Role of Host Plants on the Biological Aspects and Parasitism Levels
of Eretmocerus mundus Mercet (Hymenoptera: Aphelinidae), a Parasitoid
of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae)
Nagdy F. Abdel-Baky
Impact of the host plant type on certain biological
aspects and parasitism level of E. mundus under laboratory and
semi-field conditions was evaluated. The parasitoid biological aspects
were greatly differed within the host type. Parasitoid life cycle was
shorter on squash, followed by common beans and sweet pepper, which lasted
27.6Â±1.9, 25.9Â±1.3 and 23.7Â±1.1 days, respectively.
In contrary, female longevity was shorter on the sweet pepper (9.9Â±1.6
days), followed by the common bean (10.8Â±1.1 days) and was longer
on squash (11.7Â±1.3 days). Additionally, E. mundus life
span and female fecundity were also studied and varied among the studied
hosts. Effect of host plants on both colonization of pest nymphs and parasitism
percentages, were also evaluated. Greater numbers of young and old nymphs
and higher parasitism rates were observed on squash followed by common
beans, whereas, sweet pepper was last in this respect. Subsequently, the
reproduction and biological characteristics of E. mundus have been
shown obviously to be influenced by host plant. Efficiency of releasing
parasitoids was greatly affected by host plant type as well as releasing
rates. Releasing the parasitoid with constant numbers against various
population densities of the pest achieved different pest control levels.
Releasing rates of 1:5 and 1:10 (parasitoid:pest) gave good control measures,
whereas, moderated control levels were fulfilled with ratios 1:20 and
1:30. Increasing pest densities negatively correlated with good control
to cite this article:
Nagdy F. Abdel-Baky and M.A. Al-Deghairi, 2008. Role of Host Plants on the Biological Aspects and Parasitism Levels
of Eretmocerus mundus Mercet (Hymenoptera: Aphelinidae), a Parasitoid
of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). Journal of Entomology, 5: 356-368.
Currently, Bemisia tabaci species complex is considered to be
one of the worst world`s top 100 invasive species, because of its serious
damage to agricultural production and associated industries. Chemical
control of Bemisia species complex provides only short-term solutions
and the insects has provoked the development of resistance to those chemicals
(Dittrich et al., 1990; Damásio et al., 2007). So
far, several biological control strategies have been involved and evaluated
for the pest management, as the use of hymenopteran parasitoids, either
native or exotic (Goolsby et al., 1998; Ardeh et al., 2005)
and could be an attractive management alternative for whiteflies (Gerling
et al., 2001). At least 13 species B. tabaci parasitoids
in the genus Eretmocerus have been identified from the New World
(Zolnerowich and Rose, 1998) and many are important in biological control
of B. tabaci (Gerling et al., 2001). The Eretmocerus
spp., with at least two species (E. eremicus Rose and Zolnerowich
and E. mundus Mercet) are commercially available (Zolnerowich
and Rose, 1998; Hoddle and Van Driesche, 1999).
Whitefly parasitoids formed one of the important backbone of IPM programs.
When IPM was practiced, E. mundus was found as the most predominant
parasitoid species against whiteflies, particularly in the open field
(Rodriguez-Rrodrigez et al., 1994). Biological characteristics
of E. mundus have bean also studied under laboratory conditions
(Sarhan 1976; Abdel-Baky and Ragab, 2005).
Eretmocerus mundus Mercet (Hymenoptera: Aphelinidae) has been
recorded in many countries in the Mediterranean Basin (Mound and Halsey,
1978) and world-wide making it the most important, abundant and indigenous
parasitoid attacking B. tabaci, a serious pest in greenhouses and
outdoor crops (Roderiguez-Rrodrigez et al., 1994). E. mundus
is standing out among whitefly potential biological control agents and
considered as highly promising biological control agent of whiteflies
(Goolsby et al., 1998). This may be due to many scientific facts
such as; (a) no known Eretmocerus species exhibits auto-parasitism:
a potentially deleterious trait to biological control (Van Lenteren et
al., 1997) and b: its reproductive and competitive potential on its
host (Greenberg et al., 2002; Urbaneja et al., 2007).
