Golden Tortoise Beetle

Order: Coleoptera
Family:Chrysomelidae (Leaf Beetles)
Species: Metriona bicolor

In these monthly articles, we discuss the good, the bad and the bugly. Most bugs are just bugly; they do what insect do, with little direct effect on our daily lives. This month’s insect is sometimes a pest when it eats sweet potatoes and sometimes beneficial when it feeds on bindweed, but it’s always a curiosity. It’s my favorite beetle in Whatcom County and it’s a treat to catch a glimpse of it. So this month’s article is more about appreciation than education.

The golden tortoise beetle is a very attractive beetle. Adults are about ¼ inch long, circular in shape with a flattening ridge outlining the body, concealing the head and legs much like a tortoise. What is striking about the adult golden tortoise beetle is the color. In spring and summer, the beetles earn their name when they turn the color of brilliant liquid gold. But capture one and the gold vanishes and the beetle becomes dirt brown. Here’s why: the color is produced by an optical illusion; the outer cuticle is transparent and reflects light through a layer of liquid over the next layer of cuticle. The beetles change color depending on the availability of the liquid layer. In the fall and winter, the beetles become less lustrous and are more orange and bronze with flashes of iridescent color. If you try and collect the beetle for an insect collection, the beetle soon turns dark brown as is dries, loosing the golden color. The beetles are most beautiful left alive.

The larvae hatch out in late May and June and are just as intriguing as the adults, but in a very different way. The young larvae are very small with many protuberances outlining their bodies giving them a ‘frilly’ and spiny appearance. As the larvae molts, it keeps its old skin attached to a fork-like structure hinged to its rear end. The larvae will also add feces to the cast skin causing it to appear as a black mess. A potential predator could mistake the would-be snack for bird droppings or some sort of crud. But wait, if that’s not weird enough, the larvae can operate their ‘shield’ as a defense mechanism. When they are disturbed by another insect or a curious gardener, they flip the crusty shield up in the direction of the disturbance. This ‘fecal shield’ turns out to be an unappetizing effective deterrent to predators. Who would want to eat that!

The golden tortoise beetle produces one generation per year and spends the winter as an adult beetle in protected plant debris. When warmer temperatures arise and plants begin to grow, the adults forage for food. The golden tortoise beetle feeds only on plants in the Convolvulaceae family. As beetles become nourished in the spring, their lustrous color appears and they begin to mate. One could imagine that the color is used for mate attraction since it is unlikely that they turn that color to put on a show for me! Eggs are laid on host plants in May and June. They hatch within two weeks depending on the weather. The larvae begin to feed and develop through July. Once the larvae mature, they will adhere themselves to a leaf and pupate, much like lady bugs do. The next generation of adults emerges in late summer and early fall to start the whole cycle again.

The golden tortoise beetle is not a local native, but no one knows when, where or who introduced them to the Pacific Northwest. In Whatcom County, I have only collected the beetles on hedge bindweed, Calystegia sepium. The golden tortoise beetle feeds on many species in the family Convolvulaceae, such as morning glories and sweet potatoes. Both the adults and larvae feed on foliage. Adult feeding damage looks much like multiple shot holes in the leaf. Young larvae tend to skeletonize the leaves at first but will produce larger shot holes in the leaf as they grow.

Since the golden tortoise beetle eats bindweed, will it control it in my garden? Not reliably, nor effectively. Bindweed is such a vigorous grower that the feeding damage caused by the beetles is marginal in impacting the over all plant health. However, during my first summer in Whatcom County, I collected golden tortoise beetles like crazy and released them into bindweed blanketing my yard. I began collecting in early spring when I first noticed the adults. By June, both the adults and the voracious larvae were at work on the bindweed foliage. That year, we had such extremely hot temperatures in late June that the bindweed collapsed before going to seed. Perhaps the extensive feeding by the adults assisted the collapsing of the vines. It sure seemed like they did contribute to the bindweed’s demise because bindweed still proliferated in nearby yards. However, the bindweed came back in its usual tenacious might the next year, barely dented by the previous year’s dieback.

Whether you want to try and control your own bindweed or you just want to enjoy the beautiful beetles in your yard, go ahead and grab infested leaves when you pass them around town. I bet you’ll end up like me and feeling those mixed feelings of dread and excitement when the bindweed begins to sprout every spring.

 

To reach Todd Murray please call (360) 676-6736 or tmurray@wsu.edu