The host with the most, part one

The bright, multicolored flowers of lantanas attract many insects to feed on the nectar they dispense.

A gulf fritillary (Agraulis vanillae) visiting lantana flowers.

However, a number of other insects specialize on extracting nutrients from lantanas in other, more destructive ways.  Recently, I observed many lantana leaves that had been hollowed out, particularly along and around the veins.

A mined lantana leaf.

Although I did not see any of the leaf miners themselves, the pattern of damage was characteristic of the herringbone leaf-mining fly.

More lantana leaves mined by the larvae of the herringbone leaf-mining fly (Ophiomyia camarae)

Meanwhile, other lantanas were suffering from an even stranger affliction -- they had thick growths of miniature stems and leaves where they should have had flowers and fruits.

Lantana flower gall mites (Aceria lantanae) made this plant produce tiny stems and leaves in place of flowers.

In this case, bud mites were the culprits.  The mites had manipulated the plants into switching from producing flowers to growing more vegetative tissue, which had developed into galls enclosing the mites.

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).

The secret to staying green

While admiring the fall foliage on the crabapple tree, I noticed that some of the leaves still had patches of green, whereas others had turned a uniform yellow.

A crabapple leaf with three green patches.

When I flipped these leaves over, I found a clue to why the patches were staying green.  Wherever there was a green patch on the top of the leaf, there was a matching mine of a tentiform leaf miner on the underside of the leaf.

Three tentiform leaf miner mines on the other side of the crabapple leaf.

Below is a close-up of another mine.  As the leaf miner feeds, it separates the layers of the leaf.  The surface layer then dries and contracts, forming wrinkles on the bottom of the leaf and a tented shape on the top of the leaf.

It seems quite convenient for these leaf miners that the patches they are eating remain nutritious, even as the leaves are senescing everywhere else -- but how does this happen?  Fascinatingly, a study has found that it is bacteria (living within the leaf miners) which are responsible for producing these "green islands" and that they do so by manipulating the plant's own hormone signalling.

Explore some more: Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts

Saved by the camera

I was standing near the crabapple tree when a dark shape on one of the leaves caught my eye.  As I moved closer, I saw that there were actually two dark shapes -- a parasitoid wasp and a jumping spider. 

A jumping spider sneaking up on an ichneumon wasp.

The jumping spider was stalking the parasitoid wasp, which was preoccupied with laying an egg.  With the spider poised to strike, I thought the wasp had no chance to escape.

The jumping spider closes in on the ovipositing parasitoid wasp.

Mysteriously, though, this was as close as the spider got.  After looking at the wasp for a couple of seconds, the spider abruptly turned and moved back down the leaf.  Perhaps it was spooked by the camera, or perhaps the wasp was simply too large for the spider.  Meanwhile, the wasp continued laying its egg.  A couple of minutes later, it turned to face the site where it had been laying an egg or eggs, then spent some time apparently chewing.

The ichneumon wasp chewing nearby its oviposition site.

Once the wasp had flown off, I turned the leaf over to see what had been parasitized.  What I found was the mine of a tentiform leaf miner, with a hole where the wasp had been facing.

A tentiform leaf miner mine with a hole (on the left) where the wasp appeared to be chewing.

Home sweet home

Inside a leaf, an insect can gain food and shelter simultaneously.  The challenge is that leaves are nearly flat.  It seems like it should be very difficult for insects to live inside them -- at least not without making a gall.  Yet, the larvae of numerous species of moths, flies and other insects have overcome this challenge.

These larvae are known as 'leaf miners' and they leave distinctive marks on the leaves they inhabit.  Some leaf miners take long, winding paths through their leaves.  The resulting 'serpentine' trails are often filled with the frass (droppings) of the larvae, which turns the trails black behind the leaf miners.  To me, this seems like a big drawback of living inside your food.

The trail left in a rose leaf by a leaf miner, possibly a larva of the moth Stigmella rosaefoliella.

Other leaf miners consume large patches of leaves, creating opaque 'windows'.  Sometimes, as in the picture below, the leaf miner can be seen through the window.

A leaf miner is visible through a 'window' in a common morning glory (Ipomoea purpurea) leaf.

To get a better look at the leaf miner pictured above, I flipped the leaf over.  I found that this morning-glory leaf miner (Bedellia somnulentella) was keeping its feeding area clean by expelling its frass through a hole in the leaf surface.  It would seem that, in the case of leaf miners, outdoor plumbing is the significant innovation.

A morning-glory leaf miner (Bedellia somnulentella) in a common morning glory (Ipomoea purpurea) leaf.