E. mundus parasitizes all four nymphal instars of Bemisia tabaci,
with 2nd instars being the preferred stage. E. mundus is a primary
parasitoid that oviposites underneath the nymphal whitefly host (Gerling
and Fried, 2000). To our knowledge, two Eretmocerus types used
on the international scale, one species is arrhenotokous, but the other
is thelytokous type, which known as the Australian strain (McAuslane and
Nguyen, 1996). De Barro et al. (2000) considered thelytokous populations
as the best candidates for biological control of whiteflies world-wide.
The suitability of whitefly nymphs as a host was recognized (Jones and
Greenberg, 1998). Jervis et al. (2001) recognized two alternative
life-history strategies in parasitoid life in respect of development mode.
Kionobiont expresses when 2nd or 3rd instars parasitized by E. mundus,
in which the parasitized larva continues to feed, grow and complete its
development. Whereas, idiobiont expresses when B. tabaci 4th instars
used as a parasitoid`s host, in which the parasitized larva evidently
stop development and no life cycle occurred.
Releasing rates for natural enemies in greenhouses are generally expressed
in numbers per unit area. Releasing rates of E. mundus were initially
set to those recommended for E. eremicus of 1.5-3 wasps/m2/week
as a preventative treatment and 3-9 wasps/m2/week as curative
treatments (Stansly et al., 2005). These rates largely contrasted
with those tested for control of B. tabaci by Hoddle et al.
(2001) and Van Driesche et al. (2001).
Therefore, the objectives of the present study are to evaluate the impact
of host plant type on some biological parameters of E. mundus and
determine its potentiality as a bio-control agent for whitefly. All these
measures were designed with objective to identify optimal release rates
for whitefly control.
MATERIALS AND METHODS
Whiteflies and Parasitoids
Separated cultures from B. tabaci and its parasitoid E.
mundus were originally initiated from individuals collected from squash,
cucumber and pepper plants insecticides free at Qassim region, KSA. Whitefly
colony was kept on common bean (Phaseolus vulgaris L.) under laboratory
conditions at the room temperature (30±2°C) and photoperiod
12:12 (L:D). At least three generations of B. tabaci was gained
under these conditions, to serve as host of the parasitoids. In respect
to the parasitoid`s colony, nymphs of B. tabaci with recognizable
parasitoid`s pupae and/or had a parasitization symptoms were removed and
stored out at 25°C and kept until adult emergence and then collected
in small tubes.
Three plant hosts namely; squash Cucurbita pepo L. (Cucurbitaceae),
sweet pepper, Capsicum anuum L. (Solanaceae) and common bean, Phaseolus
vulgaris (Fabaceae) planted into 5.4 cm2 peat moss. Plants
were trimmed to 2 or 3 stems supported by strings attached to an overhead
wire. These plant hosts served as a whitefly host to evaluate their influence
on the biological aspects of E. mundus and its efficiency as a
bio-control agent against B. tabaci under laboratory conditions.
Effect of the Plant Type on the Biological Aspects of E. mundus
Fifteen pots of each tested host plants were grown in screened
cages until reaching a good vegetative development and free of whitefly
infestation at 27±1°C and 72±2% RH. For each host plant,
three full expanding leaves were selected from each pot, a couple of B.
tabaci adults were introduced to each leaf under a clip cage and left
for 24 h, then adults were removed. The numbers of deposited eggs were
counted on each leaf of the three host plants. Since E. mundus
prefers to parasitize on the 1st or 2nd instars of B. tabaci (Jones
and Greenberg, 1998), once B. tabaci nymphs reached to the 2nd,
the desire number of nymphs was left on the leaves and then one couple
of E. mundus was introduced and kept for only 24 h. Whitefly infested
leaves along with its parasitoids were left until emergence of whitefly
adults or parasitoids. The life cycle, life span and female fecundity
and longevity were recorded.
