Ecology Of Croppest Interactions In Intercropping And Cover Cropping

The interaction of weed species, crop type and the presence of insect pests has been summarized by Schellhorn and Sork (1997). Where the weeds are closely related botanically to the crop type, for example cruciferous weeds in a Brassica crop, these will encourage specialist feeding insects such as flea beetles. Where the flora is a mixture of botanical types, this depresses the numbers of specialist feeding insects and encourages more generalist feeders. Botanically mixed flora also encouraged natural insect predators such as coccinellids, carabids and staphylinids; in turn, these could be responsible for the reduction in the presence of specialist feeding pests such as imported cabbage worm (small cabbage white butterfly; P. rapae) and diamond back moth larvae (Plutella xylostella). Polycultures of cash, inter-row and cover crops often support fewer insect pests at lower densities compared with those found in monocultures (Risch, 1981). Various biotic, structural and microclimatic factors in multispecies plant communities work synergistically in producing pest control. Root (1973) suggested two ways by which this could be achieved. The first is the 'Enemies Hypothesis' which predicts that the increased abundance of insect predators and parasites in species-rich associations can better control herbivore populations. Richer plant associations supposedly supply a more favourable environment for predators and parasites, reducing the probability that they will leave or become locally extinct.

These conditions include: (i) greater temporal and spatial distribution of nectar and pollen sources - both of these attract natural enemies and increase their reproductive potential; (ii) increased ground cover provided by a more diverse environment, particularly valuable to nocturnal predators; and (iii) improved herbivore richness, providing alternative hosts and prey when other hosts and prey are scarce or at inappropriate stages of their life cycle.

The second hypothesis is the 'Resources Concentration Hypothesis' (Root, 1973) and involves changes in the behaviour of the herbivorous insects themselves. Visual and chemical stimuli from the host and non-host plant affect both the rate at which herbivores colonize habitats and their behaviour in those habitats. The total strength of attractive stimuli for any particular herbivore species determines what Root (1973) called 'resource concentration' and it is the result of the following interacting effects: (i) number of host species present and the relative preference of the herbivores for each; (ii) the absolute density and spatial arrangement of each host species; and (iii) interference effects from the non-host plants.

An herbivorous insect will have greater difficulty locating a host plant when the relative resource concentration is low. Relative resource concentration may also influence the probability of the herbivore staying in a habitat once it has arrived. For instance, a herbivore may tend to fly sooner, farther or straighter after landing on a non-host plant than a host plant, resulting in a more rapid exit from those habitats with lower resource concentrations. Finally, reproductive behaviour can be affected, for example, when a herbivore tends to lay fewer eggs on host plants in an environment of lower resource concentration.

The results of Risch's experiments suggest that the 'Resource Concentration Hypothesis' is correct as opposed to the 'Enemies Hypothesis'. The factors of importance were differences in levels of beetle colonization and residency time.

The evidence for this was as follows:

  • There were no differences in parasitism or predation of beetles among treatments - hence no 'enemy' effects.
  • There was higher beetle emergence from monocultures than polycultures in only one instance (Acalymma thiemei (Baly) from squash), and even in this case the difference was not large enough to account for the observed difference in adult abundance.
  • The number of beetles per host was lower in the polyculture than the monoculture only when there was a non-host present in the polyculture. When two host plants were present in the polyculture and no non-host plant, the numbers of beetles per host plant were in fact higher in the polycultures than in the monocultures. This pattern of abundance could be predicted only if beetle movements, and not mortality due to natural enemies, primarily determined beetle abundance.
  • Field measurements of beetle colonization and experimental studies directly showed that differences in resource concentration between monocultures and polycultures affected patterns of beetle distribution.

The impact of ground cover mulches on yield and quality is illustrated in Table 6.10.

In the particular case of brassicas, cover crop mulches affect insects by interfering visually or olfactorily with host plant selection, thus reducing pest dispersal, reproduction and colonization of Brassica crops. Beneficial insects such as ground beetles are favoured by reduced tillage. It was concluded that rye mulch, for example, offers significant levels of weed suppression for the most critical stages of cabbage and diminishes the populations of several important insect pests. These improvements, however, are at the expense of yield losses due to difficulties in crop management. The lower populations of diamond back moth (P. xylostella), imported cabbage worm (small cabbage

Table 6.10. Effects of ground covers on the quality of cabbage (Brassica oleracea var. capitata) heads.

% Heads unmarketable

Table 6.10. Effects of ground covers on the quality of cabbage (Brassica oleracea var. capitata) heads.

% Heads unmarketable

Total number

% Heads

Ground cover



Tip burn



Bare ground


















1Worm = small white butterfly (Pieris rapae) caterpillars.

2Includes heads damaged by thrips, black rot and other pests and pathogens.

Means separation by Duncan's multiple range test; means followed by the same letter are not significantly different at P = 0.05.

