Antheraea mylitta Drury is a commercial silk producing forest insect in India and Xanthopimpla pedator Fabricius is its larval-pupal endoparasitoid, which causes pupal mortality that affects seed production. Effects of host plants, rearing season and their interactions on parasitic behaviour of X. pedator were studied here, as influence of these factors on biological success of X. pedator is not known. Seven forest tree species were tested as food plants for A. mylitta, and rate of pupal parasitization in both the rearing seasons were recorded and analysed. Results showed that rearing season and host plants significantly affected the rate of pupal parasitization in both the sexes. Pupal mortality was found significantly higher (14.52%) in second rearing season than the first (2.89%). Likewise, host plants and rearing seasons significantly affected length, diameter, and shell thickness of cocoons in both sexes. Out of all infested pupae, 85.59% were found male, which indicated that X. pedator chooses male spinning larva of A. mylitta for oviposition, but we could not answer satisfactorily the why and how aspect of this sex specific parasitic behaviour of X. pedator. Multiple regression analysis indicated that length and shell thickness of male cocoons are potential predictors for pupal parasitization rate of X. pedator. Based on highest cocoon productivity and lowest pupal mortality, Terminalia alata, T. tomentosa, and T. arjuna were found to be the most suitable host plants for forest based commercial rearing of A. mylitta in tropical forest areas of Uttarakhand state, where it has never been reared earlier. Sex and season specific interaction of X. pedator with its larval-pupal host, A. mylitta is a novel entomological study to find out explanations for some of the unresolved research questions on parasitic behaviour of X. predator that opens a new area for specialised study on male specific parasitization in Ichneumonidae.
Tropical tasar silkworm,
In outdoor forest rearing, many pest and predators attack the larvae and pupae of
The ecological specialization coupled with wide ranges of hosts and biology, makes
In current study, effects of different forestry host plants, seasons of rearing and their interactions on parasitic behaviour of
The broad objectives of this paper was: (1) to assess the effect of rearing season and host plants on sex wise parasitization of
The study was conducted between 9 April 2012 to 25 July 2013 at New Forest, Forest Research Institute (FRI), Dehradun, Uttarakhand, situated at 30° 19’ 56.21’’ N to 30° 21’ 5.35’’ N and 77° 58’ 56.81’’ E to 78° 0’ 59.73’’ E at 640.08 AMSL (Fig. 3). New forest, laid down in 1925, is spread in an area of 1600.62 ha, which is divided into seven blocks and classified into 21 forest covers and land use classes, each representing a particular group of forest trees (Gupta
The climate of New forest, Dehradun is moderate due to its location at the foot of the Himalayas (Anonymous, 2011 b). Summer starts by March and lasts up to the mid of June, when the south-west monsoon sets in over Uttarakhand. Month of May and early part of June is hottest with mean temperatures of 36.2℃ with maximum temperature of 42℃ (Fig. 4a). Winter starts in November and continues up to February with mean daily maximum and minimum temperature of 19.1℃ and 6.1℃, respectively. The average annual precipitation at the site was 2123.8 mm (Fig. 4b) and most of the rainfall was received during July to September (1937.60 mm). July and August are the wettest months, and in November and January, site received winter rainfall (67.6 mm).
The evaluation of different forest tree species as resource material in respect of the traits that contribute to the growth and development of
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Test Insect and its brief life cycle
Bivoltine (BV) Daba ecorace of
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Forest rearing of A. mylitta
Tasar silkworm being wild in nature, its outdoor rearing was conducted on 5-6 y old bushes of seven forest tree species, each tree species was taken as a treatment. Experiment was run in six replications and two crops were taken. 75 DFLs (disease free layings, also called seed,1 DFLs contains 200 eggs) of
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Host selection behaviour of X. pedator on different forestry host plants
When fifth instar larvae of
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Identification of the parasitoid
The dead adults ichneumon fly found in parasitized pupae of
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Statistical design and data analysis
The percent mortality data of male and female pupae required an angular transformation to normalize the data, which was carried out by using angular transformation table of Fisher and Yates (1963). The distributions of all other variables were examined and none required a transformation to achieve normality. A two-way completely randomized block factorial design was used to test the significance of difference in the means of variables. We did factorial ANOVA by using STATISTICA 10 (Stat Soft Inc, UK) to analyse the data. Rearing seasons and host plants were treated as the main (fixed) effects; however, mortality rate in male and female pupae, cocoon length, cocoon diameter and cocoon shell thickness of male and female cocoons served as dependent variables. The level of significance for the study was fixed at P=0.05. Post HOC test was carried out by using Tukey’s HSD test to compare the homogeneous pairs of means. We also carried out multiple regression analysis for two reasons; first, to examine the kind of existing relationship between biometric parameters of the cocoons (independent variables) and parasitization rate of male and female pupae (dependent variable) and second; to find out the potential predictor(s) for pupal parasitisation. Graphs on the effects of rearing season and host plants on measured parameters were developed by using Microsoft excel.
