I looked more closely at the hairs on this plant, there were little tiny droplets of clear sticky fluid at the tip of each hair.  How can I have spent 25 years or more looking at prairie plants and not noticed that?  I looked online and in my copy of the Flora of Nebraska book and didn’t find any reference to those droplets in either place, but surely other people know of this.  I’ll have to look harder.  In the meantime…

…as I looked at nearby plants, I saw lots more dead or dying insects glued to them.  The most common of those were lightning bugs, followed by Japanese beetles.

Lightning bugs were the most abundant of the insects I found stuck to tick clover stems. I must have seen at least 20 within a few minutes.

Japanese beetles (invasive species) were also a common victim of the sticky tick clover plants. This one appeared to have become stuck on a leaf, so I guess it’s not just the stems that have adhesive qualities.

I’ve written before about insects getting stuck to the bracts beneath thistle flowers and discussed the possibility that the sticky bracts helped keep ants and other non-flying nectar thieves from stealing floral resources.  Do tick clovers do the same thing?  If so, why haven’t I noticed?

This little gnat (midge? something else? I can’t see the antennae) was the smallest insect victim I found.  If you click on this photo you can zoom in and see the droplets on the tips of the hairs.

This mosquito lost its life when it apparently tried to take a rest break on this tick clover plant.

This picture-winged fly was still struggling when I found it.

I’m really curious to know if others have noticed insects losing their lives to tick clover plants, and whether or not it happens with other Desmodium species.  Does the plant produce the sticky droplets of liquid throughout its growth period, or just when it is flowering?  WHAT IS GOING ON HERE?

Thanks for any help.

Photo of the Week – August 1, 2013

I first noticed the dead ants about a year ago, when James Trager was visiting our Platte River Prairies to help us see our grassland through the eyes of ants.  As we poked around the prairie together we kept seeing ants that were stuck and dying on the flowers of two native thistle species (Flodman’s thistle – Cirsium flodmanii and Wavyleaf thistle – Cirsium undulatum).

This ant got stuck on the lower portion of this Flodman's thistle flower and eventually died there.  Helzer family prairie - south of Aurora, Nebraska.

This ant got stuck on the lower portion of this Flodman’s thistle flower and eventually died there. Helzer family prairie – south of Aurora, Nebraska.

The ants didn’t appear to be getting stuck on the numerous spines on the flowers.  Instead, they were getting caught by the flypaper-like stickiness of the lower portion of the flower as they tried to crawl up toward the nectar and pollen at the top.  While ants are definitely the most common victim of these flower traps, I’ve also seen bees, flies, and a few crab spiders get caught as well.

A small bee is among the victims on this particular flower.  The red ant on the left was still alive when I photographed it.  Flodman's thistle at the Helzer family prairie.

A small bee is among the victims on this particular flower. The red ant on the left was still alive when I photographed it. Flodman’s thistle at the Helzer family prairie.

It’s interesting to speculate whether or not ant trapping is a strategic adaptation the flowers have developed over time or just an unlucky accident for the ants.  Ants are not particularly effective pollinators, so it would make sense for thistles to evolve a strategy that helps prevent ants from “stealing” their pollen and nectar.  Since more effective pollinators such as bees and flies come to the flowers by air, stickiness of the lower portions of flowers wouldn’t affect them much (the above photo notwithstanding).  Ants have to get to the flowers by crawling, so a sticky zone between the ground and the pollen might be a very effective strategy for ant prevention.

This thistle "caught" three ants.

This thistle “caught” three ants.

The stickiness of thistle flowers is an interesting counterpoint to the extrafloral nectar produced by plant species such as sunflowers.  Sunflowers seem to produce that substance to ATTRACT ants – presumably to help ward off potential herbivores.  Thistles may be producing a substance to keep ants AWAY.  Very interesting.

Or it could just be a complete accident – which is not a very good story, and sort of sad.

(I vote for the defense barrier theory.)