After Roberts and Cartwright (1991).

white butterfly, P. rapae) and aphids in rye mulch may have been related to the much smaller size and lower head weights of the crop plants. Cabbage planted in rye mulch and treated with Bt-insecticide (Bacillus thuringiensis) had the lowest insect damage ratings of any of the treatments, but yields were still less than those obtained by conventional tillage. A major yield constraint in the rye residue treatments was probably initial soil compaction and later competition with rye and red clover. Soil compaction was caused by equipment movement on wet soil necessary to mow the cover crop prior to planting of the cash crop.

Cabbage aphid (Brevicoryne brassicae) is a major pest of broccoli (B. oleracea var. italica) (Chapter 7); it colonizes the developing florets rendering them unmarketable. The effects of living mulch on aphid abundance are directly proportional to the amount of inter-row vegetation present; the aphids colonize more heavily plants surrounded by bare soil compared with those planted in vegetation (A'Brook, 1964, 1968; Gonzales and Rawlins, 1968; Costello, 1994), as illustrated in Table 6.11.

Flea beetle populations are generally lower on brassicas in weedy habitats compared with bare ground monocultures. This is possibly related to their movement and host-finding behaviour. Flea beetles are extremely mobile and their host-finding ability is impeded by non-host odours (Tahvanainen and Root, 1972). Non-host foliage may inhibit movement, resulting in faster leaving rates and lower colonization rates in living mulch plots. The response of the aphid B. brassicae to mixtures of host and non-host has been even more consistent than that of Phyllotreta cruciferae. Compared with monocultures, aphid populations were lower in cole crops with weeds in experiments in both the USA and the UK.

The response of P. rapae to mixtures of host and non-host plants has been variable in both the USA and UK experiments. The specific relationship between the physical and chemical structure of the cropping system and the precise host-

Table 6.11. Effect of living mulches and bare ground culture on insect population densities in cabbage (Brassica oleracea var. capitata) cv. Excel.

Cropping system - living mulches

Creeping Kentucky Kent wild white

Insect and stages Bare ground bentgrass bluegrass clover

Phyllotreta cruciferae

Adultsa 70.0 (4.3) 36.1 (7.7) 31.5 (5.4) 55.0 (7.7)

Pieris rapae

Eggsb 7.45 (1.45) 9.05 (2.16) 9.00 (1.14) 5.35 (1.59)

Larvaec 1.65 (0.45) 2.65 (0.17) 2.85 (0.35) 2.25 (0.43)

Population density = numbers per plant; data are the means of four replicates with standard errors in parentheses.

aPopulation density on cabbage in bare ground was significantly greater than on the living mulches (P = <0.0001).

bPopulation density on cabbage on bare ground was not significantly different from that on the living mulches; the density on cabbage in clover was significantly lower than on cabbage in grass (P = 0.05).

cNot significant.

searching behaviour of P. rapae may be critical, since the oviposition behaviour of P. rapae is sensitive to plant size and development, plant water content and plant dispersion. Clover inhibited oviposition in the late summer generation of P rapae but had no effect on oviposition in the mid-summer generation.

Cabbages grown in a polycropped system showed less infection by thrips (Thrips tabaci) in terms of pest incidence and reduced population size and, in consequence, lower levels of physical damage were sustained. Limitation of damage by cabbage root fly (Delia radicum) and caterpillar (mainly Mamestra brassicae with smaller populations of P. rapae and P. xylostella and the occurrence of Plusia gamma in one experiment only) was quite substantial. The rates of infestation of heads by cabbage gall midges (Confarinia nasturtii) were low and evenly distributed across treatments. Feeding damage from flea beetles (Phyllotreta spp) was low and concentrated in the monocropped plots. These effects are illustrated in Table 6.12.

Living mulches compete with cabbage plants for resources within 2 weeks of transplanting. Living mulches could not control flea beetle populations below economic thresholds by themselves. Commonly used early season chemical treatments for flea beetles might be eliminated when living mulches are used. The lower incidence of insect pests, however, may be offset by cabbage yield reductions from competition between cabbage and living mulch. The market may demand smaller heads, however, and the increased quality noted where Kent wild white clover was used could be an added incentive to using these husbandry systems (Table 6.13).

Table 6.12. Causes of non-marketability in white cabbage (Brassica oleracea var. capitata) grown in monocropping and polycropping husbandry systems.


Table 6.12. Causes of non-marketability in white cabbage (Brassica oleracea var. capitata) grown in monocropping and polycropping husbandry systems.




T. repens

T. subterraneum

Cabbage root fly












Cabbage gall midge




Flea beetles




Cabbage aphid




Figures with different letters in rows are significantly different at P <0.05; small percentages have not been taken into account. After Theunissen et al. (1995).

Table 6.13. Effect of cropping systems using living mulches on cabbage (Brassica oleracea var. capitata) cv. Excel yields and quality.

