Sergey Izraylevich

Mite parasitization on armored scale insects: host suitability

Suitability of three armored scale insects (Homoptera: Diaspididae), their stages, the host plants and their parts, for the parasitic miteHemisarcoptes coccophagus Meyer was investigated. The studied scales were chaff scales,Parlatoria pergandii Comstock andP. cinerea Hadden, infesting grapefruit and orange orchards, latania scale,Hemiberlesia lataniae (Signoret), and oleander scale,Aspidiotus nerii Bouche occurring on acacia (Acacia cyanophylla) branches and leaves. Host suitability was evaluated by mite survival, mean intensity (active mites/attacked scales) and fecundity. It was highest on latania 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 3× 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, are 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 ofH. coccophagus to control pestiferous diaspidids may depend on the specific host scale, its age structure and the specific host plant. The implications of these findings for the biological control of armored scale insects are discussed.

Sex ratio of Hemisarcoptes coccophagus, a mite parasitic on insects: density-dependent processes

Hemisarcoptes coccophagus Meyer (Acari : Astigmata : Hemisarcoptidae) is an obligate parasite of armoured scale insects (Homoptera : Diaspididae). We tested the following hypotheses : (1) the sex ratio of H. coccophagus is affected by its population density ; (2) this effect is due to an asymmetric performance and dispersal of males and females at different density levels ; (3) the effect of density on sex ratio depends on the developmental stage of the mite exposed to it. Sex ratio (proportion of females) of H. coccophagus adults in the field fluctuated between male and female biases and was negatively correlated with mite density. Laboratory experiments showed that during immature development mite survival and sex ratio decreased with increasing density, indicating that at high density levels immature females suffered higher mortality than males. At high density levels females, but not males, had smaller bodies and forelegs (which, according to the host-quality model, may affect the sex allocation decision of reproducing females). Longevity of adult males at low density was higher than that of females, while at high density, and while starving, females lived longer. Adult dispersal was affected by density : both sexes stayed on their hosts when density was low, while at high density levels females dispersed more than males. We conclude that population density may cause fluctuations in the sex ratio of H. coccophagus in either male or female directions, depending on the density level and on the mite stages exposed to its influence. The operative function underlying the effect of population density on the sex ratio might be intraspecific competition.

Sex allocation by a mite parasitic on insects: local mate competition, host quality and operational sex ratio

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 mites 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.

Frequency-dependent host selection by parasitic mites: a model and a case study

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.

The mite Hemisarcoptes sp. (Astigmata: Hemisarcoptidae) parasitizing willow oyster scale (Homoptera: Disapididae) on poplars in Northern China

Willow 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, Quadraspidiotus gigas (Thiem and Gerneck), was also attacked on poplars by Hemisarcoptes sp.

Karyotype and the sex determining mechanism of the mite Hemisarcoptes coccopahgus Meyer (Acarifomes : Astigmata : Hemisarcoptidae)

Abstract Hemisarcoptes coccophagus Meyer (Acariformes: Astigmata: Hemisarcoptidae) is a parasite of armored scale insects (Homoptera: Diaspididae). This mite can adjust the sex ratio of its progeny. We found that H. coccophagus has a diploid karyotype with 2n=14 in both sexes; the sex determining mechanism is either diplo-diploidy or parahaploidy. This provides one of the few examples, and the first in the Acariformes, of a non-arrhenotokous arthropod genetic system with the ability of facultative sex ratio adjustment.

Population dynamics of Hemisarcoptes coccophagus Meyer (Astigmata: Hemisarcoptidae) attacking three species of armored scale insects (Homoptera: Diaspididae)

The parasitic miteHemisarcoptes coccophagus Meyer (Astigmata: Hemisarcoptidae) feeds on armored scale insects (Homoptera: Diaspididae). Three host scales were studied: chaff scale (Parlatoria pergandii Comstock andP. cinerea Hadden) infesting citrus (grapefruit and orange) orchards, latania scale [Hemiberlesia lataniae (Signoret)] and olcander scale (Aspidiotus nerii Bouche) occurring on acacia (Acacia cyanophylla) branches and leaves. The overall dynamics of mite populations were similar on the different scale species, although the level of attack on each sampled diaspidid host was different. Rates of young mite stages were highly variable on all three hosts throughout the sampling period while adult rates were stable, constituting around 20% of all mites. No oviposition by mites occurred in mid-winter on any scale species, on any plants or parts, at all sampling sites. Of the three diaspidids, highest mite prevalence (parasitization rate) occurred on latania scale. Mite prevalence on oleander scale infesting mature leaves was higher than on young leaves.H. coccophagus apparently first attacks latania scale on branches, it then disperses to the same host inhabiting mature leaves, from there to co-occurring oleander scale and finally to oleander scale infesting young leaves. Mite prevalence on chaff scale was influenced by host plants and climatic regions: in the Negev it was higher than in the coastal plain, with scales on grapefruit being more heavily attacked at the former site but not on coastal plain. Our data suggest thatH. coccophagus prefers ovipositing scale females. The decreasing order of preference for other scale stages was: young females immature males, 2nd-instar nymphs females.

A sex-specific tri-trophic-level effect in a phoretic association

Hemisarcoptes coccophagus (Acari: Astigmata: Hemisarcoptidae) is an obligate parasite of armored-scale insects (Homoptera: Diaspididae). Hypopodes (non-feeding heteromorphic deutonymphs) of this mite cannot complete their development without a phoretic sojourn on the coccinellid beetle Chilocorus bipustulatus. We tested whether the feeding history of the beetles affects the probability of the hypopodes completing their development. Two diets were offered: armored-scale insects (Homoptera: Diaspididae) (suitable for both beetles and mites) and soft-scale insects (Homoptera: Coccidae) (suitable for the beetles but not for the mites). The hypopodial survival was similar for mites which had been on beetles reared on the different scales. However, female hypopodes which had stayed on beetles reared on soft scales suffered higher mortality than those from Chilocorus reared on armored scales. Male survival was not affected. A tri-trophic-level effect on sex-specific survival in a phoretic association was thus demonstrated.