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B. Use of Natural Enemies to Control Pests on Deciduous Fruit Trees
Hiroshi Amano and Akio TakafujiGraduate School of Agriculture, Kyoto University
Kyoto 606-8502, Japan, 2001-09-01

Abstract

The use of pheromone traps to control lepidopteran pests on deciduous fruit trees opened new possibilities for IPM programs to control spider mites in Japan. The non-selective pesticides formerly used to control harmful moth larvae also wiped out native natural enemies of spider mites. Now that growers are adopting IPM programs using sex pheromone traps for moths, the conservation of natural enemies has become very important. A method of evaluating each natural enemy species in orchards has recently been proposed. New monitoring traps for predacious mites of the family Phytoseiidae are also described. These may help us understand this important group of natural enemies of mite pests. Details of the traps and their value in applied and basic agricultural science are discussed.

Recent Trends in Pest Management for Deciduous Fruit Trees

Apple, peach and Japanese pear are all deciduous fruit trees of major economic importance in Japan. All three crops share common problems in pest management. Lepidopteran larvae are the most important pests, followed by aphids and mites. For some years, heavy applications of non-selective pesticides were made to control caterpillars. These destroyed existing natural enemies of aphids and mites. To solve this problem, sex pheromone traps for harmful moths were developed, and are now widely used. Populations of native natural enemies revived, and biological complexity (and stability?) were restored to orchards.

This Bulletin looks at IPM programs carried out in Japanese pear orchards, and more specifically, at some new techniques and observations associated with natural enemies of spider mites. Each orchard has a complex of pest and non-pest species. The goal of an IPM program should be a comprehensive one that covers all these species.

Use of Natural Enemies to Control Spider Mites

It is well-known that spider mites were not primary pests prior to the introduction of wide-spectrum pesticides in the 1960s. Before this, mites were usually kept under control by a complex of natural enemies. This is shown by the many cases in which stopping the application of pesticides and acaricides was followed by a rapid recovery in the numbers of natural enemies, so that mite populations were suppressed at low densities (Amano 2001).

Natural enemies of spider mites on various agricultural crops are summarized in Table 1(1375). They include two major groups, predacious insects and predacious mites. These often occur together in the field, and form a rich biotic guild. Within each group, there is considerable biodiversity. Each species has different biological characteristics, including predation capacity and population stability in orchards ( Table 2(1240)). Each species has a different role in the natural and agricultural ecosystem.

Because of the complexity of natural enemy fauna, however, it is rather difficult to measure the predation and control capacity of each species of natural enemy in orchards. Recently, Kishimoto and Takagi (2001) used SEM (Scanning Electron Microscope), to observe details of feeding traces left by various predators on egg surfaces of the citrus red mite, Panonychus citri. They found clear differences between the feeding traces left by different natural enemies. Two coleopterous beetles, Oligota kashmirica benefica and Stethorus japonicus, each made two large holes in the egg. Adults of the former species left two holes of different sizes. Adults of S. japonicus, however, made two roughly triangular holes, each with sides around 50µm long.

The same authors also observed other predator species. They found that a phytoseiid mite, Amblyseius californicus, left a single elongated oval hole measuring 15-20µm along the major axis. A predacious dipteran larva made a single small hole (3-5µm in diameter) with a partition. Another major predator, a species of thrips ( Scolothrips takahashii), had a characteristic feeding habit which left a number of circular pores (2 µm in diameter) on the upper side of the eggs.

These findings are a key to distinguishing which spider mite eggs have been attacked by different natural enemies. They also indicate the relative importance of different natural enemies in regulating prey mite populations in orchards. Using these marks, Kishimoto and Takagi (2001) were able to estimate the total number of P. citri eggs attacked by each predator species ( S. takahashii, Oligota spp., S. japonicus and others) in Japanese pear orchards. Such indices are very useful when we want to assess the impact of different natural enemies.

Conservation of Predacious Mites

Members of the family Phytoseiidae show a remarkable ability to depress spider mite populations. Numbers increase very rapidly under favorable conditions, and populations are also very stable. Their bodies (about 0.4 mm long) are as small as those of the prey mites, but they are capable of attacking spider mites at all stages of growth (Amano 2001). About 1600 species are found in various parts of the world (Chant and McMurtry 1994). Recently, Ehara and Amano (1998) reviewed the Japanese phytoseiid fauna, and named 77 species, together with short remarks on their biology. They also categorized phytoseiid fauna into several groups, based on their food habits and habitat preferences.

The orchard environment provides a variety of habitats for living organisms. Phytoseiid mites respond sharply to these microhabitats. Even in the same orchard, phytoseiid fauna found on ground cover are quite different from those found on fruit trees. Furthermore, even on the same trees, species structure on the twigs is not the same as that on the leaves ( Table 3(1202)). These biological complexities are an important part of the orchard's biotic components, which resemble those of natural ecosystems.

Surveys were carried out on 20 sampling dates in 1988, in an unsprayed small experimental orchard in Chiba Prefecture, central Japan.

Traps to Monitor Mites

Cotton Traps

Predacious mites are less conspicuous in the orchard than insect predators, because they are so small. It is a laborious task to clarify the species structure of phytoseiid mites in the orchard, and to estimate their population levels accurately. To do so, hundreds of leaves must be observed regularly, either in the field or under the microscope. A solution to this problem was suggested by a simple and ingenuous observation of phytoseiid mites on pear trees. Koike et al. (1998) found many phytoseiid mites in the nests of spiders and leafrollers on pear trees. They speculated that predacious phytoseiid mites prefer places sheltered by the silken webs produced by spiders and leafrollers. To test this hypothesis, they invented a simple trap (the "Cotton Trap"), in which cotton wool was wrapped in a pear leaf ( Fig. 1(1197)). They set the traps in small experimental orchards which had received different pesticide treatments.

Table 4(1112) shows the number of phytoseiid mites captured on trees. Two types of traps were made, one with cotton inside and the other without. These traps were set out on the pear trees in three different orchards. (One orchard was not given any chemical treatment, one was treated with selective pesticides, and the third was treated with non-selective pesticides). The traps were opened after two weeks, and the mites counted.

In all three orchards, it was apparent that more phytoseiid mites were captured in the traps which contained cotton. Setting up such a trap is an easy task. Not much skill and experience are needed, so different individuals should get very similar results. Furthermore, the traps may provide some shelter to phytoseiid mites from pesticide sprays. Many mites were captured alive in traps set out in orchards which had been sprayed with selective or non-selective pesticides.

Invention of the Cotton Trap satisfied the primary purpose of population study, but further improvement of the trap was needed. One weakness of this trap was that it did not last long in the field. Because it used leaves of plants, which are fragile, wrapping the cotton sometimes damaged the leaves. In addition, heavy wind and rain easily blew the trap away. To solve these problems, a new trap (called the Phyto trap) was invented.

The Phyto Trap

The Phyto trap ( Fig. 2(1086)) is simple and cheap to make, and the durability is greatly improved (Koike et al. 2000). It uses a piece of transparent vinyl tape and a piece of Velcro. A piece of Velcro is stapled to the vinyl tape, and a piece of woolen yarn is attached to the hooked underside ( Fig. 3(1172)). This trap can be set out either on twigs or on leaves, with woolen yarn in the space between the vinyl tape and the plant. Traps are washable after use, and can be reused many times.

All stages of mites were observed in Phyto traps. This suggests that the traps were attractive for immature stages as well as adults ( Fig. 4(1144)). In addition, the presence of eggs in the traps indicates that the traps can even play a role as an oviposition site.

Species surveys using Phyto traps clarified many aspects of species complexity on Japanese pear trees in Saitama, central Japan ( Table 5(1268)). From four unsprayed pear trees with only six traps on each tree, a total of 439 phytoseiid mites belonging to 11 species were collected. On the basis of numbers of mites captured, Koike et al. (2000) concluded that using the Phyto traps caught about 2.4 times more mites than the Cotton Traps, from 1/33 of the number of leaves observed.

Impact of Phyto Traps on Ipm

Knowing what phytoseiid fauna are present in an orchard provides useful information. This is especially the case when we want to run an IPM program. Compared to other cropping systems, orchards are large in size, so that releases of commercial products (natural enemies) are often costly. In an orchard system, therefore, natural enemies are usually expected to colonize over a certain period of time. However, commercially available phytoseiid mites often lack this ability. Growers must thus depend on native phytoseiid species, either alone or combined with native insects which are natural enemies. An accurate assessment of native fauna of predacious mites is thus an essential part of managing spider mite populations. The Phyto trap is a valuable aid in doing this.

Research on other aspects of the Phyto trap is taking place in our laboratory. We are studying the possible function of traps as a conservation site of natural enemies in orchards. The Phyto trap does not kill mites, so that it probably acts as a shelter for natural enemies, protecting them from detrimental conditions such as chemical sprays. They may also escape from their own enemies (usually larger organisms) by hiding in the trap. Another aspect is the educational function of the Phyto trap. Because of the small size of the mites, as well as the heavy use of chemicals, few farmers recognize the presence of predacious mites in their own orchards. The Phyto traps can show farmers what predacious mites look like and motivate them to adopt IPM-oriented programs. In several prefectures in Japan, extension programs using Phyto traps are now taking place, and help farmers as well as extension staff.

References

  • Amano, H. 2001. Species structure and abundance of invertebrate natural enemies in sustainable agroecosystems. In: Structure and Function in Agroecosystem Design and Management, Shiyomi M. and H. Koizumi (Eds.). CRC Press, New York, U.S.A., pp. 167-182.
  • Chant, D.A. and J.A. McMurty. 1994. A review of the subfamilies Phytoseiinae and Typhlodrominae (Acari: Phytoseiidae). International Journal of Acarology 20, 4: 223-310.
  • Ehara, S. and H. Amano. 1998. A revision of the mite family Phytoseiidae in Japan (Acari, Gamasina), with remarks on its biology. Species Diversity 3, 1: 25-73.
  • Kishimoto, H. and K. Takagi. 2001. Evaluation of predation on Panonychus citri (McGregor) (Acari: Tetranychidae) from feeding traces on eggs. Applied Entomology and Zoology 36, 1: 91-95.
  • Koike, A., H. Nemoto and H. Amano. 1998. Species and habitat survey of phytoseiid mites on pear trees using Cotton trap. Japanese Journal of Applied Entomology and Zoology 42, 1: 21-23. (In Japanese with English summary).
  • Koike, A., H. Nemoto and H. Amano. 2000. New trap for survey of species structure and seasonal dynamics of phytoseiid mites on Japanese pear trees (Acari: Phytoseiidae). Japanese Journal of Applied Entomology and Zoology 44, 1: 35-40. (In Japanese with English summary).

Index of Images

Figure 1 Cotton Traps Attached on the Pear Leaves

Figure 1 Cotton Traps Attached on the Pear Leaves

Figure 2 Phyto Trap Attached on the Pear Twig.

Figure 2 Phyto Trap Attached on the Pear Twig.

Figure 3 Different Views of the Phyto Trap. Stages of Making the Trap Run from a to D.

Figure 3 Different Views of the Phyto Trap. Stages of Making the Trap Run from a to D.

Figure 4 Numbers of Phytoseiid Mites in Each Developmental Stage Collected in Phyto Traps on Different Pear Trees

Figure 4 Numbers of Phytoseiid Mites in Each Developmental Stage Collected in Phyto Traps on Different Pear Trees

Table 1 Major Natural Enemies of Spider Mites in Japan

Table 1 Major Natural Enemies of Spider Mites in Japan

Table 2 Biological Characteristics of Common Natural Enemies of Spider Mites in Japan

Table 2 Biological Characteristics of Common Natural Enemies of Spider Mites in Japan

Table 3 Species Structure and Abundance of Phytoseiid Mites on Water Sprouts of Non-Sprayed Japanese Pear Trees

Table 3 Species Structure and Abundance of Phytoseiid Mites on Water Sprouts of Non-Sprayed Japanese Pear Trees

Table 4 Species Structure and Number of Phytoseiid Mites (Adult Females) Observed in Two Different Types of Trap on Japanese Pear Trees in 1995

Table 4 Species Structure and Number of Phytoseiid Mites (Adult Females) Observed in Two Different Types of Trap on Japanese Pear Trees in 1995

Table 5 Species Structure and Numbers of Adult Females of Phytoseiid Mites Caught in Phyto Traps on Different Pear Trees in 1996

Table 5 Species Structure and Numbers of Adult Females of Phytoseiid Mites Caught in Phyto Traps on Different Pear Trees in 1996

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