Rodger Mitchell

The Evolution of Oviposition Tactics in the Bean Weevil, Callosobruchus maculatus (F.)

A sizeable fraction (7%-17%) of the mung beans that are the preferred food of Callosobruchus maculatus are not large enough to support two larvae. Whenever two or more eggs are on a bean, development takes 40 days rather than 33 days, and only 8% of the second eggs develop even in beans with resources enough for two larvae. This larval competition confers an advantage on females that disperse eggs uniformly. Females tend to avoid adding second eggs as long as there are unused beans. Survivorship of single eggs on beans increases with weight (1l = .005 mg + .368) and females oviposit on the largest of the unused beans first. Survivorship of the second egg (12 = .31), due principally to mortality of the first egg, is about half that of the first egg; a second or third egg is added after nearly all the beans carry one or two eggs respectively. With these constraints the beetle may maximize fitness if it knows the weights and egg loads of all available beans. The actual performance falls short of achieving maximum fitness and the oviposition of the beetle fits a computer simulation that makes decisions after com- paring the weight and egg load of the present bean with the last bean encountered. The per- formance of the beetle can be appropriately evaluated against models for unspecialized beetles that oviposit randomly and for beetles with maximum fitness. This shows that the beetle gains about 70% of the fitness that it could evolve.

Bruchid Bettles and Seed Packaging by Palo Verde

Cercidium floridum pods usually contain one or two seeds. If pods contained three or more seeds, pod biomass would be reduced by 19% and that energy could be diverted into seeds. Intrinsic factors do not explain the wasteful packaging; interactions at a node divert energy into several one— or two—seeded ponds rather than one large pod. The patterns of mortality from three seed predators are: (1) Rodents attack all pods equally; (2) All seeds are equally likely to be attacked by Mimosestes amicus (Bruchidae); (3) Stator limbatus (Bruchidae) attacks through exit holes cut by M. amicus and the entrances rise almost exponentially with seed count. Stator limbatus generally attacks all the seeds of a pod, and selects the larger pods, but pod size varies so much that pods with two seeds cannot be discriminated from each other. Stator limbatus destroys twice as many seeds in two— or more seeded pods which confers an advantage on seeds from one—seeded pods. An egg parasite, Uscana semifumipennis kills °40% of the eggs of both beetles. This mortality, plus a nearly 40°% loss to heat and desiccation, makes it advantageous for M. amicus to put one egg on top of another, thus sacrificing a second egg to double the survivorship of the first egg.

Intraspecific variation in the egg-spacing behavior of the seed beetleCallosobruchus maculatus

Females of Callosobruchus maculatustend to distribute their eggs uniformly among host seeds and, thereby, reduce competition among larvae within seeds. We investigated variability in this behavior by assaying beetles from seven geographic strains on each of three host species. To quantify egg-spacing behavior,we devised an index (U)that estimates the uniformity of the egg distribution but is independent of the number of eggs laid. Egg-spacing behavior was highly variable among beetle strains and legume hosts. Females from an Indian strain frequently achieved the most uniform distribution possible (maximum hyperdispersion), whereas females from two Brazilian strains frequently deposited eggs randomly. Differences among strains were evident throughout the period of oviposition;some strains were “sloppy” when even egg densities were low. Variation in egg-spacing behavior appears to be influenced by prior host associations and by host size. The most well-developed spacing behavior was observed in a strain associated with the smallest host, which supports only one or two larvae per seed. Realized fecundity also varied significantly among beetle strains. We found no evidence, however, of a predicted tradeoff between the number of eggs laid and the uniformity of the egg dispersion.

Growth and Population Dynamics of a Spider Mite (Tetranychus Urticae K., Acarina: Tetranychidae)

Female T. urticae deposit eggs in a circumscribed area and the young tend to mature in that area. Pheromones released by developing females are effective over very short distances; hence, sib—mating is the rule at low densities. Males grow to about three times the egg biomass; 20%—30% of the eggs are males. Mature females weight eight times as much as males but they disperse at one—half or less of their mature weight and may reduce their weight by even more when resources are poor. This suite of adaptations allows 70%—80% of the eggs to be female and delays almost half the increase in the biomass of a set of young until after the mated females disperse to new resources.

