rHRlPS BIOCONTROL: OPPORTUNITIES FOR USE OF NATURAL ENEMIES
AGAINST THE PEAR THRIPS
Nick J. ~ i l l s '
Commonwealth Agricultural Bureaux
International Institute of Biological Control
Silwood Park
Ascot, Berkshire UK
Abstract
Thrips have been considered as both target pests and control
agents in biological control. The main emphasis of this paper concerns
the natural enemies of thrips and an appraisal of the potential for
biological control of the pear thrips on sugar maple in the northeastern
United States. Previous attempts at biological control o f thrips pests
have been confined t o the Caribbean and Hawaii and have made use of
eulophid larval parasitoids and anthocorid predators as control agents.
A review of the literature indicates that while these t w o groups often
figure most strongly in natural enemy complexes of thrips, fungal
pathogens are an important, if neglected, group. For biological control
of pear thrips it is considered that synchronized univoltine parasitoids
and fungal pathogens from Europe, the region of origin of the pest,
show most promise as potential biological control agents.
Introduction
Biological control has been widely practiced worldwide as an
effective means of controlling accidentally introduced pests by the
importation and release of specific natural enemies from their region of
origin (Clausen 1977, Julien 1987). This approach led to the successful
'current address: Div. of Biological Control, Univ. of Calif., Albany, Calif.
control o f the cottony cushion scale, lcerya purchasi Maskell in
California one hundred years ago through the importation of the Vedalia
beetle, Rodolia cardinalis (Mulsant) (Caltagirone & Doutt 1989). While
chemical treatments dominated pest management in the post-war years,
environmental concerns have brought biological control back t o the
forefront of current integrated pest management practices.
Biological control of forest insects (Pschorn-Walcher 1977, Waters
et al. 1976, Turnock et al. 1976) has enjoyed a long and successful
history of natural enemy importations. The Commonwealth Institute of
Biological Control (CIBC) database of classical biological control
importation, BIOCAT, records 360 natural enemies released against 4 2
forest pests. From these importations, 29% have become established
in the target area and 34% of these have provided at least some degree, if not lasting, of control. A recent review of the suitability of forest
environments for biological control and the success or failure of key case histories is provided by Dahlsten & Mills (in press).
Thrips have figured in biological control both as target pests and
as natural enemies of weeds. Four thrips have been targets for
biological control (Table 1) and the natural enemy agents selected for
importation have included both predators and parasitoids. The results of importations have not been so successful, wi th only the anthocorid,
Montandoniola moraguesi, being credited with providing at least partial
control of the ornamental laurel thrips, Gynaikothr~j~s ficorum (Marchal)
(Cock 1985). As biological control agents of weeds, thrips have had
greater success (Table 2). Of t w o earlier projects one is a complete
success and the other is uncertain due to confounding effects of other simultaneously imported and released agents.
Table 1. Classical biological control projects against thrips, data from Clausen (1 9771, Cock (1 985) and McMurtry
(this publication) --
Target Region (Date) Agent (Origin) Result
Thrips tabaci Bermuda (1 938) Ceranisus brui (Japan) Hawaii ( 1930-32) C. russelli (USA)
C. vinctus (Philippines) Goetheana parvipennis (Ghana)
Seleno thrips rubrocinctus Caribbean (1 936-37) Goetheana parvipennis (Ghana) Hawaii ( 1936)
- Established
- Established
G ynaiko thrips ficorum Hawaii (1 964-65) Macro trachelia thripiformis (Mexico) Montandoniola moraguesi (Philippines) - Established Orius tristicolor ( US A ) Chrysopa sp. (Mexico)
Bermuda (1 960-63, Montandoniola moraguesi (Philippines) - Provides 1973) partial control
Goetheana parvipennis (Ghana ) Thripas tich us gen tilei ( B r az i I)
Helio thrips haemorrhoidalis California (1 987?) [emailprotected] semiluteus (JapanIArgentina) - Probably established
Table 2. Weed biocontrol projects using thrips as natural enemies, data from Julien (1987) and Greenwood & Mills (1 989)
Target Region (Date) Agent (Origin) Result
Clidemia hirta Fiji (1 930) Liothrips urichi (Trinidad) Good lasting control
Alternan thera philoxeroides Florida (1 967) Amynothrips andersoni (Brazil) Uncertain
Mikania micran tha Solomon Islands (1 988) [emailprotected] mikaniae (Trinidad) Not yet Malaysia (1 989) known
The Pear Thrips as a Target for Biological Control
The pear thrips, Jaeniothripsinconsequens (Uzel), is an infrequent
European thrips that was accidentally introduced into North America
around the turn of the century. I t initially caused some concern in pear
and plum orchards in California but since the early 1980s has been
responsible for severe defoliation of sugar maple in Vermont,
Pennsylvania and Connecticut (see Parker, this publication).
