Tuesday, June 9, 2009

A Closer Look: Bonsai Pests, Aphids

A Closer Look: Bonsai Pests

Part 1 - Aphids

Aphids are a ubiquitous garden variety pest in temperate regions, as irritating on our tomatoes as they are on our bonsai. Also called plant lice and greenflies, Aphids are part of the insect superfamily of Aphidoidea (which pretty much covers all of the types of aphids but two), with over four thousand species known (over two hundred considered pests to horticulturalists), ranging in size from one to ten milimeters. Soft bodied, tiny, and destructive, they are seriously irritating little buggers that feed on the soft parts of our plants. (photo of aphids feeding on a fennel stalk, open source from wikipedia)

Aphids, for the most part, cause damage to our plants in one of two ways: by feeding off of the sap of the plant, and by potentially introducing viruses to the host plant (though fortunately, this is more common with vegetables than with trees, phew). The first is easier to deal with by dealing with the infestation. And, I assure you, if you see a couple aphids now, deal with it, rather than waiting. A couple aphids might not do much, but if they like your tree, they will reproduce, and *quickly*.

Anatomy of an Aphid: Aphids have soft bodies, ranging in color from green, yellow, brown, black and pink (though green seems to be the most common). Some species may appear waxy or "wolly". Their bodies are squat, and frequently pear shaped, with a tail like protusion (called a cauda). They have two compound eyes. Aphids have antenae, like all true insects, with as many as six segments. They feed themselves through stylets, which are the sucking mouth parts we find so detrimental, which are enclosed in a sheath called the rostrum. They have long, thin legs compared to their squat body shape and size, with double clawed "tarsi", which is just a fancy name for a certain part of an arthropod's leg. Most aphids have a pair of abdominal tubes (called cornicles or siphunculi), through which they exude defensive fluids to help ward off predators. The presence of cornicles distinguishes aphids from all other insects.

Aphid Habits: Some species feed on only one type of plant (these are called monophagous), though there are others that will feed on a large number of different types of plants (the green peach aphid, Myzus persicae, as a for instance). They are passive feeders. This means rather than sucking out the sap of a plant, they use their mouth parts to puncture the phloem of their host plant, letting the natural pressure within the phloem force the sap out to them. Since the sap of most plants is relatively high in sugar (a product the aphids don't need much of) and relatively high in nitrogen (something the aphids do need a lot of), they frequently give off a waste product known as 'honeydew', drops of sweet liquid containing the sugar they didn't need. Ants on a plant can frequently be an indicator of an aphid problem, as ants are attracted to the honey dew. Some species of ants will even 'herd' aphids as food sources, moving them to new plants and protecting them from predators. When host plant quality becomes poor or conditions become crowded, some aphid species produce winged offspring (known as alates) that can disperse to other food sources.

Aphid Reproduction: Interesting note- Aphids give birth to live young, called Nymphs. At least, some do, others lay eggs. There are also species that can reproduce asexually, which, considering how invasive these things are and the fact that they can be spread via wind, goes a long way toward explaining why this family of insects is so darn pervasive. Different species of aphids have different methods of reproduction, which makes it difficult here to give a brief over view. But the general gist is that most aphids reproduce rapidly when conditions are right. In the spring and summer the females (asexually) give birth to live young (all female), some of which may have wings to help them spread. At the end of the summer, males and females are produced, and eggs are laid and fertilized from this generation which will hatch the following spring. But in a warm situation like the tropics or a greenhouse, female aphids can reproduce asexually for years.

How do you know if you have Aphids: Aphids come in a number of colours and sizes, but all are tiny, soft, pear shaped bodies with cauda (tail like protrusion) and cornicales (two tubes protruding from the rear end). If you have an infestation, you'll be able to see them. You can also look for ants that are traveling up your plants (they look like they are on a mission, which is pretty accurate actually). Honeydew, the sweet liquid excreted by the aphids is another sign. Black moulds can grow on the honey dew, but they are generally considered harmless. Ugly, but harmless. Aphids leave pretty distinctive signs, even beside themselves. Leaves in an infested part of a plant will often look wrinkled or folded, even outright distorted. Aphids feeding on the base of a leaf will cause the leaf to curl backward, folding over them. Flower buds may be damaged or fall off completley. Some species of aphids will form a gall (an abnormal lump on a branch) and live inside of it, to help protect them from predators. You may also see yellow, stunted growth, wilting, mottled and brown foliage. Beside the weakening caused by the removal of sap, some aphids' saliva is actually toxic to the plants they feed on.

