Reprinted from HORTSCIENCE, Vol. 14(1), February 1979
A publication of the American Society for Horticultural Science, Mt. Vernon, Virginia
The Potential Use of Weeds in the
Manipulation of Beneficial Insects'
Miguel A. Altieri2 and W. H. Whitcomb
Department of Entomology, University of Floridla, Gainzesville, FL 327611
Additional in~dexu words. pest management
Certain weed species play an important role in the biology of many beneficial
insects. Relevant examples from the literature show that many weeds contribute to
the population regulation of various insect pests of crucifer crops, beans and or-
chards. These examples emphasize the potential of weeds for implementing pest
management systems. Although there is enough information concerning weed
manipulation methods, there is a need for more formal research on how to en-
courage the presence of beneficial weeds in crop fields for the purpose of increasing
entomophagous insect populations. Demonstration of improved biological control
in weed diversified cropping systems represents the initial step toward improved
pest management systems based on ecological principles.
Subject to attempted exclusion from
agroecosystems, weeds are thought of
primarily as competitors with crops
which cause subsequent yield reductions
(11). Weeds are also focused upon as
hosts of insect pests, however certain
weed species play an important role
in the biology of many beneficial
insects (69). The objectives of this
review are: 1) to illustrate the potential
of certain weeds to serve as alternate
food sources for beneficial insects,
and 2) to propose, from an insect pest
management viewpoint, weed manage-
ment strategies based upon the response
of beneficial weeds to both environ-
mental factors and cultural practices
which encourage acceptable population
levels of certain beneficial weed species.
Weeds contribute to the population
regulation of various insect pests of
crucifer crops (Brassica spp.) and
beans (Phaseolus vulgaris L.) (2, 50,
58, 65). The achievement of such
regulation is hypothesized to be caused
by complex interacting factors such as
an increase in associational resistance
(58), reduction of crop apparency (18)
and an increase of beneficial insects
(2, 9, 15, 20).
The manipulation of weedy habitats
I Received for publication August 30, 1978
F~lorida Agricultural Experiment Station Jour-
nal Series No. 1382.
The cost of publishing this paper was de-
frayed in part by the payment of page charges.
t for postal regu ations this adpe esmust
solely to indicate this fact.
2Graduate student and Professor of Entomo-
logy. Support from Tall Timbers Research
Station, Tallahassee, Florida and critical
review by Dr. S.H. Kerr, Dr. Jerry Stimac,
Dr. J.D. Doll, Dr. Ray William, Mr. J. Luna
and Mr. Rod Gillmore and gratefully acknow.
results of several studies demonstrate
additional requirements for amino acids
and carbohydrates from plants (10,
24, 40 63). These nutrients usually
are provided by nectar and pollen and
sometimes through leaf feeding and sap-
feeding (63). Pollen and nectar may be
provided by either the crop plant such
as cotton (Gossypium hirsutum L.)
(49) and peaches (Prunus persica
B. and H.) (52), or more commonly,
from weeds within the crop or in
Wolcot (75) described how success-
ful establishment of the parasitoid,
Larra am~ericana Sause, introduced into
Puerto Rico from Brazil to control the
cricket, Scapteriscus vicinlus Scudder,
depended on the presence of two weeds,
Borreria verticillata and Hyptis atro-
ruben~s. These weeds provided nectar
for the adult wasp. Where these weeds
were absent or scarce, the wasp failed
Van Emden (68) demonstrated that
certain Ichneumonidae, such as Meso-
chorus spp. must feed on nectar for egg
maturation, and Leius (39, 40) reported
that carbohydrates from the nectar of
certain Umbelliferae are essential in
normal fecundity and longevity in three
Ichneumonid species. Syme (64), in
studies of the parasitoids of the European
pine shoot moth, (Rhyacionlia buoliana
(Schiff.) showed that fecundity and
longevity of the wasps, Exeristes com-
stockii (Cressen) and Hyssopus thymus
Girault were significantly increased with
the presence of several weeds. Syme
emphasized the role of weed nectar
sources in allowing the adult parasitoids
to bridge a critical period when the host
Umbelliferae flowers tend to be
preferred by hymenopterans. For ex-
W. H. Whitcomb and M. A. Altieri
surrounding crops appears to be bene-
ficial because of improved synchroniza-
tion between populations of pests and
their natural enemies (41, 46, 68).
