associated with earlier harvesting dates for the broccoli crop. The lowest yielding treatment was the non-cover crop, non-tillage and no herbicide.
Incorporation of each cover crop treatment and herbicide use in tilled treatments hastened the maturity of the broccoli crop.
ECOLOGY OF CROP–PEST INTERACTIONS IN
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 Brassicacrops. 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
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.
oleraceavar. 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. rapaeto 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 6.10. Effects of ground covers on the quality of cabbage (Brassica oleraceavar.
capitata) heads.
% Heads unmarketable Total number % Heads
Ground cover heads/plot marketable Tip burn Worm1 Other2
Bare ground 19.6a 87.7ab 1.9a 4.9a 5.3b
Vetch 19.1a 83.0b 0.5ab 2.2a 14.3a
Rye 21.0a 92.0a 0.3b 1.9a 4.4b
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).
searching behaviour of P. rapaemay be critical, since the oviposition behaviour ofP. rapaeis sensitive to plant size and development, plant water content and plant dispersion. Clover inhibited oviposition in the late summer generation of P.
rapaebut 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 (Phyllotretaspp) 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 6.11. Effect of living mulches and bare ground culture on insect population densities in cabbage (Brassica oleraceavar. 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.
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 6.12. Causes of non-marketability in white cabbage (Brassica oleraceavar. capitata) grown in monocropping and polycropping husbandry systems.
Polycrop
Cause Monocrop T. repens T. subterraneum
Cabbage root fly 45a 23ab 19b
Thrips 56c 3d 12d
Caterpillars 17a 2b 4b
Cabbage gall midge 6 4 5
Flea beetles 6 0 0
Cabbage aphid 1 0 1
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 Kent wild
Bare Creeping Red white Idaho
Yield and quality ground bentgrass fescue clover clover
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) Nil 674 (134) 467b 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).
selection for oviposition and larval feeding. Cover crop mulches can confuse pests visually or olfactorily, reducing colonization of Brassicacrops. 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 6.14. Studies on the effect of plant diversity (intercropping/undersowing/weeds) on oviposition by Delia radicum.
Reduction in oviposition compared with
Plant diversity monoculture (%) Observations References
Brussels sprout/cauliflower/ 29 Eggs Demster and Coaker
clover (1974)
Brussels sprout/clover 60 Eggs O’Donnell and Coaker
(1975)
Brassicas/beans, spinach, 53–77 Eggs Coaker (1980)
clover, grass
Cabbage/clover 26–65 Eggs Ryan et al. (1980)
Brussels sprout/spurry 30–99 Infestation Theunissen and Den Ouden (1980)
Cabbage/spinach 36–44 Eggs Tukahirwa and Coaker
(1982)
Rape/clover, weeds 64–89 Infestation Coaker (1988)
After Hofsvang (1991).
make such predictions in relation to the manipulation of Brassicahabitats. 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.