Longevity of E. mundus, which fed on honey solution, was determined
in air-conditioning controlling cabinet at 27±1°C, 72±2
RH and photoperiod of 12:12 (L:D). Fifty newly emerged parasitoids were
individually placed in Petri-dishes (9 cm in diameter) with screened lids.
The parasitoid`s adults were re-provisioned daily with honey smeared on
the screened lids until they died. Adult longevity was determined.
Newly emerged females of E. mundus were singly isolated in
glass chimney with the top sealed with screen cover. Common beans pots
with leaflets infested by the 2nd instars (50-80 individuals) of B.
tabaci were inserted inside the glass chimney. Bean plants were daily
replaced with new similar pots until the death or escape of the parasitoid.
Fecundity of E. mundus females were assessed by counting nymphs
showing sign of parasitism and/or counting the adult numbers of the emerged
Efficiency of Eretmocerus mundus in Controlling Bemisia
tabaci under Laboratory Conditions
Evaluations the Role of Host Plant Type on the Efficiency of E.
mundus as a Bio-Control Agent
Impact of squash, common beans and sweet pepper on the efficiency
of the parasitoid in controlling B. tabaci was evaluated. The evaluation
was conducted as replicated releases of the parasitoids into semi-field
cages. Cages were placed at the same time when the three host plants were
planted and grown in small plastic pots. Each experiment was replicated
five times with the total of 200 plants of each host plant. In the respect
of each host plant, cage frames of 1.5 mL long on each side, were constructed
of plastic PVC pipe and three-corner joints, with one cross joint in the
middle of cage top. Cage covers were mesh screened with openings small
enough to prevent passage by whitefly and parasitoids. Also, the cages
were provided by zippers sewn in one side of the cage for easy access.
Each cage spanned forty plastic pots cultivated by the plants. When the
plants reached 20 cm height with at least 5 fully expanded leaves, B.
tabaci adults were inoculated by introducing 100 pairs per cage and
left for three weeks to reach its maximum population level. Average of
whitefly populations/plant/cage/host was counted. Subsequently, 50 pairs
of E. mundus parasitoid were introduced and let for two weeks before
evaluating its role. Weekly random samples were taken by counting the
whitefly populations and its parasitoids, thereon, the parasitism percentages
were calculated. Immature parasitoids inside the pupae or older larvae
of the whitefly host could be readily detected, so leaf samples were collected
at that point reliably measured the parasitoid progeny. Leaves in cages
were weekly inspected by taken 20 leaves in random and taken to the laboratory
to record the number of whiteflies and parasitoids. Leaves were examined
in the laboratory with stereoscopic microscopes to record the production
of the parasitoid progeny for each host plant. The parasitized nymphs
were counted on all harvested leaves; this included those individuals
with displaced (asymmetrical) mycetomes that were indicative of the presence
of mid-sized parasitoid larvae which indicated large, late instars nymphs,
pupae of E. mundus and whitefly mummies with parasitoid emergence
holes. Accordingly, comparisons among host plants were also involved.
Effect of Releasing Rates on the Parasitoid Efficiency
In this experiment, plastic pots cultivated with squash plants used
as a whitefly host. Ten pairs of B. tabaci adults were introduced
and kept under the screened cages for 48 h, then removed. The plants with
B. tabaci eggs were incubated at 27±1°C until whitefly
nymphs reached the desired age for parasitism (2nd or 3rd instars). Constant
numbers of B. tabaci nymphs (200 individuals/plant) were determined.
There were 15 replicates for each releasing ratio, each was covered with
screened cage. After that, the parasitoid was released at six release
rates (parasitoid:pest): 1:5, 1:10, 1:20, 1:30, 1:40 and 1:50/plant as
well as, the check treatment with 0-parasitiod:200 B. tabaci nymphs.
Care was taken by inspecting control cages first, followed by low release
rate cages and finally high release rate cages to minimize risk of cross
contamination by parasitoids.
