Winsome invaders, lose some invaders, continued

Despite the superabundance of Japanese beetles (Popillia japonica) in the yard, I have hardly ever seen anything attempt to eat the beetles.  (One attempt, made by an eastern phoebe chick, was fatal for both the beetle and the chick.)  However, while hunting for the beetles, I have occasionally come across ones that were already dead.  In each case, the dead beetle was loosely wrapped in spider silk; in a couple of cases, the spider was still there.

A Japanese beetle captured by a female candy stripe spider (Enoplognatha ovata, ovata color morph).

The spiders making meals out of the Japanese beetles were a type of cobweb spider.  Although they belong to the same family as black widows, they have a much friendlier name: "candy stripe spiders" (Enoplognatha ovata).  Interestingly, this predator of Japanese beetles is itself an introduced species, yet it comes from Europe not Asia.

Winsome invaders, lose some invaders

Of the thousands of invasive Japanese beetles that I see each summer, a small number have white ovals stuck to them.  These white ovals, which are the eggs of the parasitic "winsome fly" (Istocheta aldrichi), are usually attached to a beetle's pronotum (just behind the head) or sometimes to its elytra (the hard coverings over the wings).  Although the eggs do not look like much, they mean that the beetle has only a few more days to live.

Mating Japanese beetles (Popillia japonica); these beetles are unlikely to have any offspring, however, since the female has several winsome fly (Istocheta aldrichi) eggs glued to her.

About a day after the eggs are laid, larvae hatch from them and bore into their beetle host.  After a few more days, the beetle dies, eaten from the inside by the fly larvae.  Like the Japanese beetles themselves, winsome flies were introduced into North America from Japan.  Unlike the Japanese beetles, the flies were introduced intentionally -- in the hope that they would restrict the growth of the Japanese beetle populations.  However, it has now been more than 90 years since the flies were first introduced, and the Japanese beetle remains a growing problem.

Explore Some More: Biological Control of Japanese Beetles

Unafraid to be yellow

Most of the crab spiders that I find are white.  Not only do I find white crab spiders on white flowers, I also find them on pink, purple, red and blue flowers.  The exception is on yellow flowers, where the crab spiders tend to be a matching yellow.  Therefore, when I found a white crab spider hunting on the very yellow flowers of a goldenrod, I decided to keep track of it for the next few days.

Day 1: A white crab spider on yellow goldenrod flowers.

On the first day that I observed it, the white crab spider was doing quite well for itself despite not matching the color of the flowers.  It had caught a large fly, thus procuring for itself a substantial meal.

Day 3: The spider is pale yellow.

Notwithstanding its previous success as a white spider on a yellow flower, the spider had soon acquired a pale yellow hue.  After another two days, the crab spider had acquired both more yellow and another fly.

Day 5: The spider has caught another fly.

How had the crab spider turned yellow?  The color change was first triggered by the crab spider seeing yellow around it.  In response, the crab spider began to synthesize yellow pigments which it then kept in cells near the surface of its body.  (To return to white, the crab spider could move the yellow pigments away from the surface to be stored until needed again.)

Day 6: The spider is the same bright yellow as the golden rod flowers.

Why had the crab spider turned yellow?  Although the answer seems simple -- to hide from potential prey and predators -- studies of crab spiders have failed to find a benefit from either increased prey capture or decreased capture by predators.  Nevertheless, the metabolic cost of synthesizing the yellow pigments suggests that a benefit must exist (or at least one must have existed until relatively recently).