October 13, 2013

Weird Insects - Part 2 - Gall-forming insects

In the past article, I discussed insects that hide themselves in unique ways, either through materials they collect or produce themselves.  In this article, I'll look at insects (or insect-like creatures) that hide through stimulation of plant cellular division and enlargement, or through the formation of galls.

Gall forming Insects
Gall forming insects (or things that we call insects incorrectly) lay their eggs into cells or tissue layers, causing a gall to form from plant growth hormones  produced by the plant’s wound response or by the production of the PGHs themselves (or something similar).  Regardless, the plant is stimulated to have a small to massive amount of cell division and enlargement, creating a gall.  In some cases, it is a tiny gall, in others, it is sizeable.  Regardless, it is interesting.

Leaf Gall Mites (different species with the family Eriophyidae)
These tiny pests are not actually insects (closer to spiders), but they lay their eggs in the leaf and a very small, erupted gall forms.  It doesn’t seem to do much damage, other than make things look a bit weird.  Some gall mites can produce quite colourful erupted galls.  Some galls are actually disrupted leaf hairs, with no penetration of the leaf itself.
Leaf gall mites on Elm - raised areas are erupted cellular growth, each containing an immature mite

Multiple galls from gall mites on Elm - this may be a different species, as the eruptions/galls are somewhat more rounded than the ones in the above picture

Many, many galls from mite egg laying on Elm
Chokecherry Fruit Gall Midge (Contarinia virginianae)
This insect can actually be quite a pest if the population gets ramped up, but generally it just causes a minor amount of damage to just a few fruit.  The normally pea-sized chokecherry swells to the size of small to medium sized grape, turns bright red (rather than green through to purple), and is full of bright orange wriggling maggoty larvae.  It is pretty revolting, but a vivid example.

Chokecherry Fruit Gall Midge-infested fruit - fruit is enlarged, bright red and a bit misshapen
Several fruit are infested with Chokecherry Fruit Gall Midge on this plant - Note, galls range in size

Heavy infestation of Chokecherry Fruit Gall Midge

Houston, we have a problem - major infestation of Chokecherry Fruit Gall Midge

Opened fruit reveals numerous bright orange larvae wriggling around inside the enlarged fruit

Willow Pine Cone Gall Midge (Rabdophaga strobiloide)
This is one weird looking example of a tiny insect making a big, showy spectacle of its shelter.  The adult is a tiny insect, called a midge.  It lays its eggs right on the tip of the growing point of a terminal (tip) bud or axillary (side shoot) bud.  The damage (and likely subsequent production of growth hormones) causes the plant to produce layers and layers of leaves, resembling a pine cone the size of a golf ball when it is done.  If you unpeel the layers, you’ll find a tiny larva exactly in the middle of the base of the “cone”.  Engineers could learn something from this insect, it is that accurate.  The larva is the size of a grain of rice.

Willow branch with a Pine Cone Gall on the terminal bud
Willow Pine Cone Gall caused by a small midge

Taking apart the gall reveals a tiny white larva at the exact centre of the mass of woody "leaves" that comprise the gall

Tiny white larva in the centre of the gall

Midge larva on the tip of the scalpel blade, having been removed from where it was sitting perfectly upright in the gall
Saskatoon berry cabbage leaf thingy from the Peace
A similar gall was observed on Saskatoon berries up in the Peace region of Alberta.  At the time, I had no idea what to make of it.  I still have no idea, but odds are, it is something similar.

Tiny cabbage leaf gall on Saskatoon berry

Bottom view of a gall on Saskatoon berry
Willow Red Gall (Pontania proxima)
Some insects lay their eggs into the leaves, resulting in a discoloured swollen area of cells, which protects the insect through its development.  In the case of this gall on willow leaves, it looks like big, hard, red lumps or blisters, but starts off as a green lump.  The immature insect is embedded deep inside the gall.  The insect is a type of sawfly, called the Willow Redgall Sawfly.
Golden Willow appears to have a bad case of "the spots"

Red swellings/galls on the leaves of Golden Willow

Galls protrude both above and below the leaf surface

Galls on willow leaves can be opened to reveal "critters"

Very small larvae were observed at the heart of the gall
Similar galls can form on the stems or branches of different species.  In this case, on willow, the hard little lumps could be seen underneath the growing point or at or near buds.  The larva was pretty small, which seems typical of many gall-forming pests.  I’m not sure of the cause.

Woody gall on the stem of a different species of willow

Removed and dissected gall reveals a small, orange larva inside

Orange speck is the larva of the gall-forming insect - it was about half the size of a grain of rice
Stem Galls
Sometimes, galls can be quite dramatic when they form on stems or trunks of trees.  If the gall develops enough, the plant could potentially be girdled (vascular system blocked) resulting in death of the plant or at least a severely stunted plant.   In the following pictures, from a grafted willow, the galls were the size of a small-ish apple and when I dug away at them, I found the pupal casing (protective developmental covering) of some sort of insect.

Large gall swellings on the trunk of a grafted willow - they were about the size of small apples (2-3 inches across)
Opened gall shows strange cellular growth

Pupal casing found inside the gall - this is where the insect developed for a period of time before emerging

Pupal casing removed from the gall - the approximate size was 1/4 to 1/2 inch in length
Poplar Gall Mite (Aceria parapopuli)
This is one of several types of stem or branch gall forming mites.  As I mentioned before, mites aren’t true insects, being more like spiders, but they are tiny and, in terms of plant galls, can make mountains out of mole hills.  In the case of the Poplar Gall Mite, they can cause some severe damage to trees, not killing them but certainly causing some weird shapes and growth.  While these pictures show only 1 or 2 galls, there were several hundred of them on the one tree.

Single gall caused by the Poplar Gall Mite -it was one of many hundreds

Multiple galls on one branch caused by Poplar Gall Mite

Large gall on a conifer - diameter of the gall is close to 12-18 inches

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