Olga E. KRIPS-1, Peter E.L WILLEMS-1, Marcel
DICKE-1, Maarten A.
POSTHUMUS-2
1- Department of Entomology, Wageningen Agricultural University, P.O.
Box 8031, 6700 EH Wageningen.
2- Department of Organic Chemistry, Wageningen Agricultural
University, Dreijenplein 8, 6703 HB Wageningen.
In the interaction between herbivorous spider mites and their natural enemies the production of volatiles by the host plant plays an important role. Plants from several species produce volatiles in response to spider mite damage. Predatory mites, which are the natural enemies of spider mites, are attracted to these volatile blends. These spider-mite-induced volatiles play a key role in the arrestment of predatory mites in a herbivore-infested patch and thereby seem to be essential in the local extermination of spider mites by their predators.
Gerbera jamesonli is an ornamental crop of which a large number of cultivars is grown commercially. Previous research by our group has revealed large differences between these cultivars in suitability for population increase of the spider mite Tetranychus urticae. This paper adresses the question whether cultivars that differ in suitability for spider mites are equally suitable for the predatory mite Phytoseialus persimilis, which is the main predator of T. urticae. We have investigated (i) whether the population growth rate of P. persimilis is influenced by host-plant resistance to T. urticae and (ii) whether cultivars differ in the production of volatiles that attract P. persimilis.
(i) Life-history studies were performed with P. persimilis to
determine the intrinsic rate of population increase, rm, on three
gerbera cultivars that differed largely in resistance to F. urticae.
The rm was nearly identical on the three cultivars, varying from 0.41 to
0.43 per day.
(ii) The chemical composition of spider-mite-induced volatiles of
several gerbera cultivars has been determined. All cultivars produce
a large number of volatiles in reaction to spider mite damage. Many
of these volatiles are produced in very large quantities. The volatile
blends do not differ qualitatively among cultivars and the blends of
all cultivars tested are attractive for the predatory mite
P. persimilis. All cultivars produce three terpenoids and a
phenolic compound which previously have been shown to attract predatory
mites when they are offered individually to the predators.
Large quantitative differences between the cultivars exist in the production
of the terpenoid compounds trans-beta-ocimene and 4,8-dimethyl-1,3(E),7
nonatriene, both compounds that attract predatory mites. The gerbera
cultivars also differ in the production of a number of other terpenoid
compounds and some phenolic compounds.
In conclusion, our data show that the population growth rate of the predatory mites is not influenced by host-plant resistance to spider mites. However the quantitative differences found in the composition of the volatile blends suggest that gerbera cultivars may differ in their ability to arrest predatory mites in a spider-mite-infested patch. This may influence the success of the predatory mites to exterminate its prey. This will be the subject of forthcoming investigations.