Plants were weekly monitored for 10 successive weeks. In each cage,
Plant leaves, infested with B. tabaci nymphs and its parasitoids
at each release rate, were investigated. All whitefly stages and parasitized
instars were counted on each leaf using a 10x-hand lens. Relative incidence
of parasitized whiteflies was expressed as the number of parasitized nymphs/total
number of nymphs/plant. Additionally, emerged whitefly adults were counted/plant
of each release rate.
In all cases, healthy nymphal instars, exuviae from parasitized and unparasitized
whiteflies and parasitized nymphs were counted. Early stages of parasitism
in nymphal instars were recognized by displaced mycetomes and later stages
by the presence of the parasitoid`s pupa.
Obtained data were subjected to one-way analysis of variance with
mean separation using LSD test in the event of a significant F (p<0.05)
(Duncan, 1951). Degree of whitefly populations suppression obtained by
parasitoid treatments was expressed using the formula 100x(1-treated/control).
Biological Aspects of E. mundus and Role of Host Plant Type
Eretmocerus mundus biological aspects were greatly differed
based on the plant host type. The results in Table 1,
indicated that faster development, shorter life cycle and higher female
progeny were obviously observed with cucumber plants, followed by common
bean, while the sweet pepper came in last. The life cycle of the parasitoid
was shorter when reared on squash than on other two hosts. The life cycle
lasted 27.6±1.9, 25.9±1.3 and 23.7±1.1 days on sweet
pepper, common bean and squash plants, respectively. Female longevity
was shorter when developed on sweet pepper which lived 9.9±1.6
days and increased on common bean (10.8±1.1), while, it was longer
on the squash and recorded 11.7±1.3 days. Both of parasitoid life
cycle and female longevity were significantly varied (p≥0.5) as shown
in Table 1.
||Effect of host plants on certain biological characters of Eretmocerus
|Means followed by the same letter within a column are
not significantly different at (0.05%) (Duncan Multiple Rang test)
||Population of B. tabaci nymphs, colonization percentages,
number of parasitized nymphs and emerged parasitoid of E. mundus
|Means followed by the same letter within a column are
not significantly different at (0.05%) (Duncan Multiple Rang test)
Life span of E. mundus also varied among the studied hosts, which
recorded 34.6±1.7, 32.3±1.5 and 28.8±1.8 days on
sweet pepper, common bean and squash, respectively. This means that the
parasitoid life span was shorter on squash than on other two hosts. Statistical
analysis showed no significant difference in parasitoid life span when
reared on sweet pepper and common beans, but it significantly varied on
Regarding female fecundity, host plant type showed significant variation
in this respect when the parasitoid reared on three different hosts (Table 1). The daily deposited eggs/female was 3.6±0.21 on sweet pepper,
4.1±0.47 on common beans and 4.7±0.66 on squash. Collectively,
parasitoid female laid more eggs on squash (58.7±2.3 eggs/female),
followed significantly by common beans (48.3±2.1 eggs/female) and
the sweet pepper came in last which recorded only 43.1±2.5 eggs/female
Impact of Host Plants on Both Bemisia tabaci Nymphs and Eretmocerus
mundus Parasitism Rates
Bemisia tabaci Nymphs
The type of host plant directly affected whitefly population (as a
mean of nymphs/inch2/leaf) and colonization percentages. A
great number of whitefly nymphs had been recorded on squash plant, which
averaged 114.93±4.87 nymphs/cm2/leaf for young nymphs
and 83.95±4.91 nymphs/inch2 of leaf for old nymphs (Table
2). The common beans came next where number of young nymphs reached
87.18±3.98 and 59.8±3.32 nymphs/inch2 for old
nymphs. On the other hand, the sweet pepper harbored the lowest population
level of B. tabaci nymphs (74.87±3.11 and 46.2±2.65
for young and old nymphs, respectively). Statistically, number of young
nymphs (LSD at 0.05% = 9.87; Calculated F = 35.24) and old nymphs significantly
varied (LSD at 0.05% = 10.64; Calculated F = 25.09) among the three host
plants (Table 2).
