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Background and Objective:
Species belonging to the genus
Lygus are important pests in a variety of crop systems in
Canada. Losses in canola in the Prairies were estimated to be $140
million in 1994 and in each of 1996, 1997, and 1998 infestations were
above the then newly developed threshold in several parts of the
Prairies, resulting in insecticide spraying of canola. Other field
crops including field peas are also susceptible to attack. Although
actual monetary losses are not currently available, damage to fruit
crops such as strawberries, is also of significant importance as
deformed fruits are unacceptable for marketing. The use of chemical
insecticides to control these pests adds to input costs for producers,
especially in seed alfalfa where producers spray prophylactically. The
importance of Lygus as a pest in the Prairie region appears to be
increasing.
The
Lygus plant bug species found in Canadian crops are species native to
North America. In addition, the closely related alfalfa plant bug,
Adelphocoris has been accidentally introduced from Europe to North
America. However, not all of the potential natural enemies of these two
genera are present in North America. There are relatively
few species of some of
the parasitic wasps, compared to the situation in Europe. Therefore
there exists the possibility of reducing the pest status of Lygus
and Adelphocoris in North America through the introduction of
additional natural enemies which would lower the population levels on a
continuing basis, and reduce the need for insecticidal control measures.
There are 4 major species of parasitoids belonging to the genus
Peristenus (Hymenoptera: Braconidae) which attack nymphs of these
species in alfalfa in western Europe. Host preferences of the
parasitoids for Adelphocoris and the different Lygus
species are poorly known. Some species of the parasitoids appear to
have a single generation a year, while others have two. It is also
possible that some Peristenus species attack early instar nymphs
while others may attack late instar nymphs. In North America two
species of Peristenus have been documented from Adephocoris
and from Lygus species. This therefore is a complex system
with tremendous potential for successful biological control, but also
with tremendous potential for counter-productive introductions. There
is nothing to be gained by introducing a parasitoid which will compete
directly with one already here, or by introducing two species which have
the same host preferences. In addition, it would be counter-productive
to introduce parasitoids which would also attack closely related
predatory bugs, since this could reduce levels of natural control in
other insect pests. Proper evaluation of the potential of a parasitoid
species for use as a biological control agent requires that the life
cycle, aspects of behaviour, and host range of the candidate in its
native habitat be clearly known. This information is critical to
determining if an agent is appropriate for the niche to be filled and
for establishing a priori, the potential impact of the agent on the
target ecosystem.
The overall objective of this
project was therefore to investigate the biology of the species of
Peristenus parasitoids attacking Adelphocoris and Lygus
spp. in Europe to
provide information that would aid in determining the impact of
candidate Peristenus species on target and non-target species of
plant bugs in Canada.
Specific
objectives were to:
-
Study the
life cycle of major species to determine the seasonal succession of
species.
-
Determine
the host instar preference for each species of the nymphal parasitoid
guild.
-
Examine the
host specificity of the Peristenus species being considered for
introduction.
-
Study the
population dynamics of the plant bug/Peristenus complex in
several crops.
-
Examine the
strain differences within species of key Peristenus to select
the optimal biocontrol agent against Lygus species under
Canadian conditions.
Procedure and
Project Activities:
Each year for three years, for
5-6 months from April to September, Heather White worked with Dr. Ulrich
Kuhlmann of CAB-BES in Switzerland to carry out the field components and
some aspects of the laboratory components of the project. From
September to April Ms. White attended the University of Manitoba as a
graduate student under my supervision. Dr. Bruce Broadbent from the
Agriculture and Agri-Food Canada Research Centre in London, Ontario,
whose laboratory cultures some of the parasitoids, including both
quarantined and nonquarantined species, visited her in Switzerland
during her field studies, as did Dr. Peter Mason of Agriculture and
Agri-Food Canada, Ottawa, who has responsibilities in the area of
approval of introductions of biological control agents. On October 25,
2001 Heather White successfully defended her thesis. The major
components of the thesis are summarized briefly below
Results and Discussion:
Observational field studies
were made on species of Peristenus parasitoids and their mired
hosts from 1998-2000 in Germany and Switzerland. The intent of this
research was to provide new ecological information which could be used
to evaluate the potential of selected European Peristenus species
as biological control agents for pest Lygus and Adelphocoris
lineolatus in North America. The plant bug species, Lygus
rugulipennis, Lygus pratensis and Adelphocoris lineolatus,
were collected in fields of alfalfa, red clover, mixed clover,
asparagus, barley, rape seed and mustard, to determine the host's
phenology and field levels of parasitism. Plant bug nymphs that were
collected were used to compare the detection of parasitoids in nymphs by
rearing and dissection methods. In this study, rearing was found to be
more accurate in detecting parasitoids. Parasitoids cocoons obtained
from the rearing of plant bug nymphs were monitored for adult parasitoid
emergence. All parasitoid adults were preserved and identified.
Information on the emergence patterns for Peristenus digoneutis,
Peristenus stygicus and Peristenus rubricollis is
presented. Emergence information is also presented for other species of
Peristenus, Leiophron, and Mesochorus parasitoid
species. Surveys were carried out in Germany and Switzerland in many
different habitats, ranging from agricultural fields to mountain
meadows, to collect mirids to rear for parasitoids to investigate the
ecological host range of Peristenus species.
Conclusions:
The above results will form one
of the cornerstones of the long term effort to improve biological
control of pest plant bug species. Once decisions have been made, based
on this and any additional information still necessary, on which species
are appropriate for introduction, those species must be imported and
reared in large numbers for release. If they subsequently become
established, there is the very real prospect that over several seasons
we will experience an ongoing reduction in the numbers of pest plant
bugs as the natural enemies increase in numbers and in impact on the
plant bug populations. The result could be a permanent reduction in the
frequency and intensity of economically significant plant bug
infestations in North American crops of all sorts.
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