Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

4.16 Trichoplusia ni 345
Voria ruralis Dip.: Tachinidae
Adults mate soon after eclosion and oviposition commences about 9 days
later. Eggs laid on the host surface hatch within a minute and young larvae
penetrate the cuticle and enter a muscle fibre. After about 3 days at 24¡C the
larva pierces a hole in the dorsal wall of the host abdomen through which it
inserts its posterior spiracles into the air. After rapid growth of the parasitoid,
the host dies and the parasitoid larva pupates within the host integument
(Thompson 1915; Brubaker 1968). When V. ruralis oviposits on 1st instars,
development is slower and mortality higher than in later instars, except the
late 5th instar. Development was rarely completed when eggs were laid on
5th instars, unless they were laid on newly moulted individuals. Females laid
an average of 310 eggs (Elsey and Rabb 1970a). Development time from egg
to puparium ranged from 5.4 to 12 days, depending upon the temperature,
and for the pupa 7 to 8 days at 24¡C. Time from egg to adult varied from 19.4
days at 20¡C to 10.7 days at 30¡C (Brubaker 1968; Jackson et al. 1969).
Parasitisation by V. ruralis causes large larvae to eat less than normal (an
average of 47% reduction (Soo Hoo and Seay 1972). Up to 85%
parasitisation was observed in field cages, depending upon the numbers of
mated V. ruralis released, with significant superparasitism at high parasitoid
densities (Soo Hoo et al. 1974).
V. ruralis is one of 3 major parasitoids of T. ni on crops in Florida (Martin
et al. 1982) and cotton in Arizona (Werner and Butler 1979) but was present
only to the extent of 0 to 0.1% in larvae on lucerne in New Mexico (Gordon
et al. 1987). In northwestern USA, it was the only parasitoid recovered and
occurred in 0 to 14% of T. ni larvae (Biever et al. 1992). In Virginia,
V. ruralis was present in 27% of larvae in 1981 and 17% in 1982
(Chamberlin and Kok 1986).
V. ruralis can survive, develop in, and emerge from virus infected larvae.
However, it does not act as a vector, except occasionally as a mechanical one
under very restricted conditions (Vail 1981).
V. ruralis is a widespread parasite and has been recorded as far north as
Finland and as far south as Trinidad. It has been recorded from a range of
Lepidoptera. In the United States it is known mainly from larvae of various
Noctuidae, especially T. ni, but less frequently from the beet armyworm,
Spodoptera exigua, and other associated species (Jackson et al. 1969; Ehler
1977a). Ehler and van den Bosch (1974) considered V. ruralis to be host
specific to T. ni in Californian cotton.