The insect pest colonization also varied among plant host types and nymph
age. Young nymphs averaged 41.50, 31.46 and 27.04% on squash, common beans
and sweet pepper, respectively. The same trend was also obtained with
old nymphs, which lasted 43.95% on squash, 31.61% on common beans and
24.43% on sweet pepper as shown in Table 2.
Eretmocerus mundus Populations
To evaluate the impact of host plant on biological aspects of E.
mundus two biological parameters were tested (Table
2). These parameters were signs of parasitized nymphs and percentage
of emerged parasitoids. Number of B. tabaci that showed signs of
parasitism by E. mundus varied based on the plant type. Whitefly
nymphs that harbor squash plant were more preferred for parasitism followed
by common beans and finally the sweet pepper. Nymphs mean (inch2
of leaf) that showed sign of parasitism was 80.8±4.63, 54.6±3.89
and 41.1±3.41 for squash, common beans and sweet pepper, respectively.
The statically analysis revealed that number parasitized B. tabaci
nymphs varied significantly (LSD at 0.05% = 8.63; Calculated F = 43.86)
among the tested plants (Table 2). Moreover, the results
showed that squash supported 43.95% of total parasitized nymphs, followed
by common beans (30.08%) and sweet pepper (24.28%).
Additionally, the number of emerged parasitoids was also varied according
to plant host. On squash, 93.47% parasitoid adults have emerged from parasitized
nymphs, followed by common beans (87.02%) and sweet pepper (72.51%). This
means that some plants could support a large number of the parasitoids
than others. Therefore, these hosts should be used in rearing the pest
and its parasitoid. It could be concluded that the squash as a host was
the preferred one for rearing E. mundus followed by common beans
and sweet pepper plants came in last.
Table 3, clearly shows the correlation coefficient
among B. tabaci nymphs and its parasitoid, E. mundus. The
relationship between young nymphs and old nymphs of B. tabaci,
parasitized nymphs and parasitism percentage was high with R values 0.551±0.13,
0.653±0.12 and 0.429±0.14 for old B. tabaci nymphs,
parasitized nymphs and parasitism %, respectively (Table
3). Furthermore, old nymphs of B. tabaci relation with both
of parasitized nymphs and parasitism % was also strong and their R values
were 0.746±0.10 and 0.509±0.13, respectively. This means
that not all young nymphs complete their development and transformed to
the 3rd or 4th instars as a result of some mortality factors. The sign
of parasitized nymphs was clearly noted on both young and old nymphs,
but it was obvious with old nymphs (Table 3). The relation
between nymphs with signs of parasitism and emerged parasitoid % was moderate
but showed significant correlation and its "R" value was 0.471±0.13.
The non-parasitized nymphs had a strong positive linear correlation with
the parasitized nymphs (R = 0.999±0.005), as well as, a strong
positive linear correlation between the parasitized nymphs and parasitism
% (R = 0.999±0.005).
||Correlation coefficient parameters among B. tabaci nymphs
and its associated numbers and percentages of E. mundus
Evaluation of Eretmocerus mundus Role in Suppressing Bemisia
Impact of Host Plant Type
Host plants obviously affected E. mundus behavior in controlling
B. tabaci. On squash plants, average number of B. tabaci
was 1016.5/plant, but it declined to 448.5/plant after releasing the parasitoid
(Fig. 1). This means that the parasitoid was able to
reduce pest population by 55.9%. In this study, the parasitism percentages
ranged between 11.08 to 30.64% with an average of 20.62% (Fig.
2). On common beans, the average B. tabaci number was 321.4
individuals/plant but it was reduced to 186.1 individuals/plant after
releasing the parasitoids. Data revealed that E. mundus reduced
the pest populations by 42.1% on the common beans (Fig.
