Summer 1993 (v5n4)
Bugg, Robert L. and Carol Waddington
Agriculture, Ecosystems
and Environment 41. In press. 1993
Cover crops have various
effects, both beneficial and deleterious, on orchard pest problems. Ideally,
cover crops might be selected or managed so as to: 1) not harbor important
pests; 2) divert generalist pests; 3) confuse specialist pests visually
or olfactorally and thus reduce their colonization of orchard trees; 4)
alter host-plant nutrition and thereby reduce pest success; 5) reduce
dust and thereby reduce spider mite outbreaks; 6) change the microclimate
and thereby reduce pest success; and 7) increase natural enemy abundance
or efficiency, thereby increasing biological control of arthropod pests.
This review article (65 references) examines various studies of cover
crops for tree nuts, pome fruits, stone fruits, and citrus, with examples
from the United States and Canada. Only the Californian cases are detailed
in this summary.
Almond
Two ant species (Hymenoptera:
Formicidae) are particularly damaging to almonds in central California:
pavement ant (Tetramorium caespitum [L.]) and southern fire ant
(Solenopsis xyloni McCook). These feed on nutmeats after the nuts
have fallen to the ground, and in a study by Barnett et al. (1989) were
especially damaging amid cover crops of soft chess (Bromus mollis L.,
cv 'Blando', Poaceae), strawberry clover (Trifolium fragiferum L.,
cv 'Sauna', Fabaceae), and resident weedy vegetation, as compared with
residual herbicide. However, the same study showed that cover crops of
soft chess, strawberry clover or resident vegetation accelerate the decomposition
of unharvested almonds, which otherwise represent an overwintering niche
for navel orangeworm (Amyelois transitella Walker, Lepidoptera:
Pyralidae). Breakdown prompted by moisture in cover crops and by flail
mowing greatly decreased the spring emergence of the moths. Thus, cover
cropping may enhance the control of one pest while exacerbating others.
This example illustrates the trade-offs implicit in different management
options. An ongoing comparison of organic and conventional almond orchards
by Lonnie Hendricks (1991) confirms that cover crops can be an important
tool in managing arthropod pests and their natural enemies.
Walnut
Sluss (1967) found that convergent lady beetle (Hippodamia convergens Guerin-Meneville) and Olla abdominalis (Say) (now termed 0. v-nigrum [Mulsant]) were the two most abundant Coccinellidae (Coleoptera) attacking walnut aphid (Chromaphis juglandicola [Kaltenbach], Homoptera: Aphididae) in Persian walnut orchards of northern California. Sluss believed that ground cover (of unspecified composition) can assist biological control by convergent lady beetle because it harbors aphids (which serve as alternate prey) from late February through
April or May. If ground cover
is disked or chopped too early, i.e., before walnut aphid is available
as food, beetles will disperse from the orchards. Such disengagement of
the predators may lead to subsequent eruptions of the aphid. Chopping
of understory cover killed many lady beetles, but some (including pupae)
survived. Olla v-nigrum, a principally arboreal lady beetle, tolerated
lower threshold densities of walnut aphid than did convergent lady beetle
(Sluss, 1967), and remained in trees during the late spring and early
summer, when the latter species dispersed to other habitats.
Apple
In an unreplicated trial
in northern California, Altieri and Schmidt (1985, 1986) found that over
a two-year period, codling moth infested 36.1 percent of the apples in
an organic apple orchard with a cover crop of bell beans (Vicia faba
L.), whereas a nearby clean-cultivated organic orchard suffered a
45 percent fruit loss from this pest. Bell bean has extrafloral nectaries
that attract beneficial insects.
Cherry
In California, mountain leafhopper
(Colladonus montanus [Van Duzee], Homoptera: Cicadellidae) transmits
a myco-plasma-like organism that causes buckskin disease (X-disease) of
cherry. This leafhopper reproduces on cool-season Medicago spp.
and Trifolium spp. (Fabaceae), which may harbor the pathogen (Purcell
et al., 1987). Thus, there is circumstantial evidence that cover crops
of these groups should be used only with care in cherry orchards. There
are, however, no field studies showing increased damage to cherry trees
with undersown leguminous covercrops (A. H. Purcell, pers. comm.). Apparently,
the legume must survive at least a full year to be an effective reservoir
of the pathogen, an atypical scenario for annual clovers and cool-season
medics.
