· Adv. Zool. 2005: 26(1) : 7-19
THE REPRODUCTIVE BEHAVIOUR OF LIPOLEXIS SCUTELLARIS MACK. (HYM.:BRACONIDAE), A PARASITOID OF APHIS GOSSYPII GLOVER (HOM.: APHIDIDAE): ADJUSTMENT OF SEX RATIO IN RESPONSE TO HOST SIZE S. Pandey and R. Singh Aphid Bio-control Laboratory, Department of Zoology, DDU Gorakhpur University, Gorakhpur-273 009, U.P., India Email:
[email protected] ABSTRACT: As the basic necessity in biological control programmes. using parasitoids, is the mass propagation of maximum quantity of high quality females, it is necessary to know the factors that could maximise the production of daughters. Several factors have been recognised that influence the progeny production as well as their sex ratio in parasitic wasps. Host size had a greater effect on the reproductive success of several aphid parasitoid. In case of Upo/exis scutellaris Mackauer (very little known parasitoid) to deposit fertilised (diploid) eggs in large hosts (third instar nymph) and unfertilised (haploid) egg in small host (first and second instar nymphs). Unpreferred fourth instar nymphs and apterous aphid adults also received more haploid eggs despite being larger in size than the preferred third instar nymphs. However, the perception of host size by the mother was dependent of the extent of temporal variation in the host size distribution and on her previous experience of host size. Developmental period, longevity, mating potential, fecundity and progeny sex ratio of L. scutetletis emerging from small versus large hosts indicated that the host size affected the fitness of the daughters more than that of sons. The present study could not provide any evidence of differential mortality of either sex of L. scutellaris in small versus large hosts. The absence of differential mortality clearly shows that while ovipositing in the growing stages of an aphid, L. scutellaris does exhibit maternal manipulation of progeny sex ratio according to the host size. KEYWORDS
: Lepolexis
scutellaris,
sex ratio, host size
INTRODUCTION Lipolexis scutellaris Mack. IS widely distributed parasitoid of aphid species particularly, Aphis craccivora Koch, Aphis gossypii Glover, Aphis nasturtii Kalt. and Myzus persicae (Sulzer) on several vegetable crops, viz., brinjal (Solanum melongena), bean I Dolichos lablab), chili (Capsicumfrutescens), pigeonpea (Cajanus cajan) and Toxoptera citricida Kirkaldy on citrus crops. L. scutellaris reproduce arrhenotokously, i.e., sons develop from unfertilised haploid eggs by parthenogensis while daughters develop from .ertilised (diploid) eggs. Virgin mothers always :.ield only sons while mated mothers are able :0 produce both sons and daughters. Thus the .mderstanding of the factors that could
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influence the mechanism of fertilisation of the eggs is essential. As it is pre-requisite not only in the development and testing the sex ratio theories and modles but also for their practical applications in biological control because it is the females that ultimately killed the hosts. The maximisation of daughters in mass-rearing programmes also requires the knowledge of such factors that favour female biased population. The host size is one of the several factors that could affect the progeny sex ratio in the aphid parasitoids+" In recent years, host size was considered much while describing the variations in population dynamics of their parasi toi ds. 3.4.7.8 i 0.16, 18.19.n.1-'.24.25.16.41.47 The size of the aphid hosts varies with the age and an increase in host size (=age) does
Adv. Zoo!. 2005.26 (I)