1). The parasitism percentages fluctuated between 7.63 to 19.78% with
an average of 12.63% (Fig. 2). Before the release of
parasitoid, the average B. tabaci numbers reached 201.9 individuals/plant
and listed 106.8 individuals/plant after releasing. The parasitoid caused
32.27% mortalities among B. tabaci population (Fig.
1) and ranged between 4.5 to 16.63 % with an average of 9.97% (Fig.
2) on sweet pepper plants.
On all host plants tested, the pest populations were higher at the beginning
of the experiments and gradually reduced after that (Fig.
3a, b, c). These figures illustrated
that the developing parasitoid population was lower in comparison with
the pest population, but it multiplied and increased as it was higher
at the end of experiments. Statistically, efficiency of the parasitoid
in controlling the whitefly was significantly differed (p≥0.05) among
||B. tabaci population as an average/plant before
and after releasing the parasitiod
||Parasitism percentage of E. mundus a parasitiod
of B. tabaci among three plant hosts
||Role of E. mundus in suppression of B. tabaci
population (a) squash, (b) common bean and (c) sweet pepper plants
Impact of Releasing Rates
Releasing constant numbers of E. mundus, against varied pest
densities, affected the controlling levels (Table 4).
Good control levels could be achieved when the parasitoid faces low pest
populations. About 71.4% mortalities of B. tabaci population (average
of parasitized nymphs = 142.20±4.45/plant) were achieved when the
parasitoid was released by one parasitoid against five individuals of
the pests. The parasitism percentages were over 50% when releasing one
parasitoid against 10 individuals of the pests (58.57%, with an average
of 117.13±3.93 parasitized nymphs/plant) and one parasitoid against
20 individuals of the pests (53.6%, with an average of 107.02±3.76
parasitized nymphs/plant). Releasing the parasitoid with ratios of one
parasitoid against 30, 40 and 50 individuals of the pests caused 42.57,
33.07, 25.53% mortalities among the pest populations, respectively (Table
4). The average of parasitized nymphs in the pervious releasing ratios
was 85.13±3.71, 66.13±3.54 and 51.07±2.53 individuals/plant,
||Efficiency of E. mundus in controlling B. tabaci when
released against different pest densities
Classical biological control of whiteflies, which depends upon parasitoids,
becomes the primary objective and seen as an important component of integrated
management (Faust, 1992; Pickett et al., 2004). Controlling a serious
pest on different plant hosts with one parasitoid species and even different
populations of the same species, often vary as a result of changes in
parasitoid characteristics such as host acceptance, fecundity, development
time, etc. and these are often highly dependent on the host plant type
and the surrounding environmental conditions (Hoelmer and Goolsby, 2002).
The biological characters of E. mundus are affected by many important
factors such as an environmental factors, host type, density, size or
age and host plant type (Jones et al., 1999). The previous studies
indicated that the single female of E. mundus laid approximately
81.1 to 247.5 eggs, during its life time of 10-16 days under laboratory
conditions (Gerling and Fried, 2000). The reproduction and biological
characteristics of E. mundus have been shown obviously to be influenced
by host plant. Previously, Eretmocerus spp. was found to lay more
eggs on the glabrous leaves of collard than on the hirsute leaves of eggplant,
cucumber and tomato. Host plant morphology, in particular leaf hair density
and structure was found to have a major effect on the searching efficiency
of parasitoids by slowing or inhibiting its walking speeds and in turn
reducing parasitism rates (Van Lentern et al., 1987; Headrick et
al., 1996; McAuslane and Nguyen, 1996; Qiu et al., 2005).
However, the parasitoid fecundity, in this study (Table 1), was lower
compared with the previous studies. The differences in parasitoid fecundity
may be due to its oviposition behavior, which lays its eggs externally
beneath nymphs and not within them (Buncker and Jones, 2005). Where in,
Foltyn and Gerling (1985) considered E. mundus as an eco-endoparasitiod,
oviposition under 2nd and/or 3rd nymphal bodies of whiteflies. Once the
egg hatched, the larva burrows through the body. A delay in penetration
and/or slower developmental rates after penetration could be responsible
for the extended period of development.
