Gall forming Insects
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.
|Several fruit are infested with Chokecherry Fruit Gall Midge on this plant - Note, galls range in size|
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|
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|
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 GallsSometimes, 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)|
|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|
|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|