Most fruit trees are susceptible to various pests. Some of these insects and invertebrates serve as little more than a minor nuisance, while others can ruin a whole crop – some may even weaken the trees themselves.
Some species, varieties, and cultivars are susceptible to a wide variety of pests, while others are only troubled by a relative handful. Apple trees fall into the latter category, as they are primarily threatened by three rather notorious insects: the apple maggot, the codling moth and the plum curculio. These represent the greatest biological threats to most apple trees, and those which must be addressed for a successful crop.
Protecting your trees from these threats is entirely possible, but you’ll need to understand the biology and lifecycles of these insects to have a reasonable chance at success.
The apple maggot (Rhagoletis pomonella) is a small fly – generally less than ¼ inch in length — that causes damage to a variety of crops, including pears and hawthorn fruit. The adults are easily identified by noting the “F” shaped markings on their wings and the white dot on their thoraxes. Damage often manifests in the appearance of numerous dimples in the skin; if opened, small tunnels and areas of discoloration are often visible.
Apple maggot larvae are responsible for the damage to apple crops, but obviously, it is important to understand every aspect of their life cycle to be able to mount the most effective defense. Apple maggots spend the winter underground in the pupal stage; by mid-summer, they begin emerging as adults. The adults feed for about one to two weeks before attaining sexual maturity, breeding and ultimately depositing eggs under the skin of apples. The eggs usually hatch within one week. The larvae begin consuming the apple’s flesh shortly after hatching, and they will continue to do so for two to seven weeks. Once the larvae complete their development, they exit the fruit, make their way into the soil and enter the pupa stage, thus beginning the cycle anew.
Apple maggot populations are typically managed through a combination of monitoring, trapping and spraying efforts. Biological control has been tried with various parasitic wasps, but it has yet to prove effective in large-scale management of the pest.
The codling moth (Cydia pomonella) is an introduced species, historically native to Europe. A small insect, codling moth adults are about an inch long, and adorned with brown and gray wings, while their larval form is represented by a half-inch-long pink caterpillar with a brown head. Coddling moths can ruin significant amounts of a given crop, with infestation levels approaching 90 percent in some cases.
Throughout most of their range, coddling moths produce two generations each season. Development, as with many other invertebrates, is significantly affected by local temperatures, with warmer weather hastening the development of the insects.
Codling moth management is often difficult – particularly in areas where they’ve been allowed to achieve high population densities. None of the biological control methods tried thus far have been successful on a large scale, and most apple tree cultivars appear to be susceptible to the insects.
Bagging the individual apples growing on your trees may be helpful, along with the diligent and prompt removal of any apples showing signs of codling moth damage. Affixing cardboard tubes or wraps around the trunks of apple trees may provide additional protection, as it reduces the ability of the larvae to climb the trees. However, chemical treatment strategies are often necessary to resolve the issue, or at least bring the local population under control.
The plum curculio (Conotrachelus nenuphar) is a tiny and infuriating pest of apple trees – without prompt action, they can quickly overrun your grove, ruining your fruit in the process. About one-quarter inch in length, these mottled brown and black beetles are true weevils that possess an elongated beak-like mouth and rough wing covers.
Plum curculios damage apple crops through both their egg-laying habits and feeding behavior. In the spring, adult females seek out young apples (as well as other fruits) which will serve as the home for their deposited eggs. Females typically create a small cavity in the developing fruit’s flesh and then free a flap of skin to prevent the growing pressure inside the apple from killing the larvae inside. They then deposit an egg inside the cavity and move on, having caused a half-moon shaped wound on the fruit. Curculio larvae cannot feed on hard, healthy apple flesh – they can only feed on apples that fall from the tree and begin to soften. Adults feed on the mature apples by piercing the skin with their beaks. This leaves a series of very small puncture wounds in the apples, about one-tenth of an inch in diameter.
Managing curculio populations is often difficult. Pesticides sprayed after the initial emergence of the adults is often the most effective strategy, although promptly removing infested fruits also helps keep population levels low. Some growers have partial success by shaking the limbs of their trees over a sheet, tarp or piece of cardboard. Any curculios dislodged will remain motionless, allowing you to easily pick them up by hand and dispose of them properly. Hand removal is unlikely to keep your trees completely free of these pests, but it is a viable component of an integrated strategy.
No matter which of these three pests begin attacking your apple trees, it is important to ensure your trees are as healthy and vibrant as possible, to help give them the best chance of fighting off these infestations on their own. It is therefore imperative that you plant your apple trees in a proper location, water them well, cover their root zones with a thick layer of organic mulch and keep them properly pruned.
But above all else, it is important to act promptly whenever you notice one of these pests feeding on your apples. Doing so will give you the best chance to nip the problem in the bud.
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