Alebeek, F.A.N., van

Belder, E., den

Elderson, J.

Rijn, P., van

Vlaswinkel, M.E.T.

English

Wageningen Yield 

Perspectives for functional agro biodiversity in Brussels sprouts
Paul van Rijn1, Eefje den Belder2, Janneke Elderson2, Marian Vlaswinkel3, Frans van 
Alebeek3
1: Netherlands Institute of Ecology (NIOO, Heteren, p.c.j.vanrijn@uva.nl), 2: Plant Research 
International (PRI, Wageningen), 3: Applied Plant Research (PPO, Lelystad).
Abstract: The commercial production of Brussels sprouts generally involves a high input of 
insecticides. Conservation biological control in this crop is hampered by the diversity of economic 
pests involved, and by the high economic losses associated with failing pest control. Within the Dutch 
Functional Agro Biodiversity (FAB) project, other methods of non-chemical pest control have also 
received attention. One of these methods is the identification and containment of local sources of 
winter pest propagation. Another method to prevent pests from entering the crop may be the growing 
trap plants in the field margins. Some possible trap plant species have been identified, but more studies 
are required to show the feasibility of this method. Finally, monitoring insecticide-free sprout plots 
with flowering field margins showed that during summer, natural enemies can contribute considerably, 
although not always sufficient, to the control of cabbage aphids and caterpillars. For cabbage whitefly
some natural enemies have been identified, but in 2007 they arrived with too little and too late to have 
sufficient impact on the fast growing whitefly populations. 
Key words: cabbage pests, conservation biological control, field margins, trap crops, winter host 
plants, regional pest control, natural enemies, diamondback moth, cabbage whitefly
Introduction
In a regional pilot on enhancing natural control in Dutch arable and vegetable cropping, called 
the Functional Agro Biodiversity (FAB) project (see Van Rijn et al., this volume), the focus 
was on three crops: wheat, potato and Brussels sprout. In Brussels sprouts the development 
and adoption of biological control methods are complicated by the high crop value and the 
number and nature of the pest species involved. The more specific problems and approaches
related to this crop are discussed in this paper.
In Brussels sprouts pest species come from a range of insect orders: in our region 
especially aphids and whiteflies (Brevicoryne brassicae, Myzus persicae, Aleyrodes 
proletella), moths (Plutella xylostella, Mamestra brassicae), and (root) flies (Delia radicum). 
In some years, thrips and slugs may also cause significant crop losses. To control these pests 
(as far as possible) the farmers used to treat the plants with Imidacloprid before planting, and 
spray the fields 8 or 9 times a year with other insecticides (e.g. Lambda-cyhalothrin and 
Dimethoate). Within the framework of FAB three approaches have been addressed that may
contribute to reduce the amount of insecticides applied: (1) Reducing pest pressure from local 
winter refuges, (2) trap cropping, and (3) enhancing biological control of pests. Whereas the 
first approach is discussed in a separate paper by Den Belder et al. (this volume), the other 
approaches are discussed below.
Trap plants for cabbage pests
One method to reduce the pest pressure onto a crop is to intercept pest insects on trap plants
around the field before entering the crop. In order to develop and test this method, several 
plant species have been tested in the field as potential trap plants for diamondback moth and 
cabbage whitefly.
For diamondback moth (P. xylostella) two trap plant species were already known from 
literature: yellow rocket (Barbarea vulgare) and Indian mustard (Brassica juncea cv 
scimitar). Field cage studies (Badenes-Perez et al. 2004) have shown that both species are 
preferred as oviposition substrate over cultivated cabbage. Yellow rocket has the additional 
advantage of being unsuitable as host plant for the larvae, making it a rare example of a dead-
end trap plant. 
In the field Indian mustard appeared to be unsuitable due to its short life cycle 
compared to the growing season of Brussels sprouts. Yellow rocket, on the other hand, 
remains low during the first year and does not flower before the second year. We did find
some Plutella eggs on these plants and only few small larvae, as was expected. Due to 
experimental problems we were unable to check the impact on pest pressure in the adjacent 
field, so further studies are required to confirm its suitability as trap crop.