The biology, life cycle and natural enemies of the pear thrips have
not been studied in detail either in Europe or the United States. General
observations from the United States (Foster & Jones 191 5; Skinner et
al., poster presentation, this publication) indicate that adults emerge
from the soil in spring to feed on the expanding buds of the host tree.
Adults oviposit on the leaves and the t w o larval stages then feed on the
foliage for 4-6 weeks before dropping to the soil to produce earthen
cells for overwintering.
Observations from Europe (Priesner 1924, Blunck & Neu 1949)
indicate that the pear thrips is widespread in northern and central Europe
and differs from the United States populations by the occurrence of
males (Bournier 1956). Food plants include trees of the genera
Aesculus, Acer, Crataegus, Juglans, Malus, Pirus, Prunus, Populus and
Salix. Adult fecundity is estimated as approximately 100-200 eggs and
adult feeding and larval development tend t o occur between mid-March
and the end of May.
Since no specific studies of the natural enemies of the pear thrips
have been made, i t is necessary to take a look at natural enemies of
thrips in general to get an appreciation of the types of organisms likely
t o be associated with the pear thrips in Europe.
Thrips Natural Enemies
Predators
The slow moving larval thrips are easy prey for a wide range of
general arthropod predators (Table 3). The majority of these accept a wide range of prey and thrips may not represent their preferred host in
many cases. Some of the more specific thrips predators are the
Aeolothripidae, the anthocorid genera Orius and Montandoniola, the
cecidomyiid genus Thripsobremia and the sphecid genus
Micros tigmus .
Anthocorids have an important impact on thrips populations in
cotton (Stoltz & Stern 1978), in soybean (Irwin & Kuhlman 19791, in
ornamental fig (Cock 1985) and in Glyricidia flowers (Viswanathan & Ananthakrishnan 1974). In addition, the coccinellid, Scymmus
thoracicus, is suggested to be able to regulate populations of
Chaetanaphothrips orchidii (Moulton) on banana (Delattre & Torregrossa
1978). Most other predators will contribute t o the control of their prey
populations but are unlikely to be useful as biological control agents.
In the context of glasshouse crops, the phytoseiid mites, Amblysieus
cucumeris and A. mackenziei are mass reared for the control o f Thrips
tabaci (Ramakers 1983). Again, these mite predators are not so specific
in their diet and can only be effective in confined situations.
Parasitoids
Thrips are attacked by both egg and larval parasitoids (Table 4).
Egg parasitoids of the genus Megaphragma are some of the smallest
known insects and have been recorded from a variety of thrips species
(Lewis 1973). They have been little studied (McMurtry 1961 1, and do
not appear to contribute significantly to thrips mortality. In contrast,
the eulophid larval parasitoids play a more dominant role with 70-80%
parasitism being recorded for Ceranisus species on bean thrips (Russell
191 21, pea thrips (Kuetter 1936), onion thrips (Sakimura 1937); for
Goetheana parvipennis on cocoa thrips (Cotterell 1927) and
Thripastichus gentilei on olive thrips (Melis 1 934).
Table 3. The range of arthropod predators known to attack thrips
Order Family Species Reference
Thysanoptera Aeolothripidae A eolothrips s p p . Bournier et al. 1978 Franklinothrips spp. Callan 1943
Hemiptera Anthocoridae
Lygaeidae Miridae
Neuroptera Chrysopidae
Coleoptera Coccinellidae
Malachiidae
Diptera Syrphidae Cecidomyiidae
Hymenoptera Sphecidae
Acari Pyemotidae Phytoseiidae
Orius spp. Mon tandoniola moraguesi Geocoris tricolor Termatoph ylidea s p p . Chrysopa spp.
Adalia spp. Coccinella s p p . Scymnus spp. Malachius viridis
Me tas yrphus corollae Thripsobremia lio thrips
Microstigmus thripic tenus
A dac t ylidium nicolae Amblyseius cucumeris A. mackenziei
Irwin & Kuhlman 1979 Muraleedharan & Ananthakrishnan 1978 Ananthakrishnan 1984 Doesberg 1964
Callan 1943
Priesner 1964 Ghabn 1948 Dyadechko 1977 Tansky 1958
Ghabn 1948 Barnes 1930
Matthews 1970
El Badry & Tawfik 1966
Ramakers 1 983
380
Table 4. Egg and larval parasitoids of thrips
Family/ Host stage Generakpecies attacked Distribution
Trichogrammatidae
Megaphragma s p p . Egg Tropical & sub - tropical
Eulophidae
Ceranisus ( = Thripoctenus) Larval Temperate - tropical
Goetheana parvipennis Larval Tropical & sub - tropical
Thripastichus Larval Temperate - tropical ( = Tetrastichus) spp.
