Mimosa trees, commonly known as silk trees, are renowned are known for large, pink, fragrant, ornamental flowers that can grow to an 1 1/2 inch long. Mimosa trees can grow to 40 feet tall in USDA hardiness zones 6 to 9, and have thin bark and fernlike leaves. However, mimosa trees require care to avoid pests that like to feed on the foliage and sap of the tree.
The mimosa webworm originated in China and can cause serious damage to the foliage of the tree. The webworm larvae spin webs around the leaves and the feed on the enclosed leaves. Webworm eggs are tiny, oval and pearly white when first laid, and then turn pink when ready to hatch. Multiple generations of webworm will eat leaves and cause defoliation of the tree. However, webworms can be easily controlled by removing the webs of larvae from the tree by either cutting off the end of the branch with the larvae attached or by spraying with pesticide.
Spider mites are common plant pests. Spider mites cause damage to plants as they feed on the leaves to suck out the sap and cause bruises to plant cell. Severe infestations of spider mites can be identified by the discoloured leaves that will appear speckled with a dull grey or bronze look to the foliage.
When a large infestation of spider mites has been established in mimosa trees webbing can be seen. Spider mites create the webbing to provide the eggs with protection. Spider mites can be controlled by introducing their natural enemies to the area. Stethorus beetles, more commonly known as spider mite destroyers, can quickly bring a colony of spider mites under control.
Cottony Cushion Scale
Adult female cottony cushion scale have a white fluted egg sack that looks similar to combed cotton. This egg sac is secreted from the body of the female. Cottony cushion scale damages a tree by sucking sap from the leaves, twigs, branches and trunk. Infestations can cause a tree to become defoliated and cause dieback of the tree. The cottony cushion scale leaves a black, sticky residue behind after feeding.
Mimosa wilt is a fungus that is transferred through the soil to infect a mimosa tree. The root system becomes infected first and then spreads to the rest of the tree. Evidence of infection of mimosa wilt first appears as a yellowing and wilting of leaves on several branches. As the wilt spreads, these yellow leaves will turn brown and fall off and shortly after the entire branch will die. The yellowing, wilting, and defoliation will continue through the crown of the tree. Trees that have been infected with mimosa wilt will rarely survive past the year of infection, and will fail to produce any leaves the following year.
While mimosa trees have a variety of pests that can cause damage to the tree the pests can be controlled. However, mimosa wilt is fatal to a tree and can not be controlled through pesticide. Once a tree has contracted a case of wilt, it should be removed and burnt to prevent the wilt from spreading. For webworms, spider mites, cottony cushion scale, it is possible to control them through natural means such as introducing beetles to an area of infestation, as they are natural predators of these pests.
Horticultural oils sprayed on the tree will control spider mites, webworms, and cottony cushion scale. However, more than one application may be needed. The oils should first be sprayed at the beginning of the budding season when pests are starting to develop webs. A second application may be needed around mid summer. Horticultural oils are a gentler way of controlling pests than traditional pesticides. An experienced nurseryman can offer advice on application of the horticultural oils.