Messelink, G.J.

English

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Biological control is more than enemy and prey

Wageningen Yield 

Crop protection
Fruit & Veg Tech 9.4 2009 -  www.Hor t iWorld.nl 19  
Biological control  is more 
than enemy and prey  
Biological control cannot simply be seen as a one-on-one relationship between the predators and their prey. Natural ene-
mies in combination with the pest organisms create a web of food within which all kinds of interactions occur that can 
influence the efficacy of control. This interaction creates more complexity, but at the same time offers opportunities to 
make a growing system more resistant to pest and diseases. A number of examples are provided here from our research 
at Wageningen UR Greenhouse Horticulture.
Biological pest control is part and parcel of modern horticulture. Vegetable growers already started using natural enemies en masse back in the 1980s. In the ornamental 
growing sector, biological control has gradually gained 
in importance and has even become essential in proper-
ly controlling certain pests. One of the consequences of 
this development is that pest control is becoming more 
complex. Several species of natural enemies are 
deployed at the same time. An extreme example is sweet 
peppers or aubergine grown organically, where an 
entire arsenal of ten different natural enemies is some-
times used. At the same time, a shift is becoming evi-
dent in the use of chemical agents from broad spectrum 
to selective, so the number of pests is increasing. 
While chemical agents are becoming more specific, the 
trend with biological methods is towards more general 
effectiveness. In the early years of biological pest con-
trol, people were more convinced that the best way to 
attack pests was with highly specific natural enemies. 
Over the years, however, users have started deploying 
more general predators that feed on several types of 
prey. The most successful generalists are found among 
the assassin bugs (Orius, Macrolophus) and the predatory 
mites. Generalists have the advantage of establishing 
more easily in crops as they can survive on a variety of 
foods, including pollen. The presence of generalist 
predators increases the resistance of crops to pests. 
It is precisely with the generalists that we see all kinds 
of interaction between predators and pests that can 
make biological control more complex. Controlling one 
pest can no longer be seen in isolation from controlling 
a different pest. Together with the plant, the pests and 
predators form a complex web of food with all kinds of 
direct and indirect interactions that may have both neg-
ative and positive effects on biological control. Greater 
insight can lead to the eradication of the negative inter-
actions and better utilisation of the positive ones. Using 
the food web model for cucumber as an example, we 
can illustrate some of the possible interactions, together 
with the main consequences for the success of biologi-
cal control. 
The cucumber model
Various pests can occur simultaneously on cucumbers. 
The western flower thrips is nearly always present. 
Spider mites, whitefly and cotton aphid also occur regu-
larly. The standard weapon against thrips is a generalist 
predatory mite, usually Amblyseius swirskii. This is also 
effective against whitefly and spider mites, although 
spider mites also have to be controlled with specialised 
predatory mites. A veteran is the specialised spider mite 
predator Phytoseiulus persimilis. Biological control of leaf 
aphid can be done using predatory wasps, such as 
Aphidius colemani against cotton aphid. The aphid midge 
Aphidoletes aphidimyza is a valuable assistant here. Both 
are rather specialised, namely limited to leaf aphids. 
By Gerben Messelink, Wageningen UR 
Greenhouse Horticulture
Empty aphid midge eggs and red A. swirskii.
Crop protection
Fruit & Veg Tech 9.4 2009 -  www.Hor t iWorld.nl 20  
Whitefly can also be efficiently combated using special-
ised predatory wasps, such as Encarsia formosa and 
Eretmocerus eremicus. Figure 1 shows an overview of the 
interactions currently known for this food web. The 
examples are based on research results from 
Wageningen UR Greenhouse Horticulture in projects 
financed by the Product Board for Horticulture and the 
Dutch Ministry of Agriculture, Nature and Food Quality.
Direct interaction between natural enemies
It is the generalist natural enemies in particular who do 
not just attack the pests they are introduced to control, 
but also attack other natural enemies. If they are of the 
same species, this is referred to as cannibalism. If they 
are other natural enemies of the same pest, it is referred 
