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Posts tagged “Galls

Pine Cone Willow Gall

Galls are abnormal plant growths that can be caused by insects, fungi, bacteria, nematode worms and mites.  Insects cause the greatest number of galls and induce the greatest variety of structures.  Galls provide both food and shelter for the organisms living within them.  Galls develop during the growing season, often in buds and on leaves.  Pine Cone Willow Galls, named for their resemblance to small pine cones, are found on willows, typically in terminal buds.  A gall midge (Rhabdophaga strobiloides) is responsible for the willow bud going haywire and developing  abnormally. (No-one has determined exactly how insects cause galls, whether it’s the act of laying eggs in or on the plant, or if it’s somehow connected to the chewing of the larvae into the plant.)    Each gall-making insect has a specific host plant, or small group of related plant.  The galls that each insect species induces and lives in while developing into an adult has a recognizable shape and size.  When you think you’re seeing pines cones on willow trees, you’re not hallucinating, you’ve just discovered the temporary home and food supply of a tiny fly, known as a midge.


Blackberry Knot Gall

12-28-10      Blackberry Knot Gall

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Winter is a good time to look for galls (abnormal plant growths caused by different agents, including insects, fungi, mites and bacteria) such as the blackberry knot gall, which is much more noticeable when there are no leaves to hide it. Whereas many galls are inhabited by a solitary insect, the blackberry knot gall contains many individual chambers, each containing the larva of the tiny wasp Diastrophus nebulosus. During the spring and summer months, this little wasp deposits eggs into the ridged stems of blackberry which stimulates the plant’s tissue into abnormal growth along the stem. This particular colonial gall can be six inches in length, although two or three inches is more typical (the more eggs that are laid, the larger the gall).  The eggs hatch and the larvae overwinter inside the gall. Adult wasps emerge in the spring and chew their way out of the gall, leaving tiny holes along the gall’s lumpy ridges. In the first photograph you can see where a hungry predator has worked its way into two of the larval chambers.  In the second, multiple chambers and larvae are exposed (sacrificed for the sake of knowledge, but popular food for chickadees on a very cold morning).

 


Larvae-seeking Downy Woodpeckers

12-3-11  Larvae-seeking Downy Woodpeckers

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When cooler days arrive and adult insects become relatively scarce, insect-eating birds are very clever at gleaning the twigs, trunks and buds of trees and shrubs for overwintering eggs, larvae and pupae.  Certain galls (abnormal plant growths that house and provide food for a variety of insects) are sought by specific birds.  Downy woodpeckers seek the larvae of the Goldenrod Gall Fly (Eurosta solidaginis), which overwinter inside Goldenrod Ball Galls (formed on Canada Goldenrod, Solidago canadensis) before emerging as adults in the spring.  A tiny1/4” to 3/8”-wide hole (and an empty gall) is evidence that a downy woodpecker had itself a meal!

   


Ribbed Petiole Poplar Gall

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If you look at enough Trembling  Aspen (Populus tremuloides) leaves (and to some degree, those of other poplar species), you are bound to come across some that have an oval swelling about the size of a pea where the leaf  and stem, or petiole, meet.  This swelling is a gall – an abnormal plant growth caused by chemicals coming from the moth (Ectoedemia populella) that laid an egg at this spot this past summer, or from the chewing of the hatched larva as it bored its way into the stem of the leaf.  This gall provides shelter and food for the developing larva, which will, after the leaf falls, go down into the ground to pupate.  An adult moth will emerge next  spring.   

 


Oak Apple Gall

Galls are abnormal plant growths that are caused primarily by insects, but also by fungi, mites, nematodes and bacteria.  Each insect has a specific plant host, and each gall a distinctive shape.  Of the 2,000 gall-producing insects in the United States, 1,500 of them are gall wasps or gall gnats. Plants in the Oak, Daisy, Rose and Willow families have the greatest number of galls, with oaks having over 800 different types.  The insect typically lays an egg in a growing part of the plant (twigs, leaves or leaf bud), which reacts to a chemical secretion, the egg or the burrowing larva by forming a growth around it.  The pictured gall, an oak apple gall, is caused by a wasp, Amphibolips confluenta.  These golf ball-size galls were named for their resemblance to apples.  One larval wasp lives in the center of  each oak apple gall, where it feeds and eventually pupates and emerges as an adult wasp.  The hole in the pictured gall was chewed by the exiting wasp.