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What is it?
The flat-headed apple borer is of the chrysobothris species of borer insects. Newly planted apple trees or those which are very young are extremely susceptible from borer damage as are apple trees which have been weakend either mechanically from pruning cuts or through prior insect infestations or infections from bacteria and fungi.
What does it look like?
Chrysobothris flat-headed borers are usually a yellowish white colour but some can appear to be more golden or a paler light green in color. Typically Chrysobothris flat-headed borers which attack apple fruits and tress grow to the length of about 3/4 inch and resemble grubs in their appearance. The mature adult form of the Chrysobothris flat-headed borers is a beetle. The mature adult chrysobothris beetles can grow in size anywhere from 1/2 to 3/4 inch in size. The beetles are usually bronze or copper colored and do not appear until late spring or mid summer. The mature adult beetles are metallic looking and will appear reflective if seen in direct sunlight in contrast to the apple fruits. Damage from these insect pests whether as borers or full grown beetles appears first as leave wilting or turning brown and dying off. The apple tree bark may become sunken or slough off in areas of flat-headed borer damage and you will see borer holes beneath this sloughed off apple tree bark. Tunnels which can be up to 3/8 inch in diameter may pepper the exposed tree trunk and you may note the presence of the heads of the chrysobothris flat-headed borers poking out in their characteristic yellowish white.
How does it manifest?
Chrysobothris flat-headed borers start their life cycle in late spring to midsummer when the adult female beetles lay their eggs in the bark of the apple tree. Typically the female beetles will choose crevices in the bark which are deeper than others because this offers their eggs some protection and also places them closer to the unexposed tree meat beneath upon which newly hatched chrysobothris flat-headed borer larvae can feed. The larvae emerge on the apple trees and bore through the bark from the outer regions into the inner meat of the apple tree which will end up resembling a tunneled ring of twisting burrow holes. These tunnels damage nutrient and water-conducting vessels in the trees. The damage to these vessels causes the apple tree to be improperly nourished which can result in dieback of apple tree twigs and branches as well as dieback or even killing off of the entire tree if the chrysobothris flat-headed borer population is large. Once they have finished tunneling and feeding bore into the heartwood of the apple tree to pupate and emerge the following spring when conditions permit to start the cycle again.
What can you do about it?
The best defense against chrysobothris flat-headed borers for your apple trees is to keep the apple trees healthy and vigorous to begin with, being careful not to inflict mechanical wounds and treating other infections and infestations which may occur as soon as they start. Weakened apple trees and those newly planted are most susceptible to chrysobothris flat-headed borer damage. In order to discourage flat-headed borers on your apple trees wrap the trunk of the apple tree with burlap or special trunk wrapping paper, using what is recommended in your geographical location by your local cooperative extension office. Any branches which show signs of damage should be pruned out carefully and destroyed. Additionally, if your infestation of chrysobothris flat-headed borers is severe in your apple trees you may wish to apply a lindane containing insecticide to help control the populations.
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