For cabbage whitefly (A. proletella) no studies on trap plants were available yet. The 
selection of plants to be tested was based on a list of reported host plant species (NPAG 
2001). We selected 10 species distributed over four plant families of which seeds were 
available. In 2007 each of these 10 species were sown or planted in two 9m2 meter plots 
adjacent to an experimental sprouts field. 
The well-known host plant, Chelidonium majus, failed to germinate, as well as two 
other species. From the remaining 7 species the 4 non-cruciferous species and the cruciferous 
wallflower (Erysimum cheiri) did not attract any whiteflies, despite the high density of 
whiteflies in the adjacent field. Chinese cabbage, B. campestris var. chinensis, attracted some 
whiteflies, but only young kale plants, B. oleracea var. acephala, attracted many whiteflies. 
The latter plant may therefore be an effective trap crop, especially since killing off the pest on 
this host plant appeared to be feasible: Treating the plants with an experimental insecticide 
against cabbage whitefly killed 95% of the eggs. 
Conservation biological control
Parasitoids are probably the main natural enemies of cabbage moths and butterflies, whereas 
(cabbage) aphids are also attacked by the larvae of hoverflies, lacewings and gall midges. As 
these natural enemies solely feed on sugar sources or pollen during their adult stage, they may 
benefit from growing suitable flowers in the field margins. 
Materials and methods
Within the FAB project a 3 meter wide annual flower strip was sown adjacent to each
target field (see Van Rijn et al., this issue). The flower species were selected for their 
suitability in providing (floral) food for the natural enemies and the low risk of supporting 
pests. The mixture included Buckwheat, Borage, Common Vetch, Coriander, Fennel, 
Cornflower, and Corn Marigold. To match the flowering period with the long growing period 
of Brussels sprouts the mixture was sown in May and supplemented with short Sunflowers.
The fields were sampled every 3 weeks in a fixed grid at various distances from the 
edge. At each monitoring row 20 Brussels sprouts plants were inspected, recording the pests 
and their natural enemies (if possible) per species and life stage. For early detection of 
diamondback moth, pheromone emitting delta traps were put in and around sprouts fields and 
checked every 2 weeks. The main results were quickly communicated with the farmers, to 
help them with pest management decisions. In addition to commercial fields, small 
experimental sprouts field with field margins have been created where no chemical 
insecticides were applied after planting, in order to evaluate the impact of natural enemies.
Results and conclusions
In the commercial fields a regular treatment with insecticides appeared to be 
inevitable. In the absence of selective pesticides, this left very little room to benefit from the 
field margins or from natural pest control in general. 
In the experimental fields the numbers of natural enemies (especially hoverflies, gall 
midges and parasitoids) were, consequently, much higher than in the commercial fields. By 
the end of August, however, their numbers declined rapidly each year. In the experimental
fields cabbage aphid (B. brassicae) levels remained low or at least stable during June, July 
and August. The numbers per plant fluctuated around 10 and 25 in 2004 and 2005 
respectively. In 2007, when the plants were treated with Imidacloprid before planting, the 
average density was even less that one per plant. However, in all years the populations started 
to increase exponentially in early September, causing economic damage to the sprouts by 
November. The resurgence of the cabbage aphids in September when natural enemies, such as 
hoverflies, virtually disappear, suggests that natural enemies can play an important role in 
keeping the aphids under control during summer, although not always at a sufficiently low 
level, when Imidacloprid cannot be applied (see also Van Rijn et al., 2006).
Of all caterpillars, those from diamondback moth were the most numerous, especially 
in 2006 (when no experimental field was available) and 2007. In all years and months about 
50% of the pupae appeared to be parasitized (mainly by Diadegma semiclausum). The 
experimental field was treated an few times with a Bt product, when the infection tended to 
surpass the action threshold. Ultimately no economic damage from this species to the sprouts 
has been observed.
Cabbage whitefly is a growing pest problem in the last five years in the Netherlands.
This species seems unaffected by natural enemies, and by any insecticide registered for this 
