Insects and Mites in Fruit
Many species of insects and mites attack various fruit crops (see Table 2.1). Fortunately, only a few are considered serious enough to prevent a good yield and high fruit quality. These pests can be divided into two groups: those that affect the fruit (direct pests) and those that affect parts of the plant other than the fruit (indirect pests). In most instances, direct pests are far more serious since they can damage or infest the harvested part of the crop. Very few of these pests can be tolerated. Moreover, in many cases by the time an infestation is noticed, extensive damage has already been done. For this reason some pest controls (such as pesticide sprays) must be applied before fruit damage is observed. These sprays are usually applied to correspond with some stage of crop development such as pink bud or petal fall. Indirect pests, on the other hand, can be tolerated to a certain degree because a healthy plant is able to withstand some feeding damage, and the fruit are not being affected directly. Fruit growers can wait until they notice that an indirect pest is present in damaging numbers before applying a control.
The keys to successful pest management in fruit crops are close observation of the plants, the ability to distinguish the few pest species from the myriad of innocuous or beneficial insects that may be present on the plant, an awareness of when pest numbers are threatening the growth of the tree or the crop, and the ability to choose the best control option when control of a pest is necessary. Some of these techniques are discussed below.
Fruit crop plants undergo a sequence of growth stages every year. Apple trees, for example, proceed from green tip (bud break) to tight cluster (flower buds visible) to pink (flower buds showing pink) to bloom (flowers open) to petal fall, etc. The rate at which this process progresses is largely due to environmental conditions such as temperature. Likewise, pests respond similarly to these environmental cues, usually appearing in concert with some developmental stage of the plant. Therefore, the plant's developmental stage can help determine when pests (especially direct pests) should be controlled.
Not all insects are bad! In fact, many are beneficial because they feed on the pest insects. For some indirect pests, beneficial insects may obviate the need for a pesticide spray or two. In addition, these beneficial insects might be killed by the pesticide sprays. Therefore, the detrimental as well as the beneficial impact of a pesticide spray must be considered. For example, syrphid fly larvae might be feeding on pest green apple aphids and control the aphids without the use of a pesticide. However, it might be the time of the year to protect the fruit from apple maggots, which have plagued the trees in the past. The fruit grower might decide that protecting the fruit from maggots is more important and might apply a pesticide that also kills many of the syrphid flies.
How many pests are too many? As stated earlier, very few direct pests (those that attack the fruit) can be tolerated, and protective sprays are often applied prophylactically. Populations of indirect pests, however, can be tolerated without harming the tree or crop. On apples, for example, an average of five or six mites per leaf can be tolerated. Only if the mite numbers exceed this threshold are sprays recommended.
Other insects also are considered beneficial in the fruit planting. Bees are beneficial since they pollinate most fruit crops. Insecticides should never be sprayed when the crop is in bloom. Also, avoid the drift of insecticides onto other blooming plants when fruit crops are being sprayed.
As mentioned above, many types of flies are predators or are parasitic on fruit pests. Syrphid fly larvae are typically maggot-shaped and legless, with a head end that is narrower than the tail end. These flies may be found foraging among colonies of aphids on the undersides of leaves.
Beetles, particularly ladybird beetles ("ladybugs") can be very beneficial. Several species of ladybird beetle are voracious predators in both the larval and adult stages. The larvae are elongate with distinct legs and are usually black with orange markings. These larvae often are found feeding on aphid colonies. The black ladybird beetle is a mite predator. This species is discussed in the European red mite section in Chapter 4. Other predators and parasites include lacewings, predatory mites, and many parasitic wasp species.
Monitoring fruit plantings frequently and on a regular basis is the key to a successful IPM program. Monitoring can take several forms, and is accomplished by using various types of traps and by weekly scouting.
Pheromone traps lure male moths onto a sticky panel by means of a synthetic sex-attractant chemical. Counting the number of male moths in these traps weekly allows the gardener to better time insecticide treatments. Other insect traps are apple maggot spheres and white sticky panels for trapping European apple sawflies and tarnished plant bugs.
Scouting is the most useful technique for monitoring most pests. It should be done weekly during the growing season, and involves inspecting a sample of fruit plants to determine the presence and severity of various pests. During the dormant season, scouting should be performed at least once to determine levels of overwintering forms of some pests such as mite and aphid eggs and of San Jose scale. Invest enough time to make reliable assessments of pest and tree conditions.