Thripobius spp. Larval Tropical & sub-tropical
Thripoctenoides gaussi Larval Temperate
The biologies of the larval parasitoids are similar (Russell 191 2, Sakimura 1937, Dohanian 1937, Bournier 1967). Adults live
approximately 3 weeks and oviposit into young thrips larvae. The
solitary endoparasitoid larvae kill and pupate within the host pre-pupae
and the life cycle is completed in about one month. The parasitoids are
generally multivoltine, although there is some evidence that, at least in
the case of Thripastichus gentilei, diapause may be controlled by that
of its host. In association with the laurel thrips both host and
parasitoid have continuous generations (Bournier 1967) but in
association with the olive thrips, which has an overwintering adult
diapause, the parasitoid remains in host pre-pupae from October
through to April (Melis 1934).
Pathogens
Allantonematid nematodes have occasionally been found infesting
thrips but records are few (Ananthakrishnan 1984). While these
nematodes are internal parasites, they do not cause the death of their
host and their impact is confined to a reduction of adult fecundity. I t is
probable that arboreal thrips, such as the pear thrips, are not attacked
by nematodes on the foliage of their hosts but may well be attacked
during the long period spent in the soil.
Raizada (1 976a) notes the occurrence of heavy microsporidial
infection of Scirtothrl)x oligochaetus on cotton but this is the only
known record. In addition, there are no known viral (Martignoni & lwai
1981 or bacterial diseases of thrips.
Thrips are attacked by fungi of many genera including Beauveria,
Neozygites (En tomoph thora) , Verticillium, Paecilomyces and Hirsutella
(Raizada 197613). Beauveria bassiana infected up to 20% of larvae of
Haplothrips tritici in Bulgaria (Lyubenov 1961 ) and probably attacks all
thrips that pupate or overwinter in the soil. Two species of
Entomophthorales have been described (MacLeod et al. 1976, Samson
et al. 1979) that infest larval hosts while feeding on foliage and a
Hirsutella sp. nov. has recently been isolated by ClBC from foliage
feeding larvae of Liothrips mikaniae (Greenwood & Mills 1989).
Verticilliurn lecanii gives good control of Thrips tabaci in glass houses
(Gillespie et al. 1983) and a water-miscible formulation reached an advanced stage of commercial development in England. While there are
no records of fungal pathogens of the pear thrips it is probable that
such natural enemies do exist, at least in their region of origin.
Appraisal and Discussion of the Potential o f Natural Enemies for
Biological Control of Pear Thrips
It is clear from the literature that pear thrips has not been a pest
of concern in Europe in recent years and that it has never been
considered a pest of broadleaf forest trees. The host range of the pear
thrips in its native Europe is not well known but it appears to be more
of an orchard than a forest species. No study of the natural enemies
of the pear thrips has been made in Europe and it is not possible,
therefore, to assess their role in maintaining this species at endemic
densities in its area of origin.
Nutritional factors may affect the abundance of thrips populations
and may be one of the factors involved in the recent outbreak of the
pear thrips on sugar maple in the United States. Fennah (1 955, 1965),
in some classic studies of the cocoa thrips in Trinidad, found that the
establishment of thrips on particular trees or parts of the trees was
related to the extent of derangement of normal host tree physiology.
Adverse factors affecting normal leaf metabolism lead to increased
nitrogen availability and higher thrips populations. Similar interactions
could occur between pear thrips and sugar maple trees, brought about
by the action of recent years of lower than average rainfall or increased
levels of acid rain. Fertilization to improve general tree health could be
experimentally investigated as a means to reduce levels of thrips
infestation.
Of the natural enemies that are known to attack thrips, predators
are the least specific and of least interest in terms of potential for
biocontrol. While some success has been achieved in Bermuda with
the anthocorid Montandoniola moraguesi (Cock 19851, temperate
predators appear less promising. These are represented by congeneric
species in both the Nearctic and Palearctic faunas and the importation
of European predators does not seem warranted.