Although there is considerable know-
ledge in weed manipulation methods
and weed population thresholds, there
is a lack of information on how to
encourage the presence of specific
weed species within crop fields for the
purpose of increasing entomophagous
WEEDS AS SOURCES OF BENE-
Supplementary food from pollen and
Although insect prey provide the
diet for most entomophagous species,
HORTSCIENCE, VOL. 14(1), FEBRUARY 1979
constitute important elements to in-
creasing the herbaceous insects and
associated predators and parasitoids.
The entomofauna of several weeds
which occur in several crops in many
parts of the world have been exten-
sively surveyed. For example, Goeden
and Ricker (22) reported the phyto-
phagous insect fauna of several species
of Ambrosia in California. Stegmaier
(61, 62) listed some insects associated
with Eupatorium coelestinum L. and
ragweed (Ambrosia artemisiifolia L.)
in Florida. In Colombia, Figueroa
(19) observed 172 insect species on 70
weeds. Needham (45) studied the in-
sects on the flowers of spanish needle
(Bidens pilosa L.) and Romney (57)
the insect community associated with
pepperweed (Lepidium alyssoides L.)
Taxonomic observations such as those
mentioned above comprise a basic step
in developing strategies to increase the
activity of certain parasites, especially
when generations of the host and
parasite fail to coincide. For example,
the effectiveness of the tachinid Lydella
grisesens Robinesis-Desvoidy, a parasite
of the European corn borer (Ostrinia
nubilalis (Hubner)) can be increased
in the presence of an alternative host,
Papaipema nebris Guenee, a stalk borer
on giant ragweed (Ambrosia spp.)
Several other authors have reported
that the presence of alternate hosts on
ragweeds near crop fields increased
parasitism of specific pests. Examples
include Eurytoma tylodermatis Ashmead
against the boll weevil (Anthonomus
grandis Boheman) (49) and Macrocentrus
delicatus against the oriental fruit
month (Grapholita molesta (Bosch))
(8). The parasite Horogenes spp. used
the caterpillar of Swammerdamia lutulrea
(Haworth) on the weed Crataegus
sp. to overwinter each year after emer-
gence from diamondback moth (Plutella
maculipennis (Curt)) (69). A similar
situation occurs with the egg parasitoid
Anagrus epos Girault whose effective-
ness in regulating the grape leafhopper
(Erythroneura elegantula Osborn) was
greatly increased in vineyards near
areas invaded by wild blackberry
(Rubus sp.). This plant hosts an alter-
nate leafhopper Dikrella cruentata
(Gillette) which breeds in its leaves in
In New Jersey peach orchards,
control of the oriental fruit moth was
increased in the presence of ragweed,
smart weed (Polygonum sp.), lambs-
quarter (Chenopodium album L.) and
goldenrod (Solidago sp.). These weeds
provided alternate hosts for the parasite
Macrocentrus ancylivorus Roh.
When Johnson grass was allowed to
grow in grape ( Vitis sp.) vineyards in
California, there was a buildup of
alternate prey mites which supported
populations of the predatory mite
ample, in Japan Tiphia popilliavora
Roh., a parasite of the Japanese bee-
tle (Popillia japonica Newman) fed
solely on the blossoms of two Umbelli-
ferae, Libanotis ugoensis Keidz. and
Patrinia scabiosaefolia Fisch. (13). Im-
ported into the U.S., this parasite fed
on flowers of another Umbelliferae,
the wild carrot (Daucus carota L.)
(13). King and Holloway (37) empha-
sized that satisfactory establishment of
T. popilliavora required an abundance
of wild carrot at the release sites.