not necessarily imply the improvement of host quality. The aphid parasitoids prefer middle age nymphs (second and third instar) for oviposition than larger older stages (fourth instar and adults) and deposit more diploid eggs into the preferred host stages 32,46,47, Aphid size determines not only the parasitoid size but also any size related components of parasitoid fitness. Experimental evidence strengthens the prediction of sex ratio models that a greater proportion of fertilised eggs will be deposited in larger hosts. This prediction was expected only in the parasitoids which oviposit in non-growing and non-feeding host stages (idiobiotic parasitoids) where the host size is considered to be a good predictor of host resources at the time of oviposition". The aphid parasitoids, in contrast, are koinobiotic parasitoids as the parasitised hosts still grow in size. Therefore, the aphid parasitoids are assumed not to have the skill to assess the host's future quality at oviposition and thus in them, host size related progeny sex ratio should not be expected". However, there is no reason that females need this knowledge as what is necessary is a consistent relationship between host size at oviposition and resources available to the developing parasitoids". Differential mortality of the progeny by sex during development in smaller and larger hosts was proposed as an alternative explanation for shifts of progeny sex ratio in parasito id wasps":". Therefore, the host size model should also include such information". There are few laboratory studies that deal with host size related progeny sex ratio in aphid parasitoids, viz. for Ephedrus californicus, a parasitoid of pea aphid, Acyrthosiphon pisum;" for Lysiphlebus delhiensis, a parasitoid of com aphid, Melanaphis sacchariP for Lysiphlebia mirzai, a parasitoid of pearl-millet aphid Rhopalosiphum maidis" and for Binodoxys indicus a parasitoid of Ascraecivora", With a view to enhance our knowledge on this aspect
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of aphid parasitoid biology the investigations concerning variation in progeny sex ratio of L. scutellaris caused by host size, the present study was conducted. In this investigation following hypotheses were examined: 1. the prediction that a greater proportion of daughters will emerge from larger hosts 2. the occurrence of differential mortality of male and female progeny exists in smaller versus larger hosts. 3, the assumption that host size has a greater positive effect on the fitness (reproductive success) of the parasitoid. MATERIAL AND METHODS Culture of Aphids and Parasitoids : The aphid Aphis gossypii and the parasitoids L. scutellaris were cultured in the laboratory on seedlings of Solanum melongena potted in clay pots at 22.22 °C in screened (625 mesh/ern") cages (45 x 45 x 60 em) at 12 hour photoperiod, 5000-10000 lux light intensity and 60-80 % r.h. only freshly emerged (0-12 hour old) and naive parasitoids fed on a mixture of honey, honeydew and water in the ratio of 30:30:40 (v/v) were used for the experiments, in the experiments requiring mated females, each female was paired in a 5 ml glass vial. All experiments were conducted at the same conditions as adopted for maintenance of aphid and parasitoid cultures in the laboratory. The experiments were carried out inside the cages (similar to those used in the laboratory cultures) having potted seedlings of S. melongena with 4-5 leaves (20-30 days old), to serve as food plant. after the exposure to the parasitoids, the aphids (hosts) were allowed to rear on the food plant until the emergence of the adult parasitoids (progeny population), whereupon they were sexed and counted, Mortality of the parasitised hosts (as mummies) that did not yield adult parasitoids was also
Ad\· Zool 100516 (/)
recorded. The data obtained were expressed 3.5 proportion for host acceptance (host parasitoid) and male emergents (progeny sex ratio]. The data were arcsine transferred before the statistical analyses (e.g., Anova and or paired t-test), Impact of host size on the progeny sex ratio of the parasitoid : To asses the influence of host size on the progeny sex ratio, following experimental sets up were designed following Singh et al," (a) No-choice condition : Five female parasitoids were introduced individually into five cages each harbouring approximately 100 aphid nymphs of either of first, second, third, fourth instars and apterous adults on the food ?lant. After 24 hour, the females were withdrawn from the cages and were transferred :0 other cages wherein they were confined for : successive days with fresh groups of similar number of aphids of the same stage offered oer day. Five replicates were performed, thus 25 female parasitoids were utilised in this set :.:p of the experiment.