to as intraguild predation. This is not by definition neg-
ative for the control of the pest. This is well illustrated 
in our model system for cucumber in controlling spider 
mites. The generalist A. swirskii can feed on young stag-
es of the spider mite specialist P. persimilis, so the popu-
lation has more difficulty in establishing. Trials run in 
greenhouses, however, showed that this had hardly any 
effect on the efficacy of spider mite control, as               
A. swirskii itself can also have a considerable impact on 
spider mites. In addition, the web of the spider mites 
protects P. persimilis; this predatory mite can move 
around easily under the web, while A. swirskii has diffi-
culty doing so. These two predators make a fine team 
together. 
It is even better if the natural enemies are mutually com-
plementary – for example, because they destroy pests in 
various stages of their development. In the case of 
cucumbers, A. swirskii (which feeds on the eggs of the 
white fly) is complemented well by predatory wasps that 
parasite on the older larval stages. This type of system is 
more resilient, as the one can replace the other in its 
absence (for example, if chemical agents have knocked 
out one of the predators). 
With other pests, however, generalist A. swirskii can dis-
turb biological control. The predatory mite has abso-
lutely no effect on leaf aphids, but they can feed on the 
eggs of the leaf aphid predator A. aphidimyza. Trials in 
sweet peppers in greenhouses showed that this delayed 
the suppression of the aphid population to an enor-
mous extent. Knowledge of these interactions is impor-
tant in determining which biological predators are used 
in certain crops.
Interaction between pests with a predator in 
common
The cucumber model shows that pests influence each 
other indirectly if they have a predator in common. In 
this case, thrips, spider mite and whitefly have an effect 
on each other via their predator A. swirskii. Greenhouse 
trials have revealed that the control of spider mites and 
whitefly improved considerably if thrips was also 
present. It was striking that the predatory mites devel-
oped better on a diet of two pests. Apparently, a combi-
nation of the pests gives the predators better nutritional 
value. In follow-up studies we will be investigating 
Figure 1. Overview OF pOssible interactiOns between pests and predatOrs On cucumbers.
Crop protection
Fruit & Veg Tech 9.4 2009 -  www.Hor t iWorld.nl 21  
P. persimilis in a spider mite web. A. swirskii preying on whitefly eggs.
whether the diet of the predatory mite in situations with 
a single pest can be supplemented using alternative 
food, helping to enhance its effectiveness. 
In the short term too, many pests can also have negative 
results. For example, an excess of whitefly appeared to 
temporarily hamper the control of thrips because of sat-
uration of the predatory mites.  Knowledge of this will 
help us understand and react to the complexity of prac-
tical situations. 
Interaction between pests via the crop
Pests that occur concurrently on the crops can compete 
for food (the plant), but they can also have a mutual 
influence via the plant if they feed on different parts of 
it. With cucumbers, the presence of larvae of the green-
house whitefly restricts the population development of 
spider mites on other parts of the plant by an average of 
25%. This is also called induced resistance. In our 
study, if the spider mite was subsequently controlled 
using spider mite predators, the effect was considerably 
better and faster on plants where spider mite develop-
ment had already been inhibited by whitefly. The pres-
ence of greenhouse whitefly therefore gave an indirect 
advantage in the battle against spider mites. In the fol-
low-up study, we will examine whether this plant reac-
tion can be induced in other ways to help improve bio-
logical control of spider mites. 
The next step
The examples we have mentioned for cucumbers show 
that pest control quickly turns into a complex food web 
with various mutual interactions (Figure 1). 
Understanding these interactions leads to an improve-
ment in biological control through better choices in the 
field or to the development of new measures based on 
this knowledge, such as developing alternative food for 
the generalists and benefiting from induced resistance. 
The mechanisms have a wider application and can sup-
port, for example, developments in the ornamental sec-
tor. Using natural enemies does not always simplify pest 
control, but it certainly offers new openings to develop 
resistant cultivation systems. n
 



Biological control is more than enemy and prey

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