Citrus and Avocado
Various grasses produce windblown
pollen, used as an alternate food by the predatory mite Euseius tularensis
Congdon (Acari: Phytoseiidae) (Kennett et al., 1979). This species
and others in the genus attack several pests of Californian citrus and
avocado, including avocado brown mite (Oligonychus punicae [Hirst],
Acari: Tetranychidae), citrus thrips (Scirtothrips citri Moulton,
Thysanoptera: Thripidae), citrus red mite (Panonychus citri [McGregor],
Acari: Tetranychidae), and scale crawlers (Homoptera: Diaspididae) (Congdon
and McMurtry, 1985). This mite can subsist and reproduce on diets of various
pollens (Zhimo and McMurtry, 1990). Deposition of windblown pollen during
late winter and early spring has been shown to be particularly important
in hastening the seasonal buildup of populations of these mites (Kennett
et al., 1979). In California, pollens from wind-pollinated tree species
(e.g., valley oak, Quercus lobata Nee', Fagaceae) are available
as early as March (Munz, 1973), whereas annual and perennial grasses produce
pollen mainly during April and May. Pollens are attractive foods (Zhimo
and McMurtry, 1990), and, if excessively abundant, might be expected to
divert the predators from the target pests. Of the grass pollens thus
far tested, none is a sufficient diet for E. tularensis to sustain
peak reproductive rates for more than one generation. The thick exine
coats of grass pollens may render them less suitable as foods than other
pollens (e.g., apple pollen) (Ouyang et al., 1992).
Management Techniques
In California, legal and
market pressures and the development of pesticide-resistant pests are
motivating farmers to explore cover cropping as at least a partial alternative
to insecticides and acaricides. Californian orchardists are exploring
the use of cover crops to provide key resources for predators and parasites
of orchard pests, and to arrest generalist pests, such as Lygus spp.
and flower thrips (Frankliniella spp.).
The options being explored
include the use of: 1) resident vegetation that harbors abundant beneficial
arthropods; 2) alternate strip management of cover crops, including, planting,
mowing, and tillage schemes designed to ensure temporal continuity of
habitat for both beneficial and pest arthropods; 3) commercial "insectary
mixes" of plants that are attractive to beneficial arthropods; 4)
mowing and deposition of clippings as mulch to harbor generalist predators.
The last-named option is poorly understood in orchard systems.
Strip Management
The understory vegetation
in an orchard need not be managed uniformly. Different zones maybe treated
differently; we term this "strip management," because the different
treatments are usually applied linearly, and the different understory
zones appear as bands or strips running through an orchard. Strip management
of cover crops may entail: 1) sowing cover crops of different floristic
composition in different strips; 2) mowing strips at different times;
3) tilling strips at different times; and 4) combinations of 1,2, and
3. Sowing of different mixes leads to stands with differing statures and
phenologies, affording diverse resources to pest and beneficial arthropods.
A complex of stands having differing floristic compositions could remain
attractive to arthropods for longer periods of time. Paradoxically, mowing
can be used either to kill or to rejuvenate a cover crop. Close mowing
of annual cover crops in full flower may kill the cover crop. Plant species
having indeterminate growth habits can be mowed relatively high and before
peak blossoming to extend attractiveness to both beneficial and pest arthropods.
High mowing preserves vegetative and flower buds and permits regrowth
of many species. By contrast, tillage usually destroys cover crops.
Strip management can permit
a grower to attain multiple goals and balance competing aims, such as
reducing competition from the cover crop or liberating nutrients for the
trees, while maintaining habitat or allowing cover crops to reseed. Byway
of illustration, a walnut orchardist in northern California (R. Lester,
Winters, California) mows or tills strips of woollypod vetch (Vicia
villosa Roth ssp. dasycarpa Ten., cv 'Lana') and common vetch
(Vicia sativa L.), while leaving alternating remnant strips to
reseed the entire alley. After seed of vetches has matured in May, flowering
will resume if soil moisture is sufficient. Thus, arthropod habitat can
be retained through time.
A prune grower (T. Turkovich,
Winters, California) sowed alternating alleys to different mixtures. One
set of alleys received a mixture of woollypod vetch, common vetch, and
oat (Avena sativa L.). The other set of alleys was seeded to a
mixture of crimson clover (Trifolium incarnatum L.), rose clover
(Trifolium hirtum All.), subterranean clovers (Trifolium subterraneum
L.), barrel medic (Medicago truncatula Gaertn., cv 'Jemmalong'),
rattail fescue (Vulpia myuros L., cv 'Zorro'), and soft chess (Bromus
mollis L., cv 'Blando'). At two-week intervals beginning in late June,
every third alley is mowed high. Thus, maturation of seed is permitted,
cover crop regrowth (1)y vetches only) is staggered, and resident warm-season
vegetation gradually supplants the cool-season annual complexes. Beneficial
insects, such as lady beetles, ants, and parasitic Hymenoptera persist
in the understory under this regime (R.L. Bugg, pers. obs.). Outbreaks
of two-spotted spider mite (Tetranychus urticae Koch, Acari: Tetranychidae)
have been avoided since 1991, when this scheme was adopted. Prior to this,
in the spring of 1990, cover crops were mowed closely throughout the orchard,
and a spider mite outbreak ensued, necessitating an application of acaricide.
