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    Reproductive plasticity in response to the changing cluster size during the breeding period: a case study in a spider mite
    (Springer Nature, 2023-10) Weerawansha N; Wang Q; He XZ
    Animals living in clusters should adjust their reproductive strategies to adapt to the social environment. Theories predict that the benefits of cluster living would outweigh the costs of competition. Yet, it is largely unknown how animals optimize their reproductive fitness in response to the changing social environment during their breeding period. We used Tetranychus ludeni Zacher, a haplodiploid spider mite, to investigate how the ovipositing females modified their life-history traits in response to the change of cluster size (i.e., aggregation and dispersal) with a consistent population density (1 ♀/cm2). We demonstrate that (1) after females were shifted from a large cluster (16 ♀♀) to small ones (1 ♀, 5 and 10 ♀♀), they laid fewer and larger eggs with a higher female-biased sex ratio; (2) after females were shifted from small clusters to a large one, they laid fewer and smaller eggs, also with a higher female-biased sex ratio, and (3) increasing egg size significantly increased offspring sex ratio (% daughters), but did not increase immature survival. The results suggest that (1) females fertilize more larger eggs laid in a small population but lower the fertilization threshold and fertilize smaller eggs in a larger population, and (2) the reproductive adjustments in terms of egg number and size may contribute more to minimize the mate competition among sons but not to increase the number of inhabitants in the next generation. The current study provides evidence that spider mites can manipulate their reproductive output and adjust offspring sex ratio in response to dynamic social environments.
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    Multiple mating and mate choice in Sitophilus oryzae (L.) (Coleoptera : Curculionidae) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Protection (Entomology) at Massey University, Palmerston North, New Zealand
    (Massey University, 2010) Flay, Casey Damian
    The rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) is a major pest of stored grains throughout the world. A recent study found a reduction in fecundity and fertility in females that are exposed to multiple males. However, the mechanism behind this is unknown. In the first experiment we examined female S. oryzae fitness decline and behaviour in response to male density (i.e. 1, 5 and 10 males) maintained with a single female. Results show that female fecundity and longevity significantly decreased with increased male density (P < 0.01). However there was no significant effect of male density on fertility. Mating time significantly increased with male density (P < 0.01) but feeding and foraging time decreased from male density 1 to male density 5 and 10 (P < 0.03). The decrease in fecundity under high male density is proposed to be caused by increased damage by male reproductive organs. The second experiment examined precopulatory sexual selection in S. oryzae. Results show that both sexes select for large genitals. In addition males select for the number of mature eggs in females. These traits may directly affect the reproductive fitness of these insects. In the final experiment we examined how multiple mating affected female reproductive fitness in the laboratory. Results indicate that over sixty days females permanently paired with males, mated twice with different males and four times with the same males did not suffer from significant fertility decline while females mated once, twice with the same males and four times with different males had significant fertility declines. It is suggested that mating once or twice with the same males is not enough for females to maximise their reproductive fitness. The significant fertility decline in females that mated with four different males may be caused by reproductive organ damage or other factors which decreases their fertility due to excessive polyandry.
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    Factors influencing selection of settling sites within plants and oviposition by greenhouse whitefly (Trialeurodes vaporariorum Westwood) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Health at Massey University
    (Massey University, 1991) Breach, Joan Elizabeth
    Orientation by adult greenhouse whitefly (Trialeurodes vaporariorum Westwood) to younger leaves is induced by negative geotaxis and positive phototaxis but there is a minor effect of features of the leaves. The selection of the lower leaf surface is predominantly the result of a preference for being upside-down (ie. a response to gravity) but leaf characteristics also play a role. Negative phototaxis has a minor effect. Adult females lay more eggs on the younger leaves and on the lower leaf surface of some plant species but not others. Leaf hairiness and leaf angle are not significant factors in selection by adults of either 1) younger leaves or 2) the lower leaf surface nor are they significant factors in the number of eggs/female/day laid on either 1) leaves of different ages or 2) the lower or upper leaf surfaces. Adult survival on sucrose sachets (aqueous sucrose solution sandwiched between two layers of Nescofilm) was optimum for 15-20% sucrose and eggs/female/day laid on the sucrose sachets was independent of sucrose concentration when it was between 10% and 30%. Eggs/female/day reached a maximum after 2-3 days and thereafter dropped sharply. The number of larvae that hatched was independent of sucrose concentration but higher concentrations appear to induce later hatch. Percent egg hatch varied from 40% to 77%. The number of eggs laid on 20% sucrose sachets in complete darkness was nearly twice that of any other light intensity. There was no graded relationship between light intensity and oviposition. More eggs were laid on 15% sucrose sachets in light/dark regimes of 8/16, 4/20 and 0/24 than of 12/12, 16/8 and 24/0 hours. No diurnal fluctuation in egg-laying occurred nor were more eggs laid in either light or dark periods. The sucrose sachet technique is a suitable tool for further studies on greenhouse whitefly behaviour. The results provide further information for incorporation into integrated pest management research.
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    Hormonal stimulation of ovarian development, ovulation and oviposition in Japanese quail : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physiology at Massey University, Palmerston North, New Zealand
    (Massey University, 2002) Bennett, Ellen Joan
    Stimulation of ovarian development and ovulation leading to production of fertile offspring using exogenous hormones has been successful in mammals, but until recently this was not the case for avian species. These techniques would be useful for increasing the reproductive output of endangered birds such as the kakapo. Pregnant mare serum gonadotropin (PMSG) was used to stimulate ovarian development in Japanese quail as it is readily available, easy to use, and equally effective as avian gonadotropins. The research examined the best method for administering PMSG, and the doses, duration and frequency of treatment required to stimulate follicular growth. Treatment with PMSG can stimulate ovarian development, ovulation and oviposition in Japanese quail held under a short day photoperiod. However, there was considerable variation in ovarian response to PMSG between birds receiving the same treatment. In birds in which large yellow follicles developed, many follicles were similar in size and were not arranged in a hierarchy. Doses of 20-80 IU PMSG were the most appropriate for stimulating ovarian development in Japanese quail. Doses lower than 20 IU PMSG stimulated little or no ovarian development in most birds, and doses higher than 80 IU PMSG led to overstimulation of follicular development in most birds. Continuous delivery of PMSG by osmotic pumps and daily treatment using injections were equally effective in stimulating ovarian development in Japanese quail. The use of daily injections is a more practical method of delivering PMSG to birds, as it does not involve surgery and allows more control over dosage and timing of treatment. Treating birds with injections of PMSG every two days rather than daily led to a rate of ovarian growth similar to that of long day birds. Treatment every four days was not sufficient to stimulate ovarian development in quail. Restricting the feed intake of quail did not have any affect on the ovarian response to PMSG treatment. Although PMSG can stimulate ovarian development and ovulation in Japanese quail, further work is required to increase the number of birds that respond to treatment, increase the number of eggs produced by an individual, and improve egg quality.