Van Emden (68), after studying 6
different habitats, found that the
proximity of certain flowering weeds
(Angelica sylvestris L., Urtica dioica
L. Rumex acetosella L., Taraxacum
officianale Weber, Cirsium vulgare (Savi)
Ten., Ranunculus repens L., Trifolium
repens L., Chenopodium album L.,
and Anthriscus sylvestris L.) increased
the activity of parasitic Hymenoptera
in wheat (Triticum vulgare Vill.),
and cabbage fields (Brassica oleracea L.).
Spectacular parasitism increase was
observed in apple (Malus domestic
Boreb.) orchards with rich under-
growths of wild flowers (41). Parasitism
of tent caterpillar eggs and larvae and
codling moth larvae was 18 times
greater in those orchards with floral
undergowths than in orchards with
sparse floral undergrowth (41). In
sugarcane (Saccharum officinarum L.)
fields of Hawaii, the tachinid par-
asitoid Lixophaga sphenophori Villen-
euve fed upon nectar from several weeds
of the family Euphorbiaceae (38).
Considerable work has been re-
ported by researchers of the Soviet
Union on the role of nectar plants in
increasing the effectiveness of biological
control agents. Telenga (66) informed
that Scolie defeani Lind. was attracted
to its grub hosts by sowing honey
plants, Phacelia and Eryngium. These
same plants have been shown to in-
crease the abundance of the wasp
Aphelinus mali (Haldeman) for the
control of apple aphids and improve
the activity of Trichogramma spp*
in apple orchards (67).
Soviet researchers at the Tashkent
Laboratory (66) cited lack of adult
food supply as a reason for the inability
of Aphytis proclia Wlker. to control
its host, the San Jose scale (Quad-
raspidiotus perniciosus Comst.). The
effectiveness of the parasitoid improved
as a result of planting Phacelia cover
crop in the orchards. Three successive
plantings of Phacelia cover crop in-
creased scale parasitization from 5% in
clean cultivated orchards to 75% where
honey plants were grown.
Russian researchers also noted that
Apanteles glomeratus L., a parasite
of 2 species of cabbage worms (Pieris
spp.), on crucifer crops, obtained
nectar from mustard flowers. The
parasite lived longer and laid more
eggs when these, weeds were present.
When qu ck-flowering mustards were
planted in fields of cole crops, parasi-
tization of the host increased from ten
to sixty percent (44).
Nectar sources appear to have a
role in parasite survival during periods
of low host density. In such cases
many female Hymenopteran parasites
(i.e. Pteromalidae) enter a long resting
period during which they eat only
carbohydrates (21). These nutrients are
usually supplied by nectar and pollen
from weeds growing in the area (39).
Weed flowers are also important
food sources for various insect predators
(69). Sugar feeding requirements have
been demonstrated in some species of
lacewings (Chrysopa spp.) (16, 24).
This requirement is satisfied by aphid
honeydew or nectar feeding. Killington
(36) reported a preference of lacewings
for flowers of the Compositae. Pollen
appears to be instrumental in egg
production of many Syrphid flies and
is reported to be a significant food
source for many predaceous Coccinell-
idae (67). Several aphidophagous cocc-
inellids, Coccinella spp., Adalia bip-
unctata L., and Cycloneda sanguinea
L. have been reported to feed on the
extrafloral nectarines of peach trees
(52). Large amounts of pollen from the
blossoms of dandelion, Taraxacum
officinale Weber have been found in
the alimentary canal of the coccinellid
Coleomegilla maculata (De Geer) (52).
Putnam (52) emphasized the role of
weed flowers in allowing the predators
to survive the midsummer period when
hosts were scarce.
Pollen is an important alternate
food for certain predaceous phytoseiid
mites (30). One species, Amblyseius
hibisci Chant was able to develop
completely on pollen alone. Clausen
(12) mentions several other examples
of predaceous insects that feed on
plant materials at least in one period
of their life cycles.
Hagen (24) reported that cattail
(Typha sp.) pollen applied in vineyards
in California increased the abundance
of non-pestiferous tyiid mites which
served as important alternate sources
of prey for the strictly predaceous
mite Metaseiulus occidentalis (Nesbitt).
This allows M. occidentalis to survive
periods of low populations of the
target pest, the willamette mite (Eote-
tranychus willamettei Ewing).
