TITLE: Establishment and Dispersal of Ichneumon promissorius, Exotic Pupal Parasitoid of Heliothis/Helicoverpa (FY: 1993-1995)
INVESTIGATORS: T J Kring, Dept. of Entomology, Univ.
of Arkansas, J E. Carpenter, USDA-ARS-IBPMRL, Tifton, GA, S.
D. Pair, USDA-ARS-SPA, Lane, OK
Introduction
The tobacco budworm, Heliothis virescens,
and the bollworm, Helicoverpa zea, are prey/host to a wide range
of beneficial insects attacking their egg and larval stages. However, our
earlier studies have shown that most (>95%) of these insects which are
able to pupate (underground), successfully emerge into moths. No parasitoids
of bollworm pupae have been collected in the US and parasitoids of budworm
pupae are extremely rare. This is unusual since 20-40% heliothine pupal
parasitism is encountered in other countries. In 1993 we initiated a program
to import and release a promissodus beneficial insect which would attack
budworm/bollworm pupae. We selected (Actual size) the known pupal parasitoid,
Ichneumon promissorius (from Australia). The very early phases of
the program (actually before 1993) documented the host range of the natural
enemy, to assure that only budworm/bollworm and their close relatives would
be attacked by the species. USDA-APHIS-PPQ issued permits (902180, 938756,
32007) to move the insect into the southern region from Australia. The
goal of the program was to permanently establish the beneficial insect
in the US. Once established, we expect the species to be able to reduce
pupal survival by ca. 20%.
Methods
A significant amount of effort and funding was required
to maintain cultures of these beneficial insects and their hosts. Ichneumon
promissorius searches the soil surface for heliothine pupation sites,
burrows into the pupal gallery, and oviposits into the host pupa. A solitary
adult wasp emerges about 15 days after oviposition. Laboratory studies
revealed that this parasitoid can be reared easily on several native species,
although budworm/bollworm complex are considered the most suitable. Bollworm
pupae were used to maintain bollworm and I. promissorius parent
colonies. Normal maintenance required 500 bollworms/day (2,500/week), but
during the peak period for releases, we produced 2,000-2,500 hosts/day,
6 days a week, or about 14,000/week. Production costs (materials and labor)
have been about 50 cents per released wasp.
Releases were made on commercial farms in Arkansas
from 1993-1997, and in Georgia, Oklahoma and Texas during 1993-1995. Mated
I. promissorius were released in ear-stage field corn on a weekly
basis, beginning with the first observations of bollworm pupation (mid-June),
for each of the five years of the project (which continued with other funding
after the SRIPM project ended in 1995). We released adults of the species
in corn because it harbors the largest uniform bollworm population and
is not the target of significant insecticide applications. A multiple release
strategy within and over subsequent seasons is recognized as providing
the greatest chance for establishment (DeBach 1974). Both sexes were released
after mating in the laboratory. We began the summer with smaller releases
and increased to larger (1,500- 2,000/week) releases when bollworm pupal
populations are highest in June and early July. Releases averaged 20,000
individuals/summer, ranging from 10,000 to 3 5,000/yr. Releases were made
by allowing mated adults to disperse directly from emergence cages. Released
insects were visually monitored for activity (searching for hosts or dispersal).
Any beneficial insect must be completely established
in an area before a formal evaluation of the impact of the parasitoid can
be made. However, it is important that establishment is monitored by attempts
to recover the parasitoid. Recoveries of this species can be made by direct
collection of the adult (it is a large, striking insect easily distinguished
from others) during the season of release or collection of immatures by
digging up budworm/bollworm pupae. This latter technique provides the best
evidence of parasitism in a certain location and allows calculation of
a percentage of the population parasitized. However, this technique required
careful exhumation of the host pupae from under 3-5 inches of soil in the
field. As there is no visual indication of where a bollworm pupa is buried,
large areas of the field were carefully excavated. Because this was an
extremely labor-intensive (and costly) process, we developed an alternative
technique. We developed a trap which is specific to the exotic parasitoid.
These traps are modified sticky Delta-type pheromone traps baited with
virgin I. promissorius females. Because these insects use a sex
pheromone to locate the females for mating, wild males are attracted and
trapped. Presence of males is an indication of successful parasitism by
released individuals, and provided an indication of the dispersal distance.
Additionally, positive trap catches of males in the year(s) following release
were verification of successful overwintering of the species and successful
spring parasitism (only females overwinter and only males are trapped).
Results
Ichneumon promissorius is long-lived (>45
days during'summer months, longer during the winter) and a strong fiyer.
Our dispersal studies documented random dispersal far from the anticipated
outer boundries of the release sites. Dispersal direction and distance
was not correlated to crop type. Year-of-release surveys were made weekly
at the release locations. In Arkansas locations, wasps from previous weeks'
releases could still be found searching the ground (females) or looking
for mates (males). Of the nearly 10,000 pupae excavated during 1995, only
8 wasps were recovered. These few recoveries verified successful host-finding
and parasitization by I. promissorius, as well as a significant
dispersal distance. No recoveries were made in Georgia, Oklahoma or Texas,
although far fewer pupae were excavated in any season. The new recovery
system using pheromone traps baited with live female L promissorius yielded
twenty-seven and twelve male I. promissorius were in southwestern
Arkansas in 1996 and 1997, respectively. During 1997 we recovered thirty
males in the central Arkansas. These traps have yet to be used in any other
state. These recoveries indicate the species has overwintered in two areas
of Arkansas. Although the numbers recovered are small, the wasp has great
dispersal capabilities and the efficiency of the trap is unknown.
Summary
The ultimate goal of the project was to permanently
establish this beneficial species in the US and to document its dispersal
based on crop type. The robust parasitoid is long-lived and is a strong
fiyer. Dispersal studies documented random dispersal from release sites
irrespective of crop type. Although most successful colonizations of exotic
biological control agents are quickly verified, evaluation of the extent
of pest suppression cannot (and should not) be fully evaluated for a number
of years following release. However, because we know that there are no
parasitoids of bollworm pupae in the US, and only rarely are budworm pupae
attacked by natural enemies, any additional mortality will be beneficial.
In our previous studies we demonstrated that >95% of bollworm pupae successfully
emerge as moths during the season. This study verified the need to find
a natural enemy to fill that niche (attacking pupae). Because in Australia
I. promissorius reduces pupal survival by about 20%, we expect similar
reductions in the southern US if it becomes fully established. This establishment
would result in potentially 20% fewer moths placing eggs in producer cotton
fields, the crop causing the greatest insecticide load in the environment.
It is important to note that like all beneficial insects, this species
seeks the pest wherever it occurs, and the benefits to producers will be
pest suppression before the eggs are laid in the crop. This aspect
of the program is particularly attractive, as successful implementation
of the parasitoid will require no modification of the cotton or other production
systems.