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Pest Status
Background
Chapter 16
Manduca quinquemaculata (Haworth),
Tomato Hornworm (Lepidoptera:
Sphingidae)
H.H. CHENG
The tomato hornworm. Manduca quillquemaculata (Haworth). is principally a pest of
tomato and tobacco plants in Canada. The distribution of this species is North and South
American. On tobacco. the first-instar and second-instar larvae make many small holes
in the leaves, later they eat larger areas of the lamina. and by the last instar the larvae
may strip the top and mid-leaves of the plants leaving only the midribs and main leaf
veins. In Canada. a moderate to high population level of this pest may cause 0.5% to
2.7% loss of the marketable tobacco in untreated fields (Cheng 1977a).
The tomato hornworm has one complete generation and part of a second generation
each year under southwestern Ontario climatic conditions. Only the first generation of
this species causes appreciable damage to tobacco. The insect overwinters as a pupa in
the soil. Moth emergence occurs from late June to September. The adults fly at dusk and
lay their eggs on the under side of the tobacco leaves during the same period. Eggs hatch
within four to six days and the larvae dig into the soil to a depth of 10 to 12 cm and pupate.
Most of the pupae remain in the soil until the following year. but some moths may
emerge from late August to September to start a second generation. Ten-year blacklight
trap studies showed that population levels of M. quillquemacu[ata in southwestern
Ontario are generally low to moderate (unpublished data).
The tomato hornworm can be successfully controlled using a number of different
chemical insecticides, however. the use of chemical insecticides on tobacco presents a
number of problems that may assume greater importance in the future. In particular.
applications of certain insecticides on tobacco during late July and August result in
insecticide residues (Cheng & Braun 1977). which have undesirable effects on the
quality of tobacco and may put the Canadian tobacco export trade in serious jeopardy.
Consequently. a search for and development of alternative control methods for hornworms
has been carried out during the past 20 years. Among the more promising of the
biological agents tested for the control of this pest are strains of the spore forming bacterium
Bacillus thurillgiensLr Berliner. which was found to be as effective as chemical
insecticides. and has been used successfully for controlling hornworms on tobacco
(Guthrie et al. 1959. Creighton et al. 1961. Begg 1964. Bucher & Cheng 1971, Cheng
1973. 1977b. 1978).
Two commercial preparations of B. tllllrillgimsis, Dipe1® and Thuricide®-HPC, have
been registered and recommended for use in controlling hornworms on tobacco since
1973 (Anonymous 1973). A survey conducted in southwestern Ontario in 1977, 1978. and
1979 indicated that about 20% of growers applied B. thuringiensis preparations for
control of horn worms. When aphids and hornwormsoccurred in the same period, most of
the growers would apply chemical insecticides or a tank mix combination of B. thurillgiensis
preparation for hornworm and a chemical insecticide for aphids because B. thuringiensis
preparations 'gave almost no control of aphids.
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