Robert F. Luck

The effect of host size on quality attributes of the egg parasitoid, Trichogramma pretiosum.

In a study of the quality of Trichogramma pretiosum Riley (Hymenoptera: Trichogrammidae), we compared female wasps emerging from natural hosts, parasitized in the laboratory or the field with those emerging from factitious hosts used for commercial mass production. Females from the natural hosts were larger, more fecund, and longer lived than those from the factitious hosts. Compared to small females, large female wasps are substantially more fecund when honey (carbohydrate) is available but marginally more fecund when honey is unavailable. The size of a female T. pretiosum depends on two factors: the size of the host egg from which it emerges even when the wasp was gregarious, and the number of conspecifics that emerge with it. The similarities in the size distribution of female wasps emerging from natural hosts, in conjunction with the mechanism by which Trichogramma measure host size and allocate eggs accordingly, suggests the hypothesis that size related components of fitness in female T. pretiosum are under strong selection in the field.

Evaluation of natural enemies for biological control: A behavioral approach

The success of biological pest control has stimulated the development of analytical models that explore the dynamics of natural enemies and their hosts or prey. These models seek to identify those general characteristics o f the natural enemy, host or prey population that lead to economic pest control. Because the models are strategic in nature, they are of limited value in identifying the specific attributes of an effective biological control agent prior to its introduction. Empirically developed criteria have also been of limited predictive value because they too provide only general guidelines. Behavioral ecology and foraging and sexratio theories may be useful adjuncts to these approaches, by identifying the evolutionary constraints and thus helping to define better the attributes of an effective natural enemy.The success of biological pest control has stimulated the development of analytical models that explore the dynamics of natural enemies and their hosts or prey. These models seek to identify those general characteristics o f the natural enemy, host or prey population that lead to economic pest control. Because the models are strategic in nature, they are of limited value in identifying the specific attributes of an effective biological control agent prior to its introduction. Empirically developed criteria have also been of limited predictive value because they too provide only general guidelines. Behavioral ecology and foraging and sexratio theories may be useful adjuncts to these approaches, by identifying the evolutionary constraints and thus helping to define better the attributes of an effective natural enemy.

Competitive Exclusion of Aphytis lingnanensis by A. melinus: Potential Role of Host Size

Aphytis melinus, an ectoparasitoid of California red scale, Aonidiella aurantii, replaced A. lingnanensis, another ectoparasitoid, in certain California citrus areas during the 1960s following the formers introduction in 1956-1957. We show that, although both species attack a similar range of host sizes, A. melinus utilizes a smaller threshold scale size for the production of daughters. Size differences of scales within a citrus tree (largest scales occur on fruits, smallest on wood, and inter? mediate on leaves), seasonal variation in scale age structure, multivoltinism ofthe scale and parasitoid populations, seasonal availability of citrus fruits (the substrate that supports the largest scales), dif? ferences in the temperature tolerances ofthe two parasitoid species (A. lingnanensis is less tolerant), and differences in their vagility (A. melinus is more vagile), probably all favor A. melinus in competition with A. lingnanensis. A. lingnanensis does not realize its fecundity advantage in female progeny when these species compete because A. melinus preempts California red scale before it grows into the size range preferred by A. lingnanensis. Collection records through 1972 show that A. melinus continued to extend its geographical range coastally in southern California through 1972.

Influence of microbe-associated parthenogenesis on the fecundity of Trichogramma deion and T.pretiosum.

Microbe‐associated parthenogenesis (thelytoky) has been discovered in nine Trichogramma species, parasitoids of mainly lepidopteran eggs. Parthenogenetic and bisexual conspecifics co‐occur in many field populations. As an initial step to understand the dynamics of these two reproductive strategies we studied the effect of microbe‐associated parthenogenesis on fecundity. The fecundity of two parthenogenetic isofemale lines of T. pretiosum and one of T. deion was compared with bisexual lines derived from them by antibiotic treatment. In all three cases parthenogenetic females were less fecund over their lifetime than bisexual females. Also, parthenogenetic females produced fewer daughters in two cases and in one case a similar number of daughters as their respective bisexual counterparts. The lack of mating and insemination was excluded as an explanation for the reduced fecundity of parthenogenetic females, because mated and virgin parthenogenetic females produce the same number of offspring. Antibiotic treatment can also be excluded because females of field‐collected bisexual line treated with antibiotics produced the same number of offspring as untreated females. The reduced fecundity of parthenogenetic females was caused by a lower number of eggs being laid rather than by a greater developmental mortality. Parthenogenetic females produced less daughters than bisexual females when host availability was not limiting, but when host availability was severely limited, parthenogenetic females produced more daughters than the bisexual females.

