Science World Blog Who Spat in my Garden?
Who Spat in my Garden?
Last Updated (Monday, 13 June 2011 21:57) Written by Raymond Nakamura
I was hacking back the plants in my garden when I noticed what looked like foamy spit on some leaves. I'd seen this before, and had some vague sense that insects or spiders had something to do with it. I decided to poke around a bit.
Hands On
The "spit" consisted of small, fairly uniform bubbles. I wiped up with my finger. It didn't feel especially slimy or smelly. On the leaf creeped a small (maybe 2 mm) green critter, which I presumed was the culprit. After washing my hands, I looked for some insights into these insects.
Who Dunnit?
It was an aptly named spittlebug. Spittlebugs come in hundreds of species, members of the order Homoptera, which includes aphids. The adult females lay eggs in the late summer and nymphs hatch in the spring. They have piercing mouthparts which they stick into the plant to drink the sap. With the leftovers, they blow bubbles out their behinds.
Why Do They Make Spit?
One explanation is that the foam provides a stable, moist environment, which is useful because spittlebug nymphs have a thin cuticle. Another explanation is that the foam hides spittlebugs from predators. It's also supposed to have an acrid taste, but I did not test this myself. One source, however, suggests they do not cover themselves with foam right away.
How do They Make Spit?
One description said spiracles pump air into the liquid as it is being expelled. Another said the liquid traps air as it passes over the abdomen of the spittlebug. I think the uniformity of the bubbles suggests a more controlled process. I wonder if anyone has compared the composition of the air in the bubbles compared to the ambient air.
One Born Every Minute
Most plant suckers go for the phloem, which carries the fluids from the leaves which produce the sugars through photosynthesis, through the rest of the plant. Spittlebugs, however, go for the xylem, which carries water and some minerals from the roots up to the leaves. The xylem is usually deeper, so I would expect that spittlebugs should have longer mouthparts. They usually feed with their head facing downward. Maybe this helps them intercept the flow better.
What's in This Drink?
Some have suggested xylem is low in nutrients and so the spittlebugs need to drink a lot of it. This might be consistent with their need for all that bubble solution.
But maple syrup comes from the xylem, so I wonder if spittlebugs are tapping into stored nutrients being redistributed in the spring. Spittlebugs survive better on plants with more amino acids in the sap. They are more likely to go for legumes which have nitrogen fixing bacteria in the roots. I found the spittlebugs on a butterfly bush that had been growing like crazy. I do add compost, so the spittlebugs may have appreciated the general fertility of the soil. Some spittlebugs are considered pests, but most gardening sources suggested they don't stay that long and the plants usually survive.
References
What is the Spit Looking Substance on My Grass?