Here are some of the pests I've seen on bonsai during my travels, garden and nursery visits and sometimes here on the trees in the garden. Obviously we can't be blind to the fact flying and crawling pests may find our trees tasty and they can appear at any time. Regular inspection helps you understand what may or may not have set up home on your trees, although sometimes the pests are out of sight or very well hidden so a tree seeming sluggish, weak or slow to grow may be the clue you need.
Here is the basic aphid - known as greenfly, black fly etc but can be orange, brown, red or white as well. A vigorous breeder that soon swamps soft new tips and leaves as spring growth occurs. They pierce the soft tissue and suck the sap - secondary problems are the residue they leave behind - it ends up attracting a black sooty mould that further weakens the tree. A tell tale sign of aphids as well as the creatures themselves is lots of ants on the tree - they like the sticky excretion and will farm / protect the aphids to keep the supply plentiful
Ladybirds eat aphids but in my observations the aphid can out breed the best efforts of a ladybird
There are many sprays and treatments ranging from water jets, soapy water, through to contact killing pesticides and systemic treatments that enter the tree and kill the aphid when it sucks the sap.
We often hear of trees having wooly aphids as white fluffy blobs are spotted in the foliage. On the underside of beech tree leaves is the only place I've seen the wooly aphid - it is usually a totally different pest mis-interpreted as the wooly
You need to look under the leaves to find these and also spray under the leaves if you are using a contact killing spray.
These white fluffy blobs are not wooly aphids but the pine bark adelgid. I see these all the time on Scots Pine bonsai - especially if they are kept too shaded or too protected . They spread very quickly through the needles and newly growing candles - they are sap suckers and weaken the tree and certainly effect back budding too. Needles discolour and drop prematurely and the tree turns a tired colour - loosing its vibrance. Contact sprays work best with washing up liquid and a little warm water to get through the waxy coating. Systemic sprays work from the inside of the tree but often the old fluffy husk needs washing off with a jet of water
This is a more unusual species of adelgid - that colonises spruce and hemlock - different species but same type of pest
Juniper scale insects
Like the adelgid I see these tiny white spots on junipers kept too protected from the elements - dark shade net, poly tunnels, over protection and poor airflow all can contribute to a weaker tree and I believe a weaker tree more susceptible to insect attacks. Sun or at least bright light, and excellent airflow reduce the likeleyhood of a tree becoming covered in these little pests
The hard shell protects the insect underneath so a systemic pesticide is usually the most effective treatment. They often appear as tiny white dots on the scales of chinensis type junipers - often 1mm or just over. A large outbreak will eventually cause browning of the foliage.
I've seen a much larger scale insect on chinese elms and hornbeams too - these get to 5mm across, usually dark brown. Effected trees will drop branches and will die back a lot if the pests are ignored
Pine sawfly Lavae
I only had one single larvae in the garden here and luckily it was spotted as the newly purchased Scots Pine was carried from the car to the bench. I have seen larger numbers of them on pine bonsai at a nursery in the past where lots of bonsai pines were in relatively close proximity to commercial plantations so I think this is a pest you are more likely to buy in than have in high numbers in the average garden. They are incredibly voracious - chewing off rows and rows of needles virtually daily and if allowed to go unchecked they will cause branch dieback once all the needles are gone. Easy to find - they wiggle about a lot and the pine shoots become stripped of needles
This is an important one to spot - it is the feeding of the beetle stage of vine weevils. The adults eat leaves, then they lay eggs in the pot that become the root eating weevils. One adult lays about 10 eggs so it doesnt take long to become a problem. Trees slow right down on growth, lack in vigour and often end up with a poor colour. As we bring in so many trees from many places to the garden I treat everything with a systemic vine weevil killer. If you can kill the beetle while it is eating the leaves you wont get the grubs eating the roots - to me thats the best way to protect against these damaging pests.
The adult beetle
The root eating grub
Garden center material can be full of grubs - some nurseries also seem to have high numbers of grubs in the pots so I regularly treat all the new stock arriving in my garden. To get the most from a systemic treatment we make up a deep bath of pesticide and submerge the entire pot. This saturates the soil and penetrates all parts of the rootball. It kills on contact and continues to give 3-4 months protection systemically. The same stuff kills off the sap suckers too
It may cost £20 to make up the dip but it treats every tree here - after every pot up to 24" is dipped I use the left over liquid poured onto the larger pots.
At least we know the trees living here and leaving here are as pest free as we can manage
Most caterpillars are loners and do very little damage to bonsai - eating holes in a few leaves etc
There is one that seems to reach epidemic levels though and it rolls itself up in the leaves
They seem to love Azaleas and will eat into flower buds too, causing the bloom to abort or be misshaped. I've seen them bunch up larch foliage, pine needles and roll up a great many different leaves. The small brown moth lays the eggs at the base of the tree and the hatched grubs crawl up into the foliage to feed. If you spray the trees in late summer with water in the evening the moths will be disturbed and start flying about - its a good way to see if they are around.
There are other pests like spruce mites, spider mites and no end of others but in my travels they appear to be very rare and unusual - this post is about the common ones so people know what they are seeing if a bonsai gets a pest attack.
I know there are many ways to treat pests, I need effective and long lasting solutions so tend to rely on systemic / contact combination products - these will usually work long enough to kill the adults and the hatching grubs from any eggs that have been layed. We need this type of product due to the amount of trees that may be here at times, and in the long term we use a lot less chemical in the garden