The traits of a biotype of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) from South India

Abstract The South India strain of Callosobruchus maculatus (F.), which has been in culture for 10 yr, cannot be distinguished from three cultures established from beetles collected in South India in 1989. These cultures represent a geographic biotype of C. maculatus that is widely distributed in the state of Tamil Nadu, India and is stable in culture. Females of the biotype disperse their eggs uniformly, lay eggs on larger beans first, have their oviposition inhibited if beans already carry eggs, and the larvae exhibit contest competition. Fecundity varies greatly within and between cultures. The extremes of fecundity have less effect on the contribution of offspring to the next generation than the elimination of intraspecific competition by selective oviposition.

Vibrations mediate prudent resource exploitation by competing larvae of the bruchid bean weevil Callosobruchus maculatus

Larvae of the bean weevil C. maculatus feeding in a single bean apparently respond to vibrations from each others chewing: one larva feeds normally, the other is inhibited. If the burrows of the larvae intersect, the inhibited larva dies. If the dominant larva pupates or dies without the burrows intersecting, the inhibited larvae then feeds and matures if enough food remains. Since females add second eggs to the largest beans after most beans carry a single egg, competition is most common in the largest available beans, precisely where inhibited larvae can benefit from avoiding a contest.

Behavioral Ecology of Callosobruchus Maculatus

Each geographic strain of C. maculatus appears to have a unique set of physiological and behavioral traits. Some strains disperse eggs uniformly, others place eggs at random, fecundity may or may not be inhibited when few beans are available, and females may select the largest available beans for oviposition. Host preferences and tolerances differ from strain to strain and respond to selection. Before this variability was understood too little attention was given to the need for careful experimental design. A protocol is suggested as providing some standard points of reference for future research. The prospects for exciting research in ecology and evolution in this astonishingly variable species seems unlimited.

A Study of Sympatry in the Water Mite Genus Arrenurus (Family Arrenuridae)

Sympatry is common among species is the genus Arrenurus; this paper considers the fauna of a small pond in which 26 species of the genus were found during a six—year period. Resources were adequate for the aquatic phase of the life cycle as evidenced by the regular success of dispersal from adjacent habitats. One—fourth of the species records were species unable to reproduce because of unfavorable conditions for the parasitic of the life cycle. This was not a matter of the host being absent but involved a complicated mite—habitat—host interaction. The dynamics of this interaction can be postulated with a modified life—table formulation which conforms with available data. The formulation defines a set of relations between host specificity, variations in host exploitation, and regulation of parasite density which can be studied to test the validity of the model.

The fecundity and oviposition behavior of a South Indian strain of Callosobruchus maculatus

The fecundity of the South India strain of Callosobruchus maculatus (Fab.) (Bruchidae: Coleoptera) is 73 eggs for females developing in 31 days and increases to 94 eggs for females emerging after 41 days. Fed females lay 45–55 more eggs which is the egg‐equivalent of the reserves unfed females had to sacrifice for activity. Competition as a larva did not reduce a females fecundity. Oviposition is inhibited when females are given 25 or fewer beans for oviposition and it is reversed if females find fresh beans. Oviposition is generally released by beans carrying a below average number of eggs, thus, the eggs are almost uniformly distributed over the beans. Eggs are preferentially placed on the largest egg‐free beans as expected if oviposition is released when a beetle goes from a smaller to a larger bean. The cues regulating host preferences, cowpea > mung » igeonpea » chickpea, are secondary to the cues producing hyperdispersion and the selection of larger beans. We present a model for the oviposition behavior that accounts for the observed responses to species, size, and egg load of oviposition sites.

FUNCTIONAL ANALYSIS OF A THERMAL SPRING ECOSYSTEM, WITH AN EVALUATION OF THE ROLE OF CONSUMERