Integrated Control Of The Alfalfa Weevil, Hypera Postica (Gyllenhal) (Coleoptera: Curculionidae), In Ontario

ABSTRACT

INTEGRATED CONTROL OF THE ALFALFA WEEVIL, HYPERA POSTICA

(GYLLENHAL) (COLEOPTERA: CURCULIONIDAE), IN ONTARIO

John F. Abu, Ph.D. Supervisor:

University of Guelph, 1976 Professor C.R. Ellis

The distribution of the established parasitoids of the alfalfa

weevil, Hypera postica (Gyllenhal) in western Ontario was determined

during 1973 to 1975. Potasson luna (Girault), a parasitoid of eggs,

was found in 13 out of 17 counties surveyed, but parasitism was only

0.7-17-0$. Tetrastichus incertus (Ratz.), a parasitoid of the larva,

was recovered from only two sites and parasitism was less than k.0%

at both places. Bathyplectes curculionis (Thomson), another parasitoid

of the larva, was widespread and parasitism ranged from to 75-0$.

Microctonus aethiopoides Loan, a parasitoid of the adult weevil, was

recovered at only one site where parasitism was 33.3% in 19739 56.6%

in 197^9 and 76.6% in 1975.

The relationships between the alfalfa weevil and each of its two

major parasitoids in Ontario, B. curculionis and M. aethiopoides, were

investigated. Parasitism by B. curculionis was only 6.3-33.3% at the

peak of the host population, but increased to 56.0-68.0% later in the

season, when host population was low. The incidence of diapause as well

as winter survival of B^ curculionis were highest among parasitoid

cocoons obtained towards the end of the season. The populations of the

first generation adults of M. aethiopoides was synchronized well with

those of the overwintered adult weevils, resulting in parasitism of

76.6-82.0%. The second generation of the adult parasitoids and the

summer adult weevils overlapped for about two weeks and parasitism

was 1+1.6-60.0$. Some adults of M. aethiopoides were present in the

field when hosts were unavailable and this may have significance in

the establishment of the parasitoid at new release sites. Parasitism

by M. aethiopoides did not significantly affect the overwintering

mortality of the alfalfa weevil.

The toxicity of five insecticides, carbofuran, carbaryl, malathion,

phosmet, and methoxychlor was determined by topical application

in the laboratory on adult B. curculionis, adult M. aethiopoides,

third-instar larvae of H . postica, third-instar larvae of H. postica

parasitized by B. curculionis, adult H. postica and adult H . postica

parasitized by aethiopoides. The effect of these five insecticides

in an integrated control program was assessed, using selectivity ratios

calculated as the of the parasitoid divided by the of weevil

larva or adult. Each of the insecticides was more toxic to the two

parasitoids than to the host larvae but, comparatively, carbofuran and

carbaryl were 2 to 3 times safer to the parasitoids than malathion,

phosmet, or methoxychlor. Carbofuran, the only insecticide which was

selective in favour of both parasitoids over the adult weevils, had

selectivity ratios 2 to i times those of malathion, phosmet, carbaryl,

and methoxychlor.

Parasitized larvae and adults of H^_ postica were more susceptible

to the insecticides than were their non-parasitized counterparts and

most of the parasitoid larvae died within the susceptible hosts. The

weevil larvae were much more tolerant to all the insecticides than were

the adults.

Based on the selectivity ratings, carbofuran and carbaryl are

preferable to malathion, phosmet, or methoxychlor in the integrated

control of the alfalfa weevil. Because adults are more susceptible

than larvae and since there is a period in the spring when adult

weevils are active in the field while adult parasitoids are not,

the feasibility of chemical control against the adult weevil should

be investigated further. Such a recommendation could have application

in alfalfa seed production where early cutting is not an

alternative and control is usually required each year.