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
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.
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.
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| Terminal leaf distortion likely caused by an infestation of Woolly Apple Aphids |
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| Distorted leaf growth, likely caused by Woolly Apple Aphid |
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| A rolled leaf edge is the result of feeding injury by Woolly Elm Aphids |
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| Underside of the rolled leaf reveals scores of aphids feeding - honeydew secretions and waxy deposits are also evident |
When aphids feed on a more succulent or tender plant, particularly at or near a growing point, you’ll find that the undifferentiated growth their feeding can cause tends to result in distortion of the plant tissues, as viewed on this Larkspur.
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| Twisted, distorted growth due to severe levels of aphid feeding |
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| Damaged and distorted growth |
Sometimes, you find something that is totally freaky, like what these aphids did to this poplar leaf.
Somehow (I have no idea how), they formed a bit of a pocket to feed within.
When you open it up, you find it packed full of aphid-y goodness.
This is close to the weirdest aphid-related thing I’ve ever encountered.
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| Odd looking swellings along a Poplar leaf vein |
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| Leaf underside showing a pocket-like swelling |
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| Opening of the swelling (from the top side) shows masses of aphids feeding inside |
Cooley Spruce Gall Adelgid (Adelges cooleyi)
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.
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| Swelling on the new growth of a Spruce caused by infestation by Cooley Spruce Gall Adelgid |
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| Gall/swelling is very old, as evidenced by grey colour of the gall |
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| Two gall/swellings caused by Cooley Spruce Gall Adelgid - open gaps indicate that the insects have left for the alternate host (Douglas Fir) |
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| The opening of one of the swellings shows cavities where the insects lived while developing - some waxy deposits are also visible |
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| Some other types of gall-forming adelgids cause slightly different swellings |
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