Egg parasitoids appear t o be infrequent and t o have little impact
on their host populations. Larval parasitoids, such as Ceranisus spp.,
have a very significant impact on their hosts but little is known of their
host range or habitat preferences. The majority of host records for
Ceranisus species are from the Thripidae (family including the pear
thrips) but most records are from hosts feeding on crops and other low
growing plants. This probably reflects the greater attention that has
been paid to thrips pests of agricultural and horticultural importance,
rather than a distinct habitat preference of the parasitoids. The most
frequent species, C. russelli, is Holarctic but a number of other species
are known from the Palearctic region. In contrast, Thripastichus species
appear t o be more associated wi th the Phlaeothripidae, and most of the
records are from arboreal hosts. 1. gentilei is an important parasitoid in
Europe and there is some evidence that diapause in this species may be
controlled by the host (Melis 1934).
The univoltinism and restricted time period in which the pear
thrips is active above ground poses difficulties for biocontrol by
parasitoids. To be able to use parasitoids effectively, a species must
be found that can be induced into diapause by the host t o provide the
necessary synchronization and independence from alternative hosts. The
currently known parasitoids are multivoltine and are better known from
multivoltine thrips hosts. However, the limited knowledge of these
parasitoids suggests that parasitoid diapause may well be controlled by
that of the host. Thus the diapause of the pear thrips may induce
univoltinism in some of the parasitoids. I f not, then it is unlikely that
parasitoids would be able to inflict significant levels of parasitism on
this host, particularly at the early stage of the season when the pear
thrips is active.
Of the known or probably occurring pathogens, the
allantonematid nematodes have little potential for use in biocontrol.
Nematodes of the genera Heterorhabditis and Neoaplectana have far
greater potential for control of the pear thrips in the soil and local
Vermont strains of these entomopathogenic genera, isolated from the
soil in sugar maple forests would be the most appropriate t o use in experimental trials.
Very little is known of the fungal and other pathogens of thrips
and there are no records from the pear thrips. Rather than indicating
an absence of important pathogens this is likely to reflect a lack of
investigations on these rather small and often insignificant hosts. In
recent years species of Entomophthorales have been discovered
attacking thrips and it is probable that a range of other thrips-specific
fungi have yet t o be discovered. Members of the Entomophthorales
and other specialized fungal pathogens infesting the pear thrips could
be considered for classical biocontrol introductions. A precedent for
this exists in the recent establishment in Australia of an introduced
strain o f Erynia radicans for control of the alfalfa aphid, an exotic pest
(Milner et al. 1982).
Other fungi, such as Beauveria, Metarhizium and Verticillium
species, can be more readily mass produced and formulated for
application as myco-pesticides. The most appropriate fungal strains t o
use in either case would probably be those isolated from the pear thrips in its region of origin.
Several precedents exist for the application of formulated
pathogens t o perennial crops and forests. Beauveria brongniartii has
been sprayed on swarming adults of the European cockchafer Melolontha melolontha (Coleoptera: Scarabaeidae) with subsequent
contamination and suppression of larval populations in the soil for
several years (Keller 1986). This success is encouraging for pear thrips
control because here too, application of the pathogen t o the insect on
the trees could lead to an increase in the soil population of the
pathogen and consequent long-term suppression of the pest.
Very large areas can be treated wi th myco-pesticides. In Brazil, 6000 ha are treated with the fungus Metarhizium anisopliae t o control
the spittlebug, Mahanarva posticata (Hemiptera: Cercopidae) (e.g.,
Ferron 1981 ). Even larger areas of Canadian forests are aerially
sprayed with commercial formulations of Bacillus thuringiensis t o control
the spruce budworm, Choristoneura fumiferana (Lepidoptera:
Tortricidae) (Morris et al. 1986). In this last case it was public concern
about environmental damage from chemical control measures that led
t o the adoption of B. thuringiensis by the Canadian authorities. The environmental safety of biological pesticides is of particular relevance
where large areas of land such as forests must be treated.
The conidia of dry-spored fungi such as Beauveria and
Metarhizium are hydrophobic and are miscible in oil. Such oil
formulations are much more infective than water suspensions because
the oil adheres t o the insects' cuticle (Prior et al. 1988). Oil
formulations would be suitable for ultra-low volume aerial spraying,
where the use of water sprays is precluded by their rapid evaporation
and the large volumes of water required. The prospects for developing
an oil-based, ultra-low volume myco-pesticide against pear thrips are
good.
Acknowledgment
It is a pleasure t o thank Chris Prior for very helpful discussions
on the use of fungal pathogens in biological control and Bruce L. Parker
and the University of Vermont for inviting me t o participate in this
Symposium.
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