While I am a big fan of spiders year-round, now is the time of year when I truly appreciate their presence in my yard as they stalk their herbivorous insect prey. One of the most beautiful spiders I recently found hanging out on the milkweed was the goldenrod crab spider (aka the flower crab spider, or scientific name Misumena vatia).
Even though the spider I found the other day was creamy white, I like to use the common name of goldenrod crab spider because this is how I know them best — as bright yellow amongst the goldenrods. Goldenrod is just starting to bloom in my area, but once you see some flowers, look carefully at them. Bright yellow goldenrod crab spiders are often lurking among the flowers, waiting to ambush prey.
These spiders have amazing color-change abilities. They change color to blend in with the flower from which they ambush their prey, ranging in color from yellow to white, often with red patches on the sides of their abdomen. The colors are thought to provide camouflage. The spiders can sense the background color and change from white to yellow and back. It takes about six days for the spider to change color from yellow to white, but 10–25 days to change from white to yellow. The length of time differs because, to turn yellow, the spider must produce and then distribute the yellow pigment. To turn back to white (the base color) the spider simply excretes the yellow pigment.
Goldenrod crab spiders look a little like miniature crabs, hence the name, and sometimes hold their large front legs curved in front of their heads in a crab-like pose. Goldenrod crab spiders are ambush predators, meaning they don’t spin webs to capture prey. Instead, they lie in wait on flowers. When a bee, fly, or other pollinating insect lands on the flower seeking nectar, the crab spider attacks, grabbing the prey, injecting venom (fatal to insects, numbing them within seconds) and then drinking its juices.
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Every goldenrod crab spider I have identified has been female — this is because goldenrod crab spiders exhibit sexual dimorphism, I can recognize a female, but not the males. Sexual dimorphism is when sexes of the same species exhibit different physical traits. Females are larger and easier to recognize than males. The male has a brown thorax and front legs with only a small white or yellow abdomen. In addition, the females hang out on or near whatever flowering plant they have staked out (rarely traveling more than 3 feet or so from home base) whereas the males wander looking for mates.
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It might be tempting to cast goldenrod crab spiders as “bad” because they prey upon pollinators. But while they do eat a fair number of bees, they also prey upon insects that eat flowers and plants, like grasshoppers and aphids. As part of our backyard ecosystems, like other predator species, goldenrod crab spiders are keeping prey populations in check.
According to Jeff Mittin (Department of Ecology and Evolutionary Biology at the University of Colorado, Boulder Camera 2012): “Experimental studies showed that plants with resident crab spiders had fewer herbivores chewing on them — that was certainly a benefit. Plants with no damage to their flower heads produced similar numbers of seeds, whether they had spiders or not. However, plants with damaged seed heads produced more seeds if they hosted spiders. So crab spiders reduce the numbers of herbivores and in some circumstances actually increase the production of seeds.”
So, look for and welcome these charismatic spiders to your backyard − check the milkweed and other flowering plants. But for a real show, as we head into late summer and fall, keep an eye on your goldenrod. If you are lucky, you will find a bright yellow goldenrod crab spider lying in wait.
Susan Pike, a researcher and an environmental sciences and biology teacher at Dover High School, welcomes your ideas for future column topics. Send your photos and observations to email@example.com. Read more of her Nature News columns online at Seacoastonline.com and pikes-hikes.com, and follow her on Instagram @pikeshikes.