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).

It's a fly eat fly world too

Spiders eat spiders.  Spiders eat flies.  So far, no surprise.  But did you know that there are some flies that eat other flies?

A robber fly (Asilidae) with another fly it has just captured.

These predatory flies are commonly --and rather euphemistically -- called "robber flies".  Robber flies (Asilidae) snatch flies and other insects in flight, inject them with a mixture of paralyzing and digesting enzymes, and then consume the liquefied remains.  Thus, they are also referred to by some by the more appropriate name "assassin flies".

Purple insect eater

In most consumer-resource interactions between plants and insects, the plant is the one that gets consumed.  However, a few groups of plants have been able to reverse the relationship.  Insects and other small animals come to these plants for a meal, only to become one themselves.

A purple pitcher plant (Sarracenia purpurea) in bloom.

One of these carnivorous plants, the purple pitcher plant (Sarracenia purpurea), lures visitors onto its leaves with colorful markings and nectar secretions.

Dead insects, including a moth, in a pitcher plant leaf.

Once on a leaf, visitors become trapped due to the leaf's waxy surface, steep angle, and downwards pointing hairs -- and possibly due to being drugged (a narcotic compound has been found in the secretions of a related species, S. flava).  Even flying insects fall into and then drown in the pool of rainwater at the base of the leaf.

Flies and springtails trapped in another leaf.

This water also contains digestive enzymes secreted by the leaf and a community of bacteria, which together break down the victims of the trap.  The leaf can then absorb nutrients that the plant might not otherwise be able to obtain from the poor quality soil in its boggy habitat.

A carabid beetle attempting to escape the pitfall trap.

Know thy host, continued

By now, it should not come as much of a surprise that I go to gardens to look for insects more than to look at the plants themselves.  The wide variety of plants in gardens attracts a diversity of insects, especially when the gardeners prioritize native plants (and refrain from spraying pesticides all over).  On this particular garden trip, I started out by looking for plants with leaf damage and soon came across a patch of plants that appeared to have been hosting a feast.  However, standing above the plants, I did not see any of the feasters; were they still there?  Spotting some frass (insect droppings) on one leaf, I crouched down to inspect the overhanging leaf.

A larva on the underside of a leaf with some frass below.

I had expected to see a caterpillar and what I found was caterpillar-like, but its spiral pose instantly made me suspicious.  When I flipped the leaf over to get a better look and saw its eyes, my suspicion that it was not a caterpillar was confirmed.  (If it had been a caterpillar, I should not have been able to see its real eyes.)  Instead, it was a sawfly -- a relative of wasps and bees.

A sawfly larva (Tenthredo grandis) curled up in a characteristic pose.

Once I had found the first one, it became easier to find even more of the sawfly larvae.  From one of these that wasn't curled up, I could see another distinctive difference: the larva had too many legs to be a caterpillar.  (Caterpillars have five or fewer fleshy "prolegs" after the first three jointed, true legs; sawfly larvae have six or more prolegs.)

Another sawfly larva (Tenthredo grandis) on a nearby leaf.

Recognizing the larvae as sawflies was an important step towards identifying their species.  Identifying their host plant would expedite the rest of the task.  Luckily, identifying this host plant as white turtlehead (Chelone glabra) only required some simple inference.

There was no need to work through a key to identify this plant!

Explore some more: Wild Gardens of Acadia and Lepidopteran vs Sawfly Larvae

Know thy host

Picking the right spot to lay eggs is a challenge shared by many insects.  While most adult insects can fly, immature insects tend to be more limited in their movement.  Therefore, in addition to the safety of her offspring, the mother must ensure the availability of the right food -- and in adequate amounts.

A common aspen leaf miner (Phyllocnistis populiella) about to run out of leaf to eat.

Perhaps the mother of the leaf miner larva above should have chosen a bigger leaf.  Yet, young (and thus small) leaves may confer their own advantages, including being more tender and consequently easier to eat.  Indeed, young leaves are often preferred by a variety of herbivores.

On a side note, how does one identify an insect that is still inside a leaf?  The trick is to know the leaf.  Since leaf miners tend to be specialized on one or a small number of plant species, identifying the plant can lead to the identity of the leaf miner.  For example, identifying the leaf above as belonging to a quaking aspen (Populus tremuloides) quickly led to the identification of the larva inside.  Even when more than one leaf miner species uses a plant, the leaf miners will usually differ in the type of mine they create (see some other examples here and here).

Pricked by the thorn, continued

As adults, treehoppers are not particularly vulnerable.  They are protected, and sometimes camouflaged, by hard, helmet-like coverings.  True to their "hopper" name, they are also capable of launching themselves away from approaching danger.  However, as eggs and nymphs, treehoppers are less self-sufficient.  In order to increase the safety of their offspring, females of the species Publilia concava remain with their eggs after laying them.

A treehopper (Publilia concava) and her eggs, which are embedded in a late goldenrod (Solidago altissima) leaf vein.

By standing over their eggs, the treehoppers can not only guard the eggs, but also recruit additional protection -- in the form of ants.  In fact, the treehoppers do not simply wait for ants to find them; the presence of ants is an important factor in the females' choice of where to lay their eggs.

European fire ants (Myrmica rubra) tending the treehopper.

Like many other insects that feed on sap, this treehopper excretes excess sap as a sugary "honeydew" that is collected by ants to feed their colonies.  Importantly for the honeydew producers, the ants will also aggressively defend their food source.

Field ants (Formica sp.) tending treehopper (Publilia concava) adults and nymphs on a roadside goldenrod.

Explore some more: Ant-dependent oviposition in the membracid Publilia concava

Pricked by the thorn

Some insects can blend into the background, but only if they pick the right spot.  For the numerous true bugs (Hemiptera) that spend nearly all their time sitting on plants and drinking sap, the best camouflage could be to look like a part of the plant.

A leafhopper nymph (possibly Xerophloea sp.) sitting at a node along a plant stem.

At first glance, I mistook the leafhopper above for a new bud.  Perhaps a potential predator would have been similarly deceived.  However, there was no chance of making the same mistake with this bug:

One of these thorns is not like the others.

The two-horned treehopper (Ceresa diceros) might pass for a thorn on some plants -- but not on this rose bush.  On the other hand, the horns (and the rest of the armor-like "helmet") might be enough to discourage many would-be predators.

The false thorn on this rose bush is a two-horned treehopper (Ceresa diceros).

Explore some more: The surreal treehoppers and The strange origin of the treehopper "helmet"

Labor Day Special

In celebration of Labor Day, I'm posting pictures but leaving the rest of the work to you: find the insects hiding in plain sight!

If you couldn't find the insect camouflaged against the lichen on the rock, try again with this closer view.

If you thought finding the insect above was too easy, here is more of a challenge.  

Could you find the insect sitting on the leaves?  Here is a close up that might help.

If you still want help finding the insects, or if you would like to know more about them, check the Natural Current Events Facebook page later this week.