crop. Two parasitoids have been identified so far: Encarsia tricolor, which occurred at very 
low percentages only, and Encarsia inaron, which have been observed in high numbers
locally in Belgium only. 
Laboratory studies with insects from commercial rearings (Koppert BV) confirmed 
that most predators are hampered by the wax on the surface the Brussels sprout plants
(Eigenbrode 2004), especially when released by the whiteflies. For the predatory mite 
Amblyseius swirskii and the bug Orius majusculus their movement and attachment was 
strongly hampered by the wax. The larvae of the green lacewing Chrysoperla carnea could 
cope with the wax slightly better and were able to feed on the eggs and nymphs of the 
whitefly. A proportion of the larvae was even able to develop into adulthood on this prey. The
legless larvae of the hoverfly Episyrphus balteatus, are apparently not hampered by the wax at 
all. These larvae were able to kill many whitefly eggs and some nymphs each day, and to
develop into pupae and adults on this diet. Moreover, adult females were triggered to oviposit
on plants with whiteflies, this in contrast to clean plants.
Field observations confirm these laboratory studies. Hoverfly eggs and larvae could be 
found on all sprout plants with high numbers of cabbage whitefly, even when aphids were 
absent. When some of the larvae were reared to adulthood, they yielded both E. balteatus and 
Platycheirus peltatus. Eggs and larvae of green lacewings could also be found on these plants. 
However, their numbers were too low, compared to the fast growing numbers of whitefly, to 
have a notable impact on this pest.
Discussion
When using Functional Agro Biodiversity as a means for pest control we can consider not 
only methods to augment natural enemies, but also methods to diminish pests directly. The 
reduction of pest refuges that can act as sources of reinfestation is one example at the 
landscape scale. Trap cropping is another example at the field level. 
Implementation of the first method may be difficult as it requires concerted action of 
various growers in the region. The second method may be applied only after some technical 
issues have been solved. This require serious studies on e.g. (1) the attractiveness of trap 
plants relative to the crop at different stages of development, (2) the level of pest reduction 
within the crop that can be obtained and (3) the type of pest management needed to prevent 
secondary spread of the pest.
The conservation of natural enemies can only be effective when pesticides that are 
harmful for natural enemies are not or only incidentally applied. In a crop such as Brussels 
sprouts, where many pests have to be controlled at the same time, this is a challenging task. 
When measures at the landscape and farm level to support natural enemies and diminish pest 
pressure, are effective for some pests only, we may consider the efficient production and 
release of natural enemies against other pests, as well as the development of more predator-
friendly (glossy) cultivars. 
Acknowledgements
The FAB project was financed by the Dutch Ministry of Agriculture, Nature Management, 
and Food Quality (LNV), the Ministry of Housing, Spatial Planning and the Environment 
(VROM), the Dutch Horticultural and Agricultural Organisations (Productschap Tuinbouw 
and Hoofd Productschap Akkerbouw) and the Rabobank NL.
References
Badenes-Perez, F.R., Shelton, A.M. and Nault, B.A. 2004. Evaluating trap crops for 
diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae).  Journal of Economic 
Entomology 97: 1365-1372.
NPAG (New Pest Advisory Group), 2001. Aleyrodes proletella, brassica whitefly. NPAG 
Data Sheet [www.pestalert.org].
Van Rijn, P.C.J., Kooijman, J. & Wäckers, F.L. 2006. The impact of floral resources on 
syrphid performance and cabbage aphid biological control. IOBC/WPRS Bulletin Vol. 
29(6): 149-152.
Eigenbrode, S.D. 2004. The effects of plant epicuticular waxy blooms on attachment and 
effectiveness of predatory insects. Arthropod Structure & Development 33 (1): 91-102. 


Aleyrodes proletella, brassica whitefly.

Evaluating trap crops for diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae).

The effects of plant epicuticular waxy blooms on attachment and effectiveness of predatory insects.

The impact of floral resources on syrphid performance and cabbage aphid biological control.

Perspectives for functional agro biodiversity in Brussels sprouts

Perspectives for functional agro biodiversity in Brussels sprouts

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