Choice conditions : The experimental cesign and the replications were basically the same as described above for test type (a) with ::-.edifference that 20 individuals each of first, second, third and fourth instar aphid nymphs z s well as of apterous adults were smultaneously exposed to the individual :·~T:alesfor only 24 hour. After parasitisation, :::~ exposed aphids were separated stage-wise ;...-:.::were placed gently on the leaves of potted :. .'~ng seedlings of the brinjal for further z evelopment until the emergence of the adult :.~5itoids where upon they were sexed and ~>.:.!ed. (b)
..:)To separate maternal effects, if any, from :':~:-5ize effect on progeny sex ratio of L. :.:-..:::lJris, the host size was categorised into ;;;- ~-. and large classes for the ANOV As. A
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single female wasp was first released into the cage having 50 small hosts (first instar) on the food plant for only 5 h. Thereafter, the parasitoid was removed, fed and re-introduced after half an hour into another cage having 50 large hosts (third instar, the stage most preferred) on the food plant for another 5 h. In each of these, the test was conducted using five separate females of the same parent. (d) The experimental design and the replications were basically the same as described above for test type (c) with the difference that the female parasitoids were introduced first into the cages having large hosts and then small hosts. Impact of size on mortality and fitness of male and female progenies during development Differential mortality of the male and female progenies was determined by comparing progeny production of mated versus virgin mothers following King." For this purpose, 10 mated and 10 virgin females obtained from the same cohort were individually given access to 100 first or third instars nymphs of the aphid, as described earlier, for parasitisation. Fresh lots of same number and stage of nymphs were provided to each such female at the end of every 24-hour till her death. Total developmental period of successfully emerged male and female individuals of the parasitoid was also determined. Impact of host size on the fitness progeny
of the
(a) Mating ability of sons that emerged from small and large hosts : Mating ability of sons that emerged from different test situations outlined above were also observed. The male and female parasitoids were allowed to mate in a glass vial. After successful mating, the males were withdrawn and allowed to rest for an hour and were again paired with another
Adv. Zoo!' 200526
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set of female-sibs. This exercise was repeated until the male progeny died. The frequency of each of the four possible mating outcomes were noted: (1) male progeny from a small host x female progeny from a small host, (2) male progeny from a small host x female progeny from a large host, (3) male progeny from a large host x female progeny from a small host and (4) male progeny from a large host x female progeny from a large host. (b) Fecundity and progeny sex ratio of progenies that emerged from small and large hosts: Five mated female progenies were withdrawn separately from all four crosses of the above experiment were individually introduced into the cages, each having a young seedling of the host plant having approximately 100 third instar hosts for 24 hours. After every 24 hour, the females were given fresh lot of hosts throughout their life span to measure fecundity and progeny sex ratio. (c) Longevity of the progenies that emerged from different test situations: The longevity of both sons and daughters resulting from the above experiments was also determined for 100 individuals of each sex. For this, the adults were put in a large glass vial (1000 ml flask) with a regular food supply consisting of a mixture of honey, honeydew and water in the ratio of3:3:4 (v/v) along with a cotton swab moist with water to keep profound humidity and their mortality was recorded every day until all the adults died. RESULTS
AND DISCUSSION
Effect of host size on the progeny sex ratio: The female parasitoid L. scutellaris attacked all the nymphal stages and apterous adults irrespective of their size variations in both nochoice as well as choice situations (Table 1). Both the proportion of hosts accepted for parasitisation as well as the progeny sex ratio were observed to increase with increase in host
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size, but only upto third ins tar nymphs. In contrast, both the parameters decreased as the host size further increased (from third ins tar to apterous adult). The two way ANOV A provides sufficient evidence that both the test types (choice versus no-choice experiments) as well as host developmental stages influence not only proportion of hosts accepted (F test type = 38.86 ' n= l , n,= 44, P< 0.001; F h est-size . = 32.66, n I=4 ' _ n2= 44, P< 0.001) but also the progeny sex ratio (F test-type =3.58, n I'=1. n 2=44 ' P < 0 .,001· F host5;ze=13.84,nl=4, n2=44, P < 0.001). Regression between proportion of hosts accepted [X] and progeny sex ratio [Y] yielded significant negative correlation coefficients (No-choice experiments: Y = 2.22-0.80X, r= -0.901, P< 0.001; Choice experiment Y= 1.91-0.33X r = -0.741, P < 0.001). Table 2 displays the data of the experiments desigened to test the previous host size encounter experience of females in sex allocation. This shows that the progeny sex ratio was highly female-biased in large hosts when they were previously exposed to small hosts (0.3349±0.0577 SD) than to large hosts (0.3479±0.919 SD). Similarly, the progeny sex ratio was highly male-biased in small hosts when the females were offered large hosts first (0.7232±0.0992 SD) than small hosts (0.5143±0.0319 SD). When the data was subjected to two-way of ANOV A, variances in progeny sex ratio caused by host size as well as caused by order- of host exposure were significant (FIiost-srze. = 289.44, n I = 1, n 2 = 17' P< . . 0.001; Fhostorder= 36.05, nl = 1, llz= 17, P
