892 M. Kostyukovsky et al. Insect Biochemistry and Molecular Biology 30 2000 891–897
tha dominica
Fabricius and
Sitophilus oryzae
Linnaeus are all economically important pests of stored products, which feed on a wide range of commodities
Arbogast, 1991. Repeated pesticide treatments have led to the increasing incidence of resistance and consequent
serious control problems of these species in certain cases Subramanyam and Hagstrum, 1996. The objective of
this study was to compare the efficacy of the juvenile hormone agonist methoprene which is in use as control
agent against insect pests of stored products, to the juv- enile hormone agonist pyriproxyfen, and two ecdysteroid
agonists, RH-5849 and tebufenozide.
2. Materials and methods
2.1. Insects The test insect strains were reared in the laboratory
over 10 years without any contact with insecticides. Sito- philus oryzae and Rhyzopertha dominica were reared on
whole soft wheat, and Tribolium castaneum was reared on wheat flour. The actellic-resistant strain of T. cas-
taneum was obtained by raising the laboratory strain on a culture medium with gradually increasing concen-
trations up to 4.5 ppm of the organophosphorus insecti- cide, actellic 50 e.c. pirimiphos-methyl, for around
20 generations. The insects in the last five generations were raised on 4.5 ppm of actellic without increasing the
concentration. All these insects were maintained in 1 l glass jars with paper covers and bred at 27
± 0.5
° C,
70 ±
5 r.h. and light:dark cycle of 8:16 h. 2.2. Insecticides
Four compounds were used in our experiments: Methoprene,
[1,isopropyl2E,4E-11-methoxy-3,7,11- trimethyldodeca-2,4-dienoate], supplied by Zoecon Cor-
poration, California, USA. Pyriproxyfen, 2-[1-methyl-2- 4-phenoxy-phenoxyethoxy]pyridine, supplied by Sum-
itomo Chemical Corporation, Ltd., Japan. RH-5849: dibenzoyl
hydrazines, 1,2-dibenzoy,1-tert-butyl
hydrazine, and Tebufenozide, 3,5-dimethylbenzoic acid 1-1,1-dimethylethyl-2-4-ethylbenzoylhydrazide, both
supplied by Rohm and Haas Corporation, Spring House, PA, USA.
2.3. Treatments Stock solutions of the tested IGRs in acetone were
prepared at a concentration of 5 mgml. Appropriate amounts of the compounds to be tested were dissolved
in 10 ml acetone100 g medium and were thoroughly mixed with the food of each of the target insects. Food
treated with acetone alone was used as a control. Care was taken that all acetone had evaporated from the
treated and control samples, by mixing and ventilating the culture medium for 24 h, before insects were added
to all treated food. For each treatment 20 adults aged 14–18 days after emergence from the pupae, to allow
mating, when used. The insects were placed in a 100 ml jar with 20 g of wheat flour T. castaneum or whole
wheat R. dominica and S. oryzae. All insects were kept at 25
± 0.5
° C and 70
± 5 r.h. at a 8:16 h light:dark cycle.
After a period of 15 days during which the insects were allowed to oviposit eggs, the parental adults were
removed and their mortality was recorded. The emerged F
1
adults were counted twice a week and removed until emergence was complete. The developmental course and
external morphology of larvae, pupae and adults of T. castaneum were observed. In the cases of R. dominica
and S. oryzae, only live adults found externally were recorded, since the larvae and pupae develop inside the
grain kernels. The standard error of the life span means and numbers of F
1
adults were calculated. Significance of differences was analyzed using ANOVA test
P,0.05. The LC
50
and LC
90
were calculated according to POLO-PC probit analysis LeOra Software, 1987.
3. Results