Hosts of alternative prey
At times it is advantageous to in-
crease non-pestiferous herbivorous in-
sects in crop fields. Such insects serve
as hosts to entomophagous insects,
thus improving the survival and repro-
duction of beneficial insects in the
The weeds present in crop fields
HORTSCIENCE, VOL. 14(1), FEBRUARY 1979
Metaseiulus occidentalis which in turn
restrained the Pacific mite (Eiotetrany-
chus willamettei Ewing) to non-econom-
ic numbers (20).
Weeds with their numerous associ-
ated aphids serve as an important
reservoir of beneficial insects. For
example, Bombosch (9) found high
numbers of various predator species
feeding on aphids of different weeds.
Pastinaca sp. and Achillea sp. were
consistently attractive for coccinellids
In north Florida, the goldenrod
(Solidago altissima L.) supports more
than 75 different predator species
that feed on aphids of the genus Ulro-
leucon in spring (Altieri and Whitcomb,
unpublished data). Similarly, in England
a wide range of natural enemies fed on
the stinging nettle aphid (Microlophium
carnosum (Bukt.) (46). The proximity
of aphid infested nettles to bean fields
increased the numbers of Coccinellidae
on bean plants (4).
Hodek (27) suggested that by plant-
ing belts of honey plants, the control
of the bean aphid (Aphis fabae Scop)
could be improved. These plants were
hosts of economically indifferent spe-
cies of aphids which in turn were
alternative hosts of effective parasitic
wasps (Hymenoptera: Aphidiidae) of
Hemenway and Whitcomb (26) found
associations between weeds, leaf beetles
(Chrysomelidae) and ground beetles
(Carabidae) of the genus Lebia
Latreille. For example, Lebia grandis
Hentz was often found in association
with Leptinotarsa decemlineata Hentz
and the weed Solanum carolinense
L. Lebia atriventris Say was found on
camphorweed (Heterotheca subaxillaris
(Lam)) feeding on larvae and eggs of
Zygogramma heterothecae Linell. Lebia
analis Dejean was associated with Dis-
onycha glabrata Fabr. on Amaranthus
spp. Lebia viridis Say fed on the larvae
of Altica foiacea Lec. on two species of
evening primrose, Oenothera laciniata
Hill and O. biennis Linn. Practical
implications from these observations
become obvious since many of these
ground beetles are predators of impor-
tant crop pests (72). Lebia viridis
Say was observed consuming eggs of
corn earworm (Heliothis zea (Boodie))
(73). L. viridis was also observed prey-
ing actively on grapevine flea-beetles
in vineyards (31). Lebia grandis Hentz
preys upon the Colorado potato beetle
(Leptinotarsa decemlineatea Hentz) and
Lebia analis Dejean preys on immature
stages of fall army worm (Spodoptera
frugiperda (J.E. Smith)) and eggs of the
bollworm (26, 73). By introducing or
removing Solanum carolinense L. or
Amaranthus sp. from potato ( Solanum
tuberosum L.) or corn (Zea mays L.)
fields respectively, L. grandis and L.
analis can be encouraged or eliminated.
This fact might be important in other
crops such as beans, soybeans (Glycine
max (L.)) corn and alfalfa (Medicago
sativa L.) since persistent populations
of Lebia marginicollis Dej., L. viridis
and L. analis have been reported to
occur in such fields (72).
In general, most beneficial insects
present on weeds tend to disperse to
crops, but in a few instances the prey
found on weeds could prevent or delay
this dispersal (69). In such cases, allow-
ing weeds to grow to assure concentra-
tions of insects and then cutting them
regularly to force movement could
be an effective strategy. For example,
by cutting patches of stinging nettle
(Ulrtica dioica L.) in May or June,
predators (mainly Coccinellidae) were
forced to move into crop fields (46).
Similarly, cutting the grass weed cover
drove Coccinellidae into orchard trees
in southeastern Czechoslovakia (27).
