Ji, L., Gerson, U. and lzraylevich, S., 1994. The mite Hemisarcoptes sp. (Astigmata: Hemisarcoptidae) parasitizing willow oyster scale (Homoptera: Disapididae) on poplars in Northern China. Exp. Appl. Acarol., 18: 623-627.WiUow oyster scale, Lepidosaphes salicina Borchsenius (Homoptera: Diaspididae) is a serious, univoltine, pest of willows and poplars in northern China. Its main natural enemy was Hemisarcoptes sp. (Acari: Astigmata: Hemisarcoptidae), which attacks eggs, nymphs, immature males and females of the scale. Mite prevalence was greatest during June, on 2nd-instar female nymphs of the pest. The numerical responses and density dependencies of the mite on L. salicina are discussed in relation to field data. More scales were parasitized on the eastern and northern sides of the trees, which had more scales, than on their souther and western aspects. Another diaspidid, Quadraspidiont9 gigas (Thiem and Gerneck), was also attacked on poplars by Hemisarcoptes sp.Willow oyster scale (WOS), Lepidosaphes salicina Borchsenius (Homoptera: Diaspididae), is an important pest of willows and poplars in northern China (Ji et al., 1994). Heavy scale infestations may stunt or even kill the trees, especially young ones, used for timber and as shelter belts (Li, 1983). Life history studies showed that the pest raises a single annual generation and that the mite Hemisarcoptes sp.(Astigmata: Hemisarcoptidae) parasitized 36.3 and 41.6% of the scales during two consecutive years (Ji et al., 1993(Ji et al., , 1994. Herein we discuss the mite's life history in conjunction with that of its host. We postulate the temporal and spatial density dependencies of Hemisarcoptes sp., show its presence on the various tree aspects, and note other natural enemies of WOS. The specific identity of this northern Chinese Hemisarcoptes sp. could not be resolved due to taxonomic difficulties, as noted before (Gerson et al., 1990); voucher specimens will be deposited in Rehovot.
Previous studies on frequency-dependent food selection (changing food preferences in response to changes in relative food abundance) have focused on predators and parasitoids. These organisms utilize several victims during their lifetime. We introduce the case of parasites which, having accepted a host, do not change it. We propose two alternative models to explain the biased occurrence of parasites on different host types: (1) through the option of rejecting less-preferred hosts prior to accepting one of them; (2) by differential parasite survival on different host types. These models predict that host rejection, but not differential survival, can create frequency-dependent parasitism (FDP). Unlike previously described factors responsible for frequency dependence of food selection, which act through changing the foraging behaviour of individual predators or parasitoids, FDP involves no adjustment of parasite foraging strategy according to previous feeding experience. The mite Hemisarcoptes coccophagus is an obligate parasite of armoured scale insects (Homptera: Diaspididae). Our field data show that H. coccophagus is found more frequently on ovipositing than on young host females. Our model, combining the effects of host rejection and differential survival, is used to estimate the relative contribution of these factors to parasite biased occurrence on different hosts. The contribution of differential survival was dominant in H. coccophagus, and overode any effect of host rejection. Nevertheless, our prediction that FDP may be found in parasites is supported by literature data about a parasitic water mite.
(Signoret). and oleander scale. Aspidiotrcs nerii Bouche occurring on acacia (Acacia c~~rophpl/a) branches and leaves. Host suitability was e\;aluated by mite survival. mean intensity (active mites/attacked scales) and fecundity. It was highest on letania scale. lowest on oleander scale and intermediate on chaff scales. Results of laboratory experiments were consistent with these field findings: while developing on oleander scale, mites suffered 3x higher mortality. and their generation time was twice as long as on latania scale; the full egg clutch on the latter was high whereas on oleander scale it was negligible. Our data suggest that ovipositing females of latania and oleander scales. but not of chaff scales. arc more suitable hosts than young scale females and the latter more suitable than immature stages. No direct effects of acacia plant parts on host suitability were detected. Citrus species affected chaff scale suitability for the mite through their effect on host diaspidid size. The ability of H. coccophagus to control pestiferous diaspidids may depend on the specific host scale, its age structure and the specific host plant. The implications of these lindings for the biological control of armored scale insects arc discussed.
The aim of this study was to test the predictions of local mate competition (LMC), host quality (HQ) and operational sex ratio (OSR) models, using a non-arrhenotokous parasitic mite, Hemisarcoptes coccophagus (Astigmata: Hemisarcoptidae). The life-history pattern of this mite meets the assumptions of these sex allocation models. Mating group size (LMC model), HQ and OSR affected the sex allocation of H. coccophagus females. Only young mite females adjusted the sex ratio of their progenies according to the predictions of LMC and HQ models; the sex allocation of old females was contrary to these predictions. We explain these patterns by the dynamic nature of the mite's population structure. When parents are young, their population distribution is patchy and progeny matings are local; hence sex allocation is in accordance with LMC theory. When parents become older, their populations shift towards panmixis; factors which had operated previously no longer exist. Consequently, females adjust the sex ratio of late progenies so that it can compensate for the earlier sex allocation, in order to make their total sex ratio unbiased, as expected in panmictic populations. Our data, expressed as the cumulative sex ratio, support this hypothesis.
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