In Part 3, we looked at insects that "defile" or misshape plants through the creation of web nests for feeding, development and protection (and maybe company?). In this final part, I'll talk about a common group of insects, aphids. However, we'll look at some of the unusual or unique reactions that we sometimes see in plants from the feeding of different species.
Aphids don’t tend to be all that subtle about their feeding. They are inefficient feeders, basically sucking in the sugar-rich sap from the plant and firing it right out the back end in a pretty similar form (this is according to my simplified understanding). They feed in big groups, don’t tend to move much and cause mostly localized damage. That being said, they can cause a pretty dramatic impact on plants.
In some cases, the degree of response will depend on the plant species being attacked. In the case of this currant, it responds to damage with the production of darker pigments and some weird cellular growth, which looks like red blisters from the top. Turning over the leaf reveals the many aphids chowing down on the plant.
|Conspicuous distortion and discolouration on the upper leaf surface of a currant bush|
|Severe discolouration and distorted growth of a currant bush|
Sometimes, the way an aphid feeds on a particular host and the resulting distortion of the plant can be used in identifying the species. On these elms leaves, we find either the Woolly Apple Aphid, which causes a terminal distortion or the Woolly Elm Aphid, which causes a bit of a leaf rolling effect. If you open up the roll, you’ll find scores of aphids feeding, with plenty of honeydew pouring out.
|Twisted, distorted growth due to severe levels of aphid feeding|
|Damaged and distorted growth|
|Odd looking swellings along a Poplar leaf vein|
|Leaf underside showing a pocket-like swelling|
|Opening of the swelling (from the top side) shows masses of aphids feeding inside|
There are a number of different adelgids, which are distinctly aphid-like. In this case, this species moves between Douglas fir and Spruce, causing a “pineapple-like” gall on the new growth of spruce trees. The galls mature with the stages of the insect inside over a period of one or two years. The insects eventually move back to the Douglas fir to complete the life cycle. The pictures show mature, empty galls.
|Swelling on the new growth of a Spruce caused by infestation by Cooley Spruce Gall Adelgid|
|Gall/swelling is very old, as evidenced by grey colour of the gall|
|Two gall/swellings caused by Cooley Spruce Gall Adelgid - open gaps indicate that the insects have left for the alternate host (Douglas Fir)|
|The opening of one of the swellings shows cavities where the insects lived while developing - some waxy deposits are also visible|