By cutting hedges of Ambrosia trifida L.
infested with the weevil Lixus scrobi-
collis Boh, a 10 percent increase of
boll weevil parasitization by Eurytoma
tylodermatis Ashm. was obtained in two
test plots of cotton adjacent to the
hedgerow (49). These practices should
be carefully timed based on the biology
of beneficial insects For example,
in California the annual cleanup of
weeds along the edges of alfalfa fields
should be delayed until after mid-March
when aggregations of dormant Coccine-
11idae have largely dispersed (67).
POSSIBLE METHODS OF
This review lists several examples
which emphasize the role of specific
weeds in the biology of predators and
parasites. Encouraging the presence of
specific weeds in crop fields seems
a logical approach to improve biological
control of certain insect pests. Naturally,
careful manipulation strategies need to
be defined in order to avoid weed
competition with crops and inter-
ference with certain cultural practices.
In other words, economic thresholds
of weed populations need to be defined>
and also factors affecting crop-weed
balance within a crop season should be
clearly understood (5).
Changes in the composition and abun-
dance of weeds in crop fields can be
accomplished by various methods of
Changes of the levels of key chemical
constituents in the soil
Hoveland et al. (29), studying in
Alabama the response of various weeds
to different levels of P and K con-
cluded that the local weed complex
can be indirectly affected by the manip-
ulation of soil fertility. Fields with low
soil K were dominated by buckhorn
plantain (Plantago lanceolata L.) and
curly dock (Rumex crispus L.), whereas
fields with low soil P were dominated
by showy crotalaria (Crotalaria spec-
tabilis Roth.), morning-glory (Ipomea
purpurea (L.) Roth.), sicklepod (Cassia
obtusifolia L.), Geranium carolinianum
L., and coffee senna (Cassia occidentalis
Corn grown in unfertile soils in
north Florida are dominated by
Richardia scabra L. and Diodia teres
Walt. Fertilized fields are dominated
by Cassia obtusifolia L. (Altieri and
Whitcomb, unpublished data). In heavily
fertilized fields in southern Illinois,
Ambrosia artemisiifolia L. plants at-
tained heights of more than 1.3 m,
whereas in poorly fertilized fields they
were usually no more than 0.6 m high
(6). Dry matter production and height
of Portulaca oleracea L. rose with an
increase in the level of applied nitrogen
in Phillipines (5). These results show
that different soil nutrient levels may
also affect the vigor and productivity
of individual weed populations which
in turn can influence insect populations.
For example, nitrogen fertilization of
wheatgrass plots increased populations
of the plant bug Labops hesperius
Uhler, because the fertilized grass was
probably of better quality than uniferti-
lized grass (33).
Soil pH can influence the growth of
certain weeds. For example, weeds of
the genus Pteridium grow in acid soils
while Cressa sp. inhabits only alkaline
soils (44). Other species (many Com-
positae and Polygonaceae) grow in
saline soils (44).
Many weeds exhibit allelopathic inter-
actions with certain crops and other
weeds. By the release of chemical
compounds from other plants, the
abundance of certain plants can be
regulated in a field (56). For example,
some strains of cucumber (Cucumis
sativus L.) many inhibit the growth of
certain weed species by 87% (53). Fresh
foliage of Tagetes patula L., A maranthus
dubius Mart, Manihot esculenta Crantz,
and Phaseolus vulgaris L. can drastically
inhibit the germination of Sorghum
vulgare L. and Ipomoea hederifolla (L.)
under greenhouse conditions (3). Barley
(Hordeum vulgare) has been used as a
smother crop for weed suppression
(56). Although allelopathy has some
general implications for weed manage-
ment, there is a need for further research
in this area, before farmers might utilize
allelopathy to increase the competitive
ability of crops over co-existing weeds.
Continuous use of certain herbicides
Horowitz et al. (28) observed weed
population shifts after 4 years of
applying repeated herbicide treatments
in the same plot. Anagallis caerulea
HORTSCIENCE, VOL. 14(1), FEBRUARY 1979
Table 1. Species composition of pioneer weed communities of lands subjected to agricultural
disturbances in different regions of the U.S.
