The three basic approaches to biological control are relevant for an
areawide IPM program for pome fruit in the west. The approaches are:
1.) classical (importation), the introduction of natural enemies from the
region of origin of an exotic pest for the permanent suppression of the
pest in a new target region; 2) augmentation, the periodic release of insectary-reared
natural enemies to either inoculate into a crop for season-long suppression
of a target pest, or to inundate the crop with natural enemies during each
generation of the pest for immediate but short-lived suppression; 3) conservation,
the modification of crop production practices to prevent the destruction
of resident natural enemies and to enhance the activity of natural enemies
in the target crop (e.g., minimizing the need for disruptive chemical intervention).
1. Classical Biological Control
Although the prevailing view is that the action threshold for codling moth in commercial orchards is too low to support natural enemy populations, classical biological control has a significant role to play in the context of regional management. Unsprayed commercial orchards are susceptible to immigration of codling moth females from the surrounding region, and so the regional abundance of codling moth is more important than its abundance in individual orchards. Currently, in the apple and pear production areas of the western U.S., codling moth can often infest all fruit in unmanaged tree fruit settings where the potential for population growth is high. In addition, a number of alternative non-commercial host plants also support codling moth populations (Barnes 1991). The best hope for "managing" these unmanaged settings is classical biological control.
Surveys of North America show that parasitism by native parasites that
have switched to attack this exotic pest is very low (less that 5%) (Flanders
1926, Lloyd 1944, Jaynes & Marucci 1947, Nick Mills unpublished observations
in apple and walnut in CA). However, parasitism rates of 20% of overwintering
larvae are not uncommon on unsprayed hosts in central WA (Tom Unruh,
unpublished observations). The dominant species in the west is the
introduced egg-larval parasitoid, Ascogaster quadridentatus Wesmael (Braconidae).
In comparison, some dozen primary parasitoids belonging to four families
(Braconidae, Ichneumonidae, Pteromalidae, and Tachinidae) attack codling
moth in Europe (Rosenberg 1934, Simmonds 1945, Geier 1957). In central
Asia, the purported origin of both apple and codling moth, an even greater
diversity of parasitoids exist and parasitism can be even higher; in some
regions it is dominated by species that are rare or absent in Europe (Abdullaev
1974, Tkachev 1974, Zlatanova & Tarabaev 1985, Babidorich & Sharov
1986). Rates of parasitism exceeding 60% and levels of fruit damage
as low as 20-30% are reported in areas where codling moth displays three
generations, indicating that natural enemy induced mortality is much greater
in central Asia than in North America. There is encouraging potential
for new natural enemies to be imported for codling moth control.
2. Augmentative Biological Control
The augmentation of natural enemies has proved an effective tactic in pest management in several crops (King et al. 1985, Knipling 1992). A 3-year trial in an experimental apple orchard in Austria (Rupf 1976) indicated that inoculative releases of the egg-larval parasitoid, A. quadridentatus, could elevate the level of parasitism of codling moth by 1-21% over pre-release levels. More recently, inoculative releases (150-200 per acre per week during the first generation, and per two weeks during subsequent generations) over a 2-year period in small plots in a commercial walnut orchard (McDougall et al. 1993, Mills & Dixon, in preparation) indicate that codling moth suppression can be comparable to that of conventional pesticide treatments. These results must be treated with caution, due to the small size of the experimental plots (1- 2 acres), but indicate the potential of parasitoid inoculation as a potential tactic in regional pome fruit IPM. Inoculative augmentation of other parasitoid species, such as L. caudatus or M. castaneus, also may be feasible and need to be considered, particularly in the context of reducing residual overwintering populations of codling moth.
Ongoing studies in California (USA) have shown parasitism of sentinel eggs of 60%, 80%, and 89% following releases of 100, 200, and 400 thousand Trichogramma per acre (McDougall et al. 1993). Codling moth fruit infestation after the first generation in June was reduced to 58%, 75%, and 86% of that in no release blocks, respectively. Current costs of Trichogramma platneri, the species native to the western U.S., are approximately $14/100,000 which makes releases of up to a million/acre potentially cost effective.
Parasitoid augmentation shows considerable promise for integration with
other selective control tactics such as mating disruption, sterile insect
release, or selective pesticides. Research is in progress in CA (Mills,
unpublished) to evaluate Trichogramma inundation in apple and pear orchards,
both as an orchard-wide first generation treatment to enhance the effectiveness
of season-long mating disruption, and as an edge treatment to replace pesticide
treatment of the vulnerable perimeter of mating disrupted orchards.
3. Conservation Biological Control
The integration of parasitism into areawide codling moth management necessitates a clear understanding of the interaction of management tactics and the selection of complementary treatments. The general objective of reducing or eliminating the use of broad-spectrum pesticides against codling moth in pome fruit crops in the western U.S. will create a more hospitable environment for natural enemies and will constitute the most important element of the conservation of imported and augmented parasitoids.
Another important component of biological control conservation is the
provision of adequate food sources and resting sites for the adult natural
enemies (Powell 1986, Altieri et al. 1993, Evans 1994). Many adult
parasitoids and predators feed on honeydew, nectar, and plant exudates,
while adult predators often feed on pollen or fungal deposits to obtain
sufficient food for the maturation of eggs. These food sources, together
with a more humid microclimate, greater shade, and resting sites, can be
introduced into pome fruit crops through orchard floor management.
The presence of wild flowers in apple orchards has been shown to increase
the abundance of parasitoids and significantly enhance the control of San
Jose scale, woolly apple aphid, and codling moth (Chumakova 1960, 1977,
4. Biological Control of Insect Pests of Apple and Pear Other Than Codling Moth
In addition to codling moth, a number of secondary insect pests inhabit apple and pear orchards in the western U.S. and their levels of abundance may change as codling moth management moves away from the use of organophosphate cover sprays. A list of the key secondary pests of apples and pears shows a surprising abundance of exotic species (see Table 2) of which only a few have been the target of classical biological control. Successful biological control has been experimentally demonstrated for woolly apple aphid, San Jose scale, and the mite complexes in many sites around the world, or in the American Northwest, in the absence of disruptive pesticides (Hoyt & Burts 1974, Croft & Hoyt 1983, Blommers 1993). Most of the other species on the list are assumed to be regulated by natural enemies (e.g., apple ermine moth, pear psylla) or to be minor pests. The most important of the secondary pests are the pear psylla, the aphid complex, and the leafroller complex.
Several studies show that use of broad-spectrum insecticides for codling moth control exacerbates pear psylla, a costly pear pest (Burts 1983, Westigard et al. 1986). In addition, broad-spectrum insecticides applied for pear psylla can cause mite infestations to reach economic levels. In the past five years, pear psylla resistance to pyrethroid insecticides has increased and appeared at a number of locations (van de Baan et al. 1989, Burts et al. 1989, Pree et al. 1990) so that control in the critical spring period has become only marginally effective. Biological control efforts against pear psylla in western North America were recently reviewed (Unruh et al., in press). Several native species of Hemiptera are abundant as predators of C. pyricola but the role of individual species has recently been demonstrated experimentally (Unruh & Higbee, unpublished). Relative abundance varies geographically; in pear, for example, Anthocoris antevolens White and A. melanocerus Reuter are most abundant in British Columbia (McMullen & Jong 1967), A. antevolens is dominant in CA (Madsen et al. 1963) and Deraeocoris brevis (Uhler) dominates in OR (Westigard et al. 1968). Anthocoris nemoralis (Fabricius) is most abundant in much of western Europe (Herrard 1986), but is only found in unsprayed orchards in British Columbia and central WA following its release and establishment 20 and 10 years ago (Unruh et al., in press). Seven parasitoid species have been reported from C. pyricola in North America of which only two are known to be obligatory primary parasitoids. These are Trechnites psyllae Ruschka which is abundant and Prionomitus mitratus Dalman which is rare. Previously, the importance of parasitoids in the suppression of pear psylla in North America was thought to be less than that of the generalist predators. In contrast, Talitski (1966) considered Trechnites psyllae to be the regulating agent of Cacopsylla pyri (L.) in Moldavia. Parasitism as high as 70-90% of the late nymphal instars by Trechnites has been observed in western North America (Westigard et al. 1968, Nickel et al. 1965) with consistent parasitism of 30-50% per generation in unsprayed orchards (Unruh, unpublished).
Leafrollers are a major concern in orchards using selective controls for codling moth. However, Helsen & Blommers (1989) have demonstrated the benefits of changing from a broad-spectrum chemical program for codling moth control in the Netherlands, where natural control of the summer fruit tortrix in IPM orchards, primarily from general predators, was found to be responsible for 99% mortality of eggs and larvae. In the western U.S., there are three main leafroller species: all three are native and have substantial parasitoid complexes (Newcomer 1958, Paradis & LeRoux 1962, Powell 1964, Simmons 1973, Mayer & Beirne 1974).
It seems probable that natural biological control, classical biological
control, and augmentative biological control all could play substantial
roles in the suppression of the most important secondary pests of apple
and pear under a selective management program against codling moth.