Factors influencing the optimum sex ratio in a structured population

W. D. Hamilton (1967, Science 156, 477-488) calculated the optimum sex-ratio strategy for a population subdivided into local mating groups. He made three important assumptions: that the females founding each group responded precisely to the number of them initiating the group; that ail broods within a group matured synchronously; and that males were incapable of dispersing between groups. We have examined the effects of relaxing each of these assumptions and obtained the following results: (1) When broods mature asynchronously the optimum sex ratio is considerably more female biased than the Hamiltonian prediction. (2) Increasing male dispersal always decreases the optimum female bias to the sex ratio, but it is of particular interest that when moderate levels of dispersal are coupled with asynchrony of brood maturation then the optimum strategy is relatively insensitive to changes in foundress number. (3) When females cannot precisely determine the number of other foundresses initiating the group then the optimum strategy is almost exactly the strategy appropriate to a group of average size. These effects can be most easily understood in terms of local parental control (LPC) of the sex ratio. Through LPC a founding female can alter the mating success of her sons by altering the sex ratio of her brood. Asynchrony in the maturation of broods within a group increases the control that a founding female has over the mating success of her sons, whereas male dispersal reduces it. We have shown that the role of LPC and the role of inbreeding, which favors a female-biased sex ratio in haploidiploid species, are independent and that their effects can be combined into a single general formula r = (1-(r2/z2) E(alpha z/alpha r]/(1 + I). The concept of LPC can also be used to interpret two factors which have been proposed to select for the Hamiltonian sex ratios: local mate competition is LPC acting through sons; and sib mating is LPC acting through daughters.

Selfish element maintains sex in natural populations of a parasitoid wasp

Genomic conflicts between heritable elements with different modes of inheritance are important in the maintenance of sex and in the evolution of sex ratio. Generally, we expect sexual populations to exhibit a 1:1 sex ratio. However, because of their biology, parasitoid wasps often exhibit a female–biased sex ratio. Sex–ratio distorters can further alter this optimum, sometimes leading to the complete loss of sexual reproduction. In the parasitoid wasp Trichogramma kaykai ca. 4–26% of females in field populations are infected with a bacterial sex–ratio distorter, Wolbachia, allowing virgin mothers to produce daughters. In some micro–Hymenoptera these infections have led to the complete loss of sex, but in field populations of T. kaykai the proportion of individuals infected remains relatively stable. We tested several hypotheses to explain this low infection level, including ineficient and horizontal transmission of Wolbachia, suppressor genes negating the effect of Wolbachia and the presence of male–biasing sex–ratio distorters. Here, a male–biasing sex–ratio distorter, a parasitic B chromosome, causing females to produce only sons, keeps the frequency of Wolbachia low. The male–biasing factor of T. kaykai is the second known case of a B chromosome manipulating the reproduction of a parasitoid wasp.

Regulation of an Insect Population Under Biological-Control

California red scale is suppressed to very low densities by the parasitoid Aphytis melinus. The system also appears stable. We report on an experimental test of the hypothesis that stability is caused by a refuge for scale. In a grapefruit grove in southern California in 1984-1985, the bark in the interior part of the tree provided a partial refuge from parasitism. Scale were -100 times denser there than in the exterior of trees. In a field experiment, we removed Argentine ants from some blocks of trees to test whether (1) ants caused the refuge by interfering with Aphytis and (2) the expected reduction in scale density in the refuge would lead to an unstable interaction in the exterior. We also tested for density- dependent parasitism, host mutilation, and predation by analyzing data from samples and from scale placed in the field. The temporal variability of the scale was at the low end of the range recorded in field populations. The experiment provided some evidence in support of the refuge hypothesis. The population in the refuge fluctuated much less than that in the exterior. Ant exclusion led to increased parasitism and lower scale density in the interior, and to increased fluc- tuations in abundance in the refuge and exterior. However, these changes were relatively small and perhaps temporary, suggesting that (1) ants are not the main cause of the refuge and that (2) we did not reduce the refuge density enough to determine whether the system would go unstable in the absence of the refuge population. Parasitism, host mutilation, and predation rates on scale showed no temporal density dependence, either direct or delayed, though detection of such patterns is difficult. Possible alternative stabilizing mechanisms include size-dependent interactions between red scale and Aphytis.

Refuge dynamics and metapopulation dynamics: An experimental test

Red scale, an insect pest of citrus, is under control by the parasitoid Aphytis melinus in many areas, and in our study area the interaction appears dynamically stable. The bark on the interior branches and trunk of trees provides a partial refuge for red scale, which are rarely attacked there by the parasitoid. In a grapefruit grove, we carried out a two-way experiment in which we manipulated the refuge population (present or removed) and either left trees connected with the rest of the grove or isolated individual trees with cages to test for metapopulation effects. The experiment ran for 17 mo, encompassing three generations of scale. Scale density in the exterior of refuge-removed trees decreased by about 60%. However, neither removal of the refuge population nor isolation of individual trees increased the temporal variability of the scale population in the exterior or led to drift in population density. Indeed, removal of the refuge population caused a decrease in temporal variability. We conclude that stability in the control population was not maintained by either refuge or metapopulation dynamics. Reduced scale recruitment and density in the exterior of trees lacking a refuge population were associated with increased (i.e., density-dependent) scale survival that did not reflect a change in parasitism.

Natural Decline of a Pine Needle Scale (Chionaspis pinifoliae [Fitch]), Outbreak at South Lake Tahoe, California following Cessation of Adult Mosquito Control with Malathion

The population dynamics of a pine needle scale infestation Chionaspis (Phenacaspis) pinifoliae (Fitch) were studied at South Lake Tahoe, California from 1969 to 1971. A uniparental population on lodgepole pine, Pinus contorta Dougl., and a biparental population on Jeffrey pine, P. jeffreyi Grev. and Balf., declined dramatically during the study period. The dominant mortality factors for the lodgepole pine population were crawlers which failed to settle, parasitization by Prospaltella bella Gahan, predation by Chilocorus orbus Casey var. monticolus Drea and Cryptoweisia atronitens (Casey), and an unknown mortality of adult female scale. In the Jeffrey pine population, the dominant factors were crawlers which failed to settle, unknown mortality of immature scale, and predation by the same cocinellid species. An intrageneration, intertip analysis of scale populations on lodgepole pine revealed that predation destroyed a constant proportion of the scale in all study areas. Prospaltella bella responded similar...

Dispersal and mating structure of a parasitoid with a female-biased sex ratio: Implications for theory

SummarySpecies of parasitic Hymenoptera that manifest female-biased sex ratios and whose offspring mate only with the offspring of the natal patch are assumed to have evolved biased sex ratios because of Local Mate Competition (LMC). Off-patch matings, i.e. outcrossing, are inconsistent with the conditions favouring biased sex ratios because they foster a mating structure approaching panmixia. Such a mating structure favours parents who invest equally in daughters and sons, assuming the production of each sex is of equal cost.Pachycrepoideus vindemiae (Rondani) is a solitary pupal parasitoid of patchily distributed frugivorousDrosophila, whose offspring manifest a female-biased sex ratio. Thus this species appears to manifest a population structure and progeny sex ratio consistent with LMC. However, preliminary observations and subsequent greenhouse experiments suggest that the males participate in off-patch matings and that this propensity is unlikely to be an experimental artefact. FemaleP. vindemiae dispersed from patches in which either the males were lacking (12% of the emigrant females), both resident (sibling) and immigrant males were present (23% of the females), only immigrant males were present (14% of the females), or their opportunity to mate could not be determined (14% of the females). Of the 12% that emigrated from a patch lacking males, an estimated 7% mated at an oviposition site and 5% remained unmated, presumably because they arrived at an oviposition site that lacked males before they were dissected to determine whether they were inseminated. Thus the degree of bias in the sex ratios of the progeny (18% males), coupled with the suggested outcrossing potential from the experiments (26–37%), is inconsistent with the assumptions of LMC or variants of it, i.e. asynchronous brood maturation. Thus the explanation for a biased sex ratio in the offspring ofP. vindemiae remains a conundrum. More importantly,P. vindemiae does not appear to be an isolated example.