Region Pioneer weed species Reference
Colorado Setaria viridis L. (green foxtail) 32
Chenopodium album L. (lambsquarters)
Solanumn nigrum L. (black nightshade)
A maranthus retroflexus L. (red root pigweed)
Verbesina encelioides (Cay.)
North Carolina Piedmont Digitaria sanguinalis L. (crabgrass) 35
Erigeron canadensis L. horseweedd)
Diodia teres Walt.
Idaho Aristida dichotoma Mich. (three awn grass) 48
Salsola kall var tenuifolia Tausch (russian
Bromus tectonrm L. (brome: grass)
Michigan Ambrosia elatior L. (ragweed) 7
Polygonum convolvulus L. (wild buckwheat)
Lactuca scariola L. (prickly lettuce)
Agropyron repens L. (quackgrass)
Poa compressa L. (bluegrass)
Central basin of Erigeron strigosus Muhl 43
Tennessee Gnaphalium obtusifolium L. (fragrant everlasting)
Ambrosia artemisiifola L. (common ragweed)
South Carolina Rumex acetocella L. (red sorrel) 74
Oenothera laciniata Hill (evening primrose)
Linaria canadenis L. (toad-flax)
Specularia perfoliata L. (venus' looking glass)
Gnaphlalium purpureum L. (purple cudweed)
Festuca subaxillaris (fescue)
Southern Illinois Solanum carolinense L. (horsenettle) 6
Abro ia arteuTi iiola L. (common ragweed)
Schreb. was eliminated with the use of
substituted ureas, whereas the popula-
tion of bindweed (Convolvulus arvensis
L.) was increased. Hausen et al. (25)
reported reducing the populations of
yellow nutsedge (Cyperus esculentus L.),
crabgrass (Digitaria sanguinalis L. Scop.)
and cocklebur (Xanthium penn-
sylvanicum Wall), but increasing the
population of spurge (Euphorbia mac-
ulata L.) after several crop rotations and
chemical methods. In Colombia, Pied-
rahita and Doll (47) obtained a 21% in-
crease in broadleaf weed cover in
soybeans after four applications of the
herbicide linuron. Buildups of morning-
glory (Ipomoea tilliacea Willd) and
Cucumis melo L. are obtained by con-
tinuous applications of alachlor in corn.
In southwestern Germany birdseye
speedwell (V~eronica persica Poiret)
became dominant in plots annually
treated with MCPA or 2,4-D, whereas
Lamium purpureum L., chickweed
(Stellaria media L.) and bedstraw
(Galium aparine L.) all increased in
abundance after MCPA-treatments only
Perhaps one of the most valuable
tools to suppress certain weeds while
encouraging others is the use of herbi-
cides used in crop-weed competition
studies. Buchanan (11) published a list
of herbicides, and recommends rates
of application safe to use in growing
certain weed species in particular
crops. For example, by applying a
maximum rate of 0.6 kg/ha of trifluralin
preplantt incorporated), populations of
velvetleaf (Abutilon theophrasti Medi-
cus), jimson weed (Datura stramonium
L.), venice mallow (Hibiscus trionum
L.), and prickly sida (Sida spinosa L.)
can be grown in cotton and soybeans
without the presence of unwanted
weed species (11). Although most
examples cited by Buchanan (11)
concern the enhancement of noxious
weeds for weed control studies, the
method implicates possibilities for en-
hancing particular beneficial weeds to
achieve early increases of predator
Direct sowing of certain weeds
in the field
In experimental plots in Colombia,
Altieri (1) attempted to regulate the
total weed cover in bean and corn
fields by differential sowing of six
weed species. A mixture of constant
volumes of weed seeds was sowed in
each plot. This amount of seeds varied
according to the area to be covered, but
the proportions were kept constant.
Twenty days after planting, the soil
coverture was corrected by differential
Lack of water (rainfall) was a definite
limiting factor in attempting to grow 1-
meter wide borders of grass weeds (Eleu-
sinle indict (Gaertn) and Leptochloa
filiformis Lamb (Beauv.) around bean
fields (1). Once the borders became
established, the colonization and repro-
ductive efficiency of leafhoppers in
bean plots (Empoasca kraemzeri Ross &
Moore) were effectively reduced (2).