Cropping system - living mulch

Table 6.13. Effect of cropping systems using living mulches on cabbage (Brassica oleracea var. capitata) cv. Excel yields and quality.

Cropping system - living mulch

Kent wild






Yield and quality






Marketable head

size (kg/head)

1.59 (0.07)

1.21b (0.12)

1.37 (0.04)

1.43 (0.12)

1.15b (0.08)

Harvest datea

22 September

6 October

14 October

6 October

14 October

% Marketable heads

92.5 (4.8)

75.0 (14.4)

87.5 (9.5)

100 (0.0)

55.0 (11.9)

Mulch dry wt (g/m2)


674 (134)


314 (78)b

407 (90)b

aPlanting date = 2 July bSignificantly different from bare ground by Duncan's procedure (P <0.05). After Andow et al. (1986).

Intercropping cabbage and beans reduced oviposition by brassica root flies (Delia radicum and D. floralis) by 29% compared with monocultures (Hofsvang, 1991). There was also a reduction in oviposition when the crops were mixed with 'weeds', where reductions of 63 and 40% in eggs per cabbage plant were recorded in two seasons. Such effects are summarized in Table 6.14.

In summary, reduced tillage in combination with cover crop mulch systems can conserve beneficial insects. For example, predatory wasps nest in the ground and tillage interferes with their reproduction. Cover crop mulches may also reduce pest dispersal, reproduction and colonization of host plants. Plant compounds released by cover crop residues may influence host plant

Table 6.14. Studies on the effect of plant diversity (intercropping/undersowing/weeds) on oviposition by Delia radicum.

Reduction in oviposition compared with

Table 6.14. Studies on the effect of plant diversity (intercropping/undersowing/weeds) on oviposition by Delia radicum.

Reduction in oviposition compared with

Plant diversity

monoculture (%)



Brussels sprout/cauliflower/



Demster and Coaker



Brussels sprout/clover



O'Donnell and Coaker


Brassicas/beans, spinach,



Coaker (1980)

clover, grass




Ryan et al. (1980)

Brussels sprout/spurry



Theunissen and Den

Ouden (1980)




Tukahirwa and Coaker


Rape/clover, weeds



Coaker (1988)

After Hofsvang (1991).

After Hofsvang (1991).

selection for oviposition and larval feeding. Cover crop mulches can confuse pests visually or olfactorily, reducing colonization of Brassica crops. Important visual cues for insects are leaf colour, area and visual prominence of the hosts. Twice as many cabbage root fly eggs were found on green and yellow models compared with red or blue ones. Cabbage root fly landings increased linearly with host leaf area. Diamond back moth has a strong preference for egg laying on dark green hosts.

Tillage had a significant effect on cabbage maggot and diamond back moth incidence. Larger numbers of both pests were associated with the tillage treatments compared with non-tillage treatments. Rye or hairy vetch can, however, reduce cabbage yields. Rye residues have a high carbon to nitrogen ratio and their decomposition could immobilize soil nitrogen, thereby reducing cabbage yields.

In several studies, particular soil properties were improved by cover cropping, and weed and insect pest populations were reduced but yields also fell. Yield reductions could have resulted from the immobilization of soil nitrogen, lower soil temperatures or allelopathy. Strip tillage, which cultivates the row where the brassicas are planted leaving residues between crop rows intact, may overcome reduced Brassica vegetable yields while combining with the advantages of both conventional tillage and cover crop mulch systems as identified by Mangan et al. (1995) (Table 6.15).

This information can be used to predict how mixtures of plants can be used to reduce pest infestation and damage. So far, it has been difficult to

Table 6.15. Incidence of cabbage maggot eggs and diamond back moth larvae in relation to tillage and cover cropping.

Numbers per plant

Cabbage maggot eggs Diamond back moth larvae

Treatment (Delia radicum) (Plutella xylostella)








Rye + vetch







No cover







Factorial F testa

Cover (C)



Tillage (T)






a NS, i.e. P> 0.05; * P< 0.05; ** P< 0.01.

Site = South Deerfield, Massachusetts, USA; 3 X 2 X 2 factorial experiment., cover crops = rye (Secale cereale) + vetch (Vicia villosa); rye alone; no cover crop; tillage = conventional = cover crop tilled in and no till = cover crop mowed; herbicide = DCPA (dimethyl tetrachloroterephthalate). After Mangan et al. (1995).

make such predictions in relation to the manipulation of Brassica habitats. If animal behaviour is the key to such predictions, however, then it may be possible to identify herbivore abundance in novel horticultural systems. This requires a basic understanding of the pest and predator's natural history (habitat preference, diet and general behaviour). This information can be used to suggest the taxonomic and structural plant diversity needed to achieve a degree of resistance developed by placing Brassica crops in association with other plants.

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