Km. Heinz et al., TOWARD PREDICTABLE BIOLOGICAL-CONTROL OF LIRIOMYZA-TRIFOLII (DIPTERA,AGROMYZIDAE) INFESTING GREENHOUSE CUT CHRYSANTHEMUMS, Environmental entomology, 22(6), 1993, pp. 1217-1233
Implementation of augmentative biological control is often hindered by
the inability to obtain accurate information on natural enemy release
rates that will ultimately yield a salable crop within economic const
raints. The purpose of this study was to develop and evaluate a comput
er model that attempts to predict augmentative releases of the parasit
oid Diglyphus begini (Ashmead) (Hymenoptera: Eulophidae) to use for bi
ological control against the serpentine leafminer Liriomyza trifolii (
Burgess) (Diptera: Agromyzidae) infesting greenhouse-grown cut chrysan
themums. The model attempts to determine the appropriate release rate
necessary to reduce leafminer densities below 1 larvae per 1,000 chrys
anthemum leaves within 40 d after planting, after which time aesthetic
ally important foliage forms on chrysanthemums. In the construction of
this deterministic model, three simplifying assumptions have been mad
e: (1) there is no insect movement between the greenhouse and outside,
(2) greenhouse temperatures are a constant 27-degrees-C, and (3) the
greenhouse represents a homogeneous environment. Model simulations ind
icated that successful biological control was unlikely when parasitoid
releases are initiated later than 14 d after planting regardless of t
he release rate. The simulations also demonstrated that D. begini rele
ase rates are not linearly related to L. trifolii densities, and that
a parasitoid release strategy is not simply the determination of a pro
per wasp/leafminer ratio. Validation studies of the model's prediction
s provided mixed results. Following the release rates generated by the
model, L. trifolii larval densities were not significantly greater th
an 1 per 1,000 leaves 40 d after planting. However, the model did not
always track the succession and magnitude of leafminer population fluc
tuations with statistical accuracy. Two factors probably contributed t
o these errors in prediction: (1) the assumptions inherent to the mode
l were not met during the validation trials, and (2) the leafminer sub
routine of the model could not accurately predict L. trifolii densitie
s in the absence of D. begini. In spite of these errors, leafminer dam
age to the harvested foliage was significantly lower in the treatments
receiving D. begini releases than in the control treatments. In addit
ion, when the model was tested in a commercial cut chrysanthemum green
house, L. trifolii was successfully controlled by releases of D. begin
i, resulting in the production and harvest of a salable cut chrysanthe
mum crop without the use of any pesticides. Finally, postvalidation re
construction of the model, to include temperature-dependent variation
in life history parameters, increased the predictive power of the mode
l.