In another experiment, the establish-
ement of strips of Amaranthus dubius
Mart. between bean rows was not
accomplished when the weed seeds were
covered by a soil layer (1). Similarly,
when deeply buried seeds of foxtail and
smartweed species were uncovered, they
germinated freely (43). One-year-old
seeds of wild carrot and redroot pig-
weed (Amaranthus retroflexus L.) pur-
chased from California failed to germin-
ate in corn fields in north Florida (Altieri
and Whitcomb, unpublished data).
Putwain (54) introduced seeds of
sorrel (Rumex acetosa L.) and red
sorrel (R. acetosella L.) in grasslands
of North Wales obtaining high germi-
nation. However, one to two weeks
after germination a crash in seedling
population density was observed.
The application of this direct sowing
method demands that certain weed
seed germination requirements must be
carefully investigated. Most weed seeds
have specialized requirements for ger-
mination, and some remain in the con-
dition of enforced dormancy and will
not germinate until certain environ-
mental conditions become available. For
this reason it is often difficult to sow
weeds for experimental purposes (71).
Buchanan (11) published a complete
review on the germination requirements
of weed seeds and methods to stimulate
their germination. Among the most
useful methods are mechanical scarifica-
tion, sulfuric acid treatment, chemical
stimulators of germination (i.e. Potassi-
um nitrate) and stratification (period
of low temperature after ripening). Be-
fore sowing the viable weed seeds in
the field it is important to make sure
that the supplies contain pure seed so
that new weeds are not introduced.
Soil disturbance is of major impor-
tance in the initiation of secondary
successions in agricultural lands (43).
Plowing destroys the existing plant
cover, creating open niches for the in-
vasion of new plants.
The role of soil disturbance in weed
seed germination is not well understood
beyond the obvious effects of tillage
on soil moisture and aeration. The actu-
al number of seeds that germinate at a
particular time of the year as a result of
a specific season of disturbance is deter-
mined by the specialized conditions that
are necessary for germination of seeds in
various stages of dormancy (43). The
germination of the exposed weed seeds
after disturbance depends on the exist-
ing environmental conditions and the
dormancy state of the seeds (71).
HORTSCIENCE, VOL. 14(1), FEBRUARY 1979
Date of soil Herbivore(s) associated Predaceous arthropod(s)
disturbance that with the weeds that serve as associated with the
enhances the weed alternate prey to herbivore(s) on the weed
Weed species population various predators
1. Genothera laciniata Hill (early evening
primrose) August Altica sp. (leaf beetle) Lebia viridis Say (ground beetle)
2. O. biennis L.. (evening primrose) December Altica sp. L. viridis
3. Amaranthus sp. (pigweed) April Disonychaglabrata Fab. Lebia analis Dej. (ground beetle)
4. Heterotheca subaxillaris (Lam.) (camphorweed) October Zygogramma heterotheca (L.) Lebia arriventris Say (ground
(leaf beetle) beetle)
Sinea sp. (assassin bug)
Pselliopus cinctus Fab.)
Perillus bioculatus (Fab.)
Stiretrus anchorage (Fab.)
Peucetia viridans (Hentz)
Oxyopes salticus Hentz
Th~eridion sp. (spider)
5. Chenopodium ambrosioides L. (mexican tea) December Z. suturalis (F~ab.) and Callida decora Fab (ground beetle)
other leaf beetles L. viridis
Hippodamia convergens Guerin-Menev.
