Identifying Leaf Damage, Part 2 - Insects

Beyond the sunburn and nutrition damage I showed last week, pests can really do a number on your plants. There are many different kinds of pests, and the damage they do shows up differently depending on what they are and how they attack the plant.

Chewing insects leave holes in the leaf. I thought I had slugs, but when I used the old “cheap beer in a shallow dish” slug catcher, I found that I had a tuna can full of drowned earwigs and not a slug in sight. This made sense because many of my damaged leaves were pretty high off the ground and earwigs are unfortunately very mobile. Also there haven’t been many slime trails – which slugs leaves as they move.

Leaf damage from earwigs.

This chewing damage happened on leaves that are 18” off the ground.

I must have a major infestation because they seem to damage a large variety of species, like this gourd seedling, which is covered with plant hair…

and this weedy plantain. 

Of course since I have put a chip mulch down as a path, I have given them perfect hiding places. Earwigs eat aphids and can therefore be considered helpful, but they are doing too much damage and I am going to begin a major trapping event. Check out this page from the U of CA on integrated pest management for more information on earwig damage and control. 

Leaf cutter bee damage to peonies. The leaf cutter bees are a native pollinator,
and usually only cause cosmetic damage, not health damage, to the plant.

These peony leaves also sport insect damage. This is most likely a leaf cutter bee. All the damage is at the edge of the leaf and that is pretty typical. This insect can make your plant look like it has been attacked by pinking shears. Usually the damage is only cosmetic and the plant doesn’t lose vigor. These bees are beneficial native pollinators and shouldn’t be nuked with pesticides.

A skeletonizing leaf. It's best to take this to an extension agent
and ask for their help, as the type of insect varies with the plant.

This kind of damage is also indicative of an insect munching on my leaves. This is the results of insects eating the top and/or bottom layers of cells on the leaf and leaving one layer and the veins. This skeletonizing is unsightly, making the plant look like it is browning out and dying. If the damage is severe, the leaf will often drop off. If you have this, take a leaf into your extension service and ask what to do. Different plants get different kinds of insects.

These next two pictures show mite damage. Mites and aphids are sap-sucking insects and this kind of splotched discoloration is typical. Both these insects are very hard to control since they reproduce rapidly and they are hard to completely remove off your plant.

  

Mites are tiny but their presence can be determined in a couple of ways. You can bang the leaf over a piece of blank white paper and they fall off and gross you out. Or you can rub the back of the leaf across the paper and find mite guts left behind. If the infestation is bad enough, you can see greyish webbing on the back sides of the discolored leaves. If you have a magnifying glass or keen eyesight, it is also possible to see them on the leaf.

Aphids are bigger than mites, but still small. There are numerous kinds but none of them are good. Some have wings, but mostly they are slowly mobile on their tiny bug legs. They also suck sap from the leaves. Some species poop out that sap, producing a sticky substance called ‘honeydew’ (yes, the name puts you off the melon, doesn’t it?) and this is what makes your car sticky if you park it under a tree. It is also an attractant to ants, so if there are aphids, you will often finds ants all over that plant.

Here is an interesting article, the kind I call "gross but cool," about honeybees and honeydew. If you want some "just gross" pictures of aphids, google "aphids and honeydew" and look at the images that pop up. EWWWW! Another good read is this article about how long it takes to get aphids under control.