Insectary Mixes
Proprietary mixes of reputed
"insectary crops" are now commercially available from several
sources. Several of the plants in these mixes are known to attract beneficial
arthropods of interest to orchardists; others have not been assessed formally.
Few have been evaluated in critical experiments on orchard biological
control. The authors suggested that most of the plants employed attract
wide ranges of entomophagous arthropods, and that increasing numbers of
farmers are using insectary cover crops.
Conclusions
Various cover crops harbor
distinctive complexes of beneficial and pest arthropods, and diverse trophic
relationships have been well documented in the literature. More study
is required to determine: 1) whether cover cropping modifies orchard microclimate
and target-crop nutritional status, thereby influencing pest dynamics;
and 2) whether and how cover crop species composition, spatial interspersion
of species, and management (irrigation, mowing, and tillage) affect build-up
and movement of arthropods, and resultant pest damage to the target crop.
Formal replicated trials are essential to evaluate these issues in the
context of overall orchard management.
References
Altieri, M.A. and L.L. Schmidt.
1985. Cover crop manipulation in northern California orchards and vineyards:
Effects on arthropod communities. Biol. Agric. Hortic. 3:1-24.
Altieri, M.A. and L.L. Schmidt.
1986. Cover crops affect insect and spider populations in apple orchards.
Calif. Agric. 40(1,2):1S-17.
Barnett, W.W., L.C. Hendricks,
W.K. Asai, R.B. Elkins, D. Boquist and C.L. Elmore. 1989. Management of
navel orangeworm and ants. Calif. Agric. 43(4):21-22.
Congdon, B.D. and J.A. McMurtry.
1985. Biosystematics of Euseius on California citrus and avocado
with the description of a new species (Acari: Phytoseiidae). Int. J. Acarol.
11:23-30.
Hendricks, Lonnie. 1991.
Sustainable almond comparison update. Components 2(1):1-3. UC Sustainable
Agriculture Research & Education Program.
Kennett, C.E., D.L. Flaherty
and R.W. Hoffman. 1979. Effect of windborne pollens on the population
dynamics of Amblyseius hibisci (Acarina: Phytoseiidae). Entomophaga
24:83-98.
Munz, P.A. (in collaboration
with D.D. Keck). 1973. A California Flora (with Supplement by RA. Munz).
University of California Press. Berkeley, Calif.
Ouyang, Y., E.E. Grafton-Cardwell
and R.L. Bugg. 1992. Effects of various tree and grass pollens on development,
survivorship and reproduction of Euseius tularensis (Acari: Phytoseiidae).
Environ. Entomol. 21:1371-1376.
Purcell, A.H., J.K. Uyemoto,
R.A. Steenwyk, W.R. Schreader, K.G. Suslow and B. Kirkpatrick. 1967. Buckskin
disease of cherry. Calif. Agric. 41(3,4):26-27.
Sluss, R.R. 1967. Population
dynamics of the walnut aphid Chromaphis juglandicola (Kalt.) in
northern California. Ecology 48:41-58.
Stern, V.M., A. Mueller,
V. Sevacherian and M. Way. 1969. Lygus bug control through alfalfa interplanting.
Calif. Agric. 23(2):8-10.
Stern, V.M., R. van den Bosch,
T.F. Leigh, O.D. McCutcheon, W.R. Sallee, C.E. Houston and MJ. Garber.
1967. Lygus control by strip cutting alfalfa. University of California
Agricultural Extension Service, AXT-241. 13 pp.
Zhimo, Z. and J.A. McMurtry.
1990. Development and reproduction of three Euseius (Acari: Phytoseiidae)
species in the presence and absence of supplementary foods. Exper. Appi.
Acarol. 8:233-242.
For more information write
to: Robert L. Bugg, Information Group, University of California Sustainable
Agriculture Research and Education Program, University of California,
Davis, CA 95616-8533, U.S.A. and Carol Waddington, TriCal Biosystems,
P.O. Box 1327, Hollister, CA 95024-1327, U.S.A.
(DEC.458)
Contributed by Robert
Bugg
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