P. biocurltus and other stink bugs
Orchelimum sp. (long horned grass-
Tetragnatha laboriosa (Hentz) and
6. Solidago altissima L,. goldenrodd) December Urotecon spp. (Aphids) H. convergens and other
Condylostylus sp. and other
long legged flies
Micromus spp. (brown lacewing)
Chrysopa spp. (lacewing)
Podabrus sp. and other soldier
P. viridans and other spiders
Zelus cervicalis Stal. and
other predaceous Hemiptera
Where natural vegetation is disturbed,
the first pioneer plants arise from dor-
mant seeds stored in the soil rather than
from fresh dispersals (7, 35). Soil dis-
turbance may alter the microenviron-
ment of seeds in many ways. For ex-
ample surface germinators may simply
respond to better aeration or to expo-
sure to normal periods of daylength
In general, recently disturbed fields
are quickly covered by a variety of
annual plants. The species composition
of pioneering weed communities is
highly variable (6, 7, 35). Infinite com-
binations exist, depending upon the
seed source, season of disturbance, pre-
vious agricultural practices, moisture,
soil conditions, and other habitat
characteristics. The colonization pat-
terns are affected by various factors, but
seem to be governed mainly by the rate
of habitat colonization by each species
and by environmental changes caused
by colonizing plants themselves (35).
Table 1 reflects the considerable
variability in species composition of
the pioneer weed communities of dis-
turbed lands in different regions of the
Understanding the relationships be-
tween soil disturbance and weed suc-
cession is an important tool for insect
manipulation. For example, after ana-
lyzing the differences in species com-
position of early weed stages in different
fields of Idaho, Peimeisel (48) suggested
that insect populations could be con-
trolled by changing and/or improving
the plant cover. He found high popula-
tions of beet leafhoppers (Eutettix
tenellus) (Baker) in fields dominated by
russian thistle (Salsola kali var tenuif~folia
Tausch), but low numbers of this insect
in fields dominated by brome grass
(Bromus tectorum L.) Of further inter-
est are the studies of Menke (42) and
Kay et al (34) in North Florida which
show that the season in which the soil
is disturbed affects both the density
and species composition of the vegeta-
tion. Such changes exert important
influences in the population of herbi-
vorous insects. For example, popula-
tions of the leaf beetle Altica sp. were
numerous in plots plowed in August and
October because these treatments en-
hanced the abundance of this insect's
preferred food supply, the evening prim-
rose Genothera laciniata Hill (42, Altieri
and Whitcomb, unpublished data). Fur-
ther research by Altieri and Whitcomb
(unpublished data) in experimental
plots at Tall Timbers Research Station
(34, 42) in north Florida, showed con-
sistent associations between weeds, her-
bivorous insects and predaceous arthro-
pods (Table 2). Specific dates of plow-
ing affected the abundance of particular
weed species, which in turn affected the
abundance of particular weed species,
which in turn affected the abundance
of their associated herbivorous entomo-
fauna. These insects served as alternate
prey to many native predators which
responded numerically to the seasonal
increases of prey on specific weeds.
This review has emphasized the role
of certain weeds as a reservoir for bene-
ficial insects. The fact that weeds are
ever present within and around crop
Table 2. Selected examples of associations between herbivorous and
ticular dates of plowing in north Florida.*
predaceous insects occurring on specific weed species which respond to par-
*Altieri and Whitcomb (umpublished data)
Description of study, plots, treatments and methods are reported in the literature (34, 42).
HORTSCIENCE, VOL. 14(1), FEBRUARY 1979
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7. Bei sitlic gy 49 19524-9E logical suc-
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indicate that certain weed species
should be introduced into crop systems,
because these weeds have potential
for greatly increasing populations of
beneficial insects, thus improving the
biological control of pests. However'
there is an urgent need for information
on weed manipulation, especially con-
cerning methods on how to enhance
the presence of "useful weeds" and how
to distribute them in the field so that
they do not compete with crops. Pro-
viding weeds as borders around small
fields or as alternate strips within
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fields at different times of the year. By
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populations of certain weeds can be
increased in the field and the phy-
tophagous insects associated with these
weeds will be increased. It also results in
an increase of predators and parasitoids
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By proper manipulation (i.e. periodic
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forced to move from the weeds into
the crops and thus act upon economic
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area might provide important clues on
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tured to minimize pest incidence,
not only within the cultivated field but
also at the regional level. It should be
emphasized that an agroecosystem must
be defined large enough to include the
crops studied but also the surrounding
matrix of uncultivated land which
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of many entomophagous arthropods
(27, 67). Weeds provide much of the
plant diversity within and out of crop
atrate fobr edtn sits Pa d sthelt r
for many important beneficial insects
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