Aphid prevention: The best prevention for aphids is healthy, happy plants. When plants aren't stressed, they have better defenses against infestations. Keep a good distance between your trees to keep good airflow, adequately water your bonsai, and make sure they are getting the right amount of light for their species preferences. Don't over fertilize. Chemical fertilizers high in nitrogen produce large amounts of just the right kind of growth that attracts aphids. Slower acting, organic fertilizers are better for helping prevent these infestations. While we go for good growth in our trees, this is another reminder for us that balance is the key to all things in bonsai. Aphids can also thrive in thick inner canopies of trees, so keep your trees from getting too over grown in the interior. Allowing light and fresh air to penetrate will help prevent other diseases as well. There are also some studies that have been done using reflective mulches (silver coloured polyethylene sheets) to help prevent aphid infestations, though I'm not sure if anyone has tried them in bonsai cultivation. Keep new plants isolated for a week or two before putting them near your other bonsai. This way, if there is an aphid problem you may have over looked at purchasing, you have a chance to catch it before it spreads to your other trees.

Aphid Removal: If despite your best efforts, you notice an aphid infesation on one of your trees, don't panic! There are several steps you can take, ranging from the simple to the more complex.

The simplest, and cheapest way to deal with a mild aphid problem is to spray the entire plant with a strong burst of water to knock the aphids off. Make sure you get everywhere, especially the bottoms of the leaves and branches where the aphids might hide. Remove any leaves that have curled in on themselves, as aphids will hide in these as well. Don't just knock them on to the soil beneath your benches though, as they will just come back. Find someplace out of the way or over a drive way where you can then wash the whole thing down again. Spray your plants early in the day so they have a chance to dry and you can check them again in the afternoon. A solution of soapy water can also be used to wash over the entire plant.

My personal favorite (an a prevenative measure I take almost every year anyway) is to release insects that feed on aphids in to my garden and amoung my bonsai. The most common are lady bugs, though there are also types of parasitic wasps, aphid lions, crab spiders, syrphid flies and lace wings amoung others. These beneficial insects will stick around as long as there is food (i.e. APHIDS!), but you can encourage them to stick around longer by planting certain things that attract them (yarrow, cilantro, parsley and sweet alyssum are all plants that lady bugs in particular like). Lady bugs can be purchased at most garden centers now, as an alternative to chemical pesticides. If you are going to try beneficial insects, make certain you don't use any pesticides, as these will kill the good bugs as well as the bad.

Insecticidal soap or neem oil can be used, and is generally safe for bonsai, but check the label, and if in doubt, do a test on a small portion of the plant to be certain it does not cause an ill effect. An even coat that gets every part of the plant (especially the underside of the leaves!) is necessary. These only work on contact, and do not provide long term prevention. If a spot is missed on the plant, the infestation can ballon again once the oils have dried, and applications may need to be repeated. Do not use oils on water stressed plants (those that have been under watered and have wilted, or those that have been over waters and may have root rot), or if the weather exceeds 90F (30C), as this can harm the plant. Supreme- or superior-type oils will kill overwintering eggs of aphids trees if applied as a delayed dormant application just as eggs are beginning to hatch in early spring. These treatments will not give complete control of aphids however, and additional controls may be needed later in the season. Earlier applications will not control aphids.

There are also many other, stronger insecticides available for aphids (malathion, permethrin and acephate to name a few). Some of these are safer for bonsai than others. Especially if a certain tree is hit consitantly by aphid infestations, a systemic pesticide might be worth considering. Maples and Elms in particular can be susceptible to repeat attacks. There are so many kinds available it is hard to go over all of them. General guidelines are to follow the instructions on the bottle for usage, including timing and dosage. If in doubt, test it on a small area of the tree. If there are no negative affects (dying leaves, blackening bark), you can probably proceed with the rest of the tree. If you can, however, avoid the use of diazinon and chlorpyrifos; beside diazinon being dangerous for a few species we use for bonsai, both have been identified as serious sources of pollution in lakes and rivers. Carbaryl is not recommended because it is not very effective against aphids. In general, check the labels and see what pests they are effective against.

Keep in mind that if you see ants in conjunction with the aphids, you also have to deal with the ants, as they will simply reinfest the plant. If they have made a nest in the pot itself, repotting and making sure to remove all of the old soil is effective. There are also some insecticides useful against ants, but many are not, so be certain to read the labels. Smaller garden centers can often lead you in the right direction for which pesticides will be the best for your area.

On large, landscape trees, Aphids are seldom a problem. But on small bonsai with limited roots, they can weaken a plant quite a bit, and even lead to death if not taken care of. While it may seem simple, or even not very worrisome if you see only a couple aphids, whether on a bonsai or another garden plant, early reaction and prevention are your best defenses again a full blown infestation.