July 11, 2017
We're making great progress on Scattergrass in the Bay Grove. We will continue there this week. And still, there are tons of Sticky Monkey and Farewell to Spring in bloom, and it's already July.
Also blooming now are the Tarweeds. These are members of the Aster (Sunflower) family that are known for the sticky drops of aromatic oil on their leaves and flower buds. The main Tarweed that is blooming right now along the Skyline Trail is the Coast Tarweed, or Madia sativa. These are tall, rank growers who often colonize open or disturbed areas. They are often the first wave of natives in such areas. They often bloom in the evening. Since Tarweeds usually bloom in the warmer months, this seems to be a successful strategy to protect their flowers from being burnt by the hot sun.
Here is Coast Tarweed in full bloom:
(photo by Bill Bouton, via iNaturalist)
Many people have wondered what is the point of the sticky tar? Up till recently, I have responded, "Imagine you are a small plant trying to flower and make seeds. There are many animals, large and small, who would like to eat you, such as deer and rabbits. What can you do to protect yourself - you can't run away?"
(Credit here to John Muir, who said: "Any fool can cut down a tree. They can't run away.")
So one way plants protect themselves against browsers is to make themselves taste bad by developing strong chemicals. Think about Sage, for example. I believe Tarweeds use this strategy, because I rarely see browse damage to mature plants.
However, some insects have become specialists in eating the flowers of Tarweeds. One of them is the Small Owlet Moth, (Heliothodes diminutiva). The adults lay eggs on Tarweed flower buds and the caterpillars then eat the Tarweed flowers and buds. Here's a picture of the caterpillar.
This picture also clearly shows the many droplets of 'tar' at the tips of the bristles on the bud, stem, and leaves. (Thanks to Ken-ichi Ueda for this picture).
Here is a picture of a caterpillar, caught in the act of eating a Tarweed flower bud:
(photo by Sam Beck)
So how do we Tarweed plants protect ourselves against these hungry ones? What if many other insects are also attracted to Tarweeds and the smaller of them, such as aphids or small flies, can get caught in our sticky tar and can't get away? And, what if predator insects are attracted by the dead ones and come to eat them?
Here is just such a predator, the famous or infamous Spine-Collared Assassin Bug (Pselliopus spinicollis):
The assassin bugs use their long proboscis to spear and inject a a lethal saliva into their prey, which also liquifies the insides of the prey so they can be sucked out. This super clear photo (thanks again, Ken-ichi) is taken of one on an Everlasting flower head. Can you see the spiny collar? The proboscis here is tucked under the body.
And here's one, in action, spearing a fruit fly on a Tarweed:
(photo by Sam Beck)
The assassin bugs are just one of several predator bugs who visit Tarweeds. So the next question is would these predators who come to eat the carrion bugs also notice and eat the caterpillars of the Owlet Moth? And would this reduce the numbers of caterpillars enough to make a difference to the Tarweeds in terms of their fertility - making good seeds?
The answer seems to be yes. This was tested by a couple of researchers at UC Davis, who placed five dead fruit flies on selected tarweed plants each week, and then compared the results with 'un-doctored' control plants. They found that the tarweeds with the extra 'food' had significantly more blossoms and increased seed production. Here's the link to a popular summary of their research:
So, we Tarweeds have adapted to protect ourselves by trapping small insects with our tar, which brings in predator bugs who also eat the caterpillars who are eating our flowers.
That's pretty subtle and complicated, and also just amazing.
Take a closer look next time you are out.
PS. Special thanks to Ken-ichi Ueda of iNaturalist for bringing this drama (and wonderful photos) to our attention.