The mechanisms of host instar suitability for whitefly parasitoids have
not been studied and only can be surmised. Gerling et al. (2001)
suggested that young host instars are less able to defend themselves with
horizontally protruding wax filaments than the third and fourth instars.
The non-penetrating E. mundus larva almost induced permanent developmental
arrest in its 4th instar of whitefly host and also caused a reduction
in whole body host ecdysteroid titers. Ghahari et al. (2005) studied
the biology of thelyotokous biotype of E. mundus as part of an
evaluation of its potential for biological control of B. tabaci.
They found that the parasitoid deposited more eggs under 2nd and 3rd nymphal
instars than 1st or 4th instars. Moreover, when females fed honey, with
no access to whitefly nymphs, they lived significantly longer (13.6±4.7
day) than those given access to nymphs with no honey (7.6±2.21
day). In this point, their results were completely in agreement with our
finding (Table 1). They concluded that E. mundus longevity on host-infested
disks was significantly less than when parasitoids were supplied with
honey. Therefore, the net reproductive rates, measured as the number of
progeny per female was within the range reported for Eretmocerus
spp. or within E. mundus biotypes (Vet and Van Lenteren, 1981;
McAuslane and Nguyen, 1996; Ghahari and Ostovan, 2002).
Consequently, Abdel-Baky and Ragab (2005) explained many scenarios that
could interprets the lower fecundity in the current study,( 1) the hatching
parasitoid larva may be unable to penetrate the nymph cuticle, due to
the host defense system or thick wax layers (Tuda and Bonsall, 1999),
(2) females of E. mundus may be unable to discriminate between
parasitized and non-parasitized nymphs, so, the same female or another
one from the same species or from others may lay more than one egg under
a parasitized nymphs which leads to inter or intra-specific competitions.
In this trend, Sarhan (1976) mentioned that E. mundus female deposited
one egg/whitefly nymph and in few cases the parasitoid laid two eggs under
the same nymph, but only one parasitoid adult was emerged; (3) females
of different species of Eretmocerus share the same host, this may
leads to super or multi-parasitism (Ardeh et al., 2005) and the
recent study added additional interpretation, (4) the parasitoid adults
may attack a non-suitable instar (4th instars) resulted to un-completed
life cycle and no parasitoids adults were obtained as in idiobiont concept
(Jones and Greenberg, 1998; Jervis et al., 2001; Ghahari et
al., 2005). The previous results may have influenced the parasitoids
biological characters and its efficiency as a bio-control agent against
whiteflies in the open field or in greenhouses.
In respect to the effect of host plant type on the biological characters
of E. mundus, Mandour et al. (2007) reported that the parasitoid
use the pest honeydew as a contact kairomone to locate its hosts. The
sweet potato whitefly produce large quantities of sugar-rich honeydew
while feeding on the phloem of their host plants. Plant sap contains amino
acids and secondary plant products in small quantities which may affect
of parasitoid attractions (Byrne and Miller, 1990; Romeis and Zebitz,
1997). The parasitoid is responded to whitefly honeydew extraction up
to 10 days and search time decreased with increasing honeydew age. In
Table 1 and 2 higher population of
the pest on squash resulted on increasing honeydew secretion and this
gave good biological characters and higher parasitism rates on the plants.
Additionally, parasitoid response to the host was not only due to honeydew
quantity but also due to the enzymatic involved and sugar concentrations
and type (Lewis et al., 1998; Salvucci et al., 1997). Hendrix
et al. (1992) reported that the Homopteran honeydew sugar composition
is determined by both the insect and plant species.
Finally, current results show that E. mundus is a promising candidate
bio-agents against whiteflies. The reproductive potential and certain
biological aspects of the parasitoid could be increased and give a satisfied
control measures when reread on the suitable host plant and insect instars.
Further studies should be applied in the future for integrate the parasitoid
with other biological control agents to enhance their role in controlling
The authors are very grateful to an anonymous reviewer for most valuable
comments improving the manuscript considerably.
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