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Host selection behaviour of X. pedator on different forestry host plants
Female ichneumon flies begin to appear at the rearing site from mid July and continued up to November until the period of hardening of the cocoons. However, its larger population was found during second rearing season, whereas in first crop their presence could be hardly traced. It was found that ichneumon fly prefers to attack the spinning larvae or freshly formed cocoons in the morning (6.30-11.00 a.m.) and evening hours (4.00 - 6.30 p.m.). Flies showed their ability to locate their preferred host accurately, even if they were concealed amidst the leaves of host plants. The time spent in selecting the larva through antennal probing while moving on cocoon hammock hardly last for 45 s and time spent in choosing the oviposition site from the movement of aligning the fly on spinning worm last for 20- 30 s, while actual time spent to deposit an egg inside the larval body was about 4-6 s. It was seen that the parasitoid attains partially inverted “U” shape during oviposition and leaves the larva after oviposition. It was observed that during oviposition, spinning worm flutters vigorously by moving its head and lover abdominal segments. In most of the cases,
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Effect of rearing season and host plant on production of cocoons
The number of harvested cocoons differe significantly between rearing seasons (DF 1, F 42.67, P<0.05) and among host plants (DF 6, F 87.62, P<0.05). Analysis of variance for the effect of rearing season and host plants on cocoon production and parasitization rate of male and female pupae is shown in Table 1. Cocoon production was found higher in first rearing season (127.71 cocoons) than second (105.42 cocoons). Overall,
Analysis of variance for effect of rearing seasons and host plants on cocoon production and parasitization rate of male and female pupae
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Effect of rearing seasons and host plants on parasitic behaviour of X. pedator
Null hypothesis (H0) for no difference on the effect of rearing seasons and host plants on parasitization rate of
Combined effect of rearing seasons and host plants on sex wise mean (SE) pupal mortality of A. mylitta due to parasitization of X. pedator
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Effect of host plants on biometric parameters of the cocoons
Analysis of variance for effect of rearing seasons and host plants on biometric parameters of male and female cocoons is shown in Table 3. Length and diameter of male and female cocoons were significantly affected by rearing seasons (DF 1, P<0.05), host plants (DF 6, P<0.05) and their interactions (DF 6, P<0.05). Likewise, shell thickness of male and female cocoons were also significantly affected by rearing reasons (DF 1, P<0.05) and host plants (DF 6, P<0.05), but effect of their interactions was found non-significant (DF 6, P>0.05)). The length, diameter and shell thickness of male and female cocoons increased from first rearing seasons to second rearing season on all the tested forestry host plants. Further, the length, diameter and shell thickness of female cocoons were found always higher than the male cocoons in both the rearing seasons on all the forestry host plants. The host plant
Analysis of variance for effect of rearing seasons and host plants on biometric parameters of male and female cocoons Parameters Source DF F value P- value
[Table 4.] Biometric assessment of male and female cocoons infested by X. pedator (mean ± SE)
Biometric assessment of male and female cocoons infested by X. pedator (mean ± SE)
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Biometric parameters of the cocoons and rate of pupal parasitization
The beta coefficient obtained from multiple regression analysis on parasitization of male and female pupae on length, diameter and shell thickness of the cocoon showed that the length and shell thickness of male cocoons, and shell thickness of female cocoons are strong predictors for pupal parasitization (Table 5 and 6). Scatter plot of parasitization in male and female pupae indicated that cocoon shell thickness of both the sexes is negatively associated with the pupal parasitization rate (Figs 7a and 7b.) whereas, length of male cocoon was found to have a positive association (Fig. 8). Further, diameter of male and female cocoons, and length of female cocoons was found to have positive association with pupal parasitization rate, but these were found non-significant.
Multiple regression summaries for dependent variable: Parasitization of male pupae (%) R= .92567246; R2= .85686950; Adjusted R2= .85150210; F(3,80)=159.64; p<0.0000; Std. Error of estimate: 2.5871 N=84
Multiple regression summaries for dependent variable: Parasitization of female pupae (%) R= .76004600; R2= .57766992; Adjusted R2= .56183254; F(3,80)=36.475; p<.00000 Std. Error of estimate: 2.3161
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Host selection behaviour of X. pedator on different forestry host plants
The fitness and success of any parasitoid is largely determined by its capacity to accurately locate its host first in a given ecosystems (Hausmann
Semiochemicals are considered as the most common cues for parasitoids (Hou and Yan, 1997), which originate from the hosts and host-damaged plants (Vinson, 1991). Parasitism of Lepidopterous species is strongly host-plant dependent and their parasitoid species are specialized with respect to their hosts’ tree species (Lill
Vision information(s) are used by many ichneumon flies to locate their host. Colour contrasts (chromatic or achromatic) of the host rather than specific colour characteristics are used in visual host detection (Schmidt
Contact stimuli (character and hardness of the host covers) also serve as the most influential cue in host selection of ichneumon parasitoids (Xiaoyi and Zhongqi, 2008). Schmidt
Vibrational sounding is an important cue for host location in many hymenopterous parasitoids. Some species of the Ichneumonidae and Orussidae have evolved a sophisticated host finding mechanism that is called vibrational sounding, which allows the wasps to detect their host under substrates like plant material or silken cocoons (Quicke, 1997). This mechanism is used as a kind of echolocation in which self-produced vibrations are transmitted onto the substrate and perceived by subgenual organ in the tibiae (Kroder, 2006).
Female parasitoids are guided by multisensory information system during host finding, however individual cues are used in an interactive or a hierarchical manner, however, the combination of vision and vibrational sounding increases the precision of host location (Fischer
We have observed that
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Instar specific solitary parasitic behaviour of X. pedator
It is known that parasitic insects choose to attack immature stages of their concealed hosts. Similarly instar (fifth) and stage (spinning) specific host selection behaviour of
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Parasitization timings of X. pedator
We found that host searching and oviposition activity of ichneumon fly was largely restricted in the morning (6.30 a.m.-11.00 a.m.) and evening (430 p.m.-6.45 p.m.) hours of the day, and their presence in rearing field reduced remarkably between high sun-shine day hours. The reasons for time specific searching behaviour and oviposition of
It has also been reported that Ichneumonoidea wasps can detect infrared induction (differences in environmental temperature) radiating from their concealed host. Richerson and Borden (1972) while studying a braconid,
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Process of oviposition in the host
After host location, the ovipositor plays an important role in host discrimination and host acceptance. It is reported that many cocoon parasitoids are stimulated to oviposit by the presence of silk of a particular age (Gauld and Bolton, 1988). Movement of host pupa within the cocoon can also be an important cue to elicit oviposition, or preventing host acceptance in many species of parasitoids (Arthur, 1981). For example, Lloyd (1940) found that movement by
Many tropical endoparasitic idiobionts of ichneumonids fly show a wide range of morphological specializations associated with manipulating the ovipositor and penetrating a substrate (Gauld, 1987). According to Scudder (1961) female ichneumon fly injects venom into the host prior to oviposition that paralyses the host without killing it and consequently host remains alive and fresh until consumed by the larva. Injected venom subtly modifies and manipulates the physiology and development of the host and restricts host reactions against ichneumon larva by physical destruction of vital organs such as the brain (Vinson and Iwantsch, 1980; Shaw, 1981). This duel function of ovipositor (egg placement/venom injection) is found in all Apocrita except the aculeate superfamilies (Gauld and Bolton, 1988). Fuhrer and Willers (1986) have demonstrated that the larva of
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Effect of rearing season and host plants on production of cocoons
Season indicates the inter-annual variation in temperature, humidity, sunshine, rain fall etc. of a particular place, which is governed by different geographical parameters. Studies have shown the importance of seasonal variations in biology and development of a given insect (Odum, 1983; Ouedraogo
The success of an insect depends significantly upon an optimal diet in both quantity and quality (Hassell and Southwood, 1978) which provides the energy, nutrients, and water to carry out life’s activities (Slansky, 1993). Forestry host plants are known to differ greatly with their nutrient profile that affects the growth and development of herbivorous insects (Kohli
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Effect of rearing seasons and host plants on parasitic behaviour of X. pedator
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Season specific parasitic behaviour of X. pedator
The extent of pupal loss in
In first rearing season, the intensity of pupal parasitization was almost negligible (2.89%), whereas in second rearing season, there was almost fivefold increase (14.52%) in parasitization rate of
In second rearing season, temperature remains favourable for effective host location by
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Sex specific parasitic behaviour of X. pedator on A. mylitta
Observation on sex wise pupal mortality indicated an interesting fact about male specific parasitic behaviour of ichneumon fly, which was confirmed by high proportion (85.59%) of male infested pupae in total parasitization. It means,
Literature survey does not indicate any specific reason for such intersexual host selection behaviour of
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Effect of cocoon biometric parameters on pupal parasitization
Klomp and Teerink (1962) concluded that hymenopteran parasitoids use their antennae to explore size, shape and surface texture of the prospective host to elicit their oviposition response. Price (1970) observed that the ichneumonid,
Further, result of multiple regression analysis (Table 5 and 6) indicated that length and shell thickness of male cocoons significantly affect the parasitization rate of
Further, it was also found that cocoon shell thickness in both the sexes were negatively associated with the parasitization rate of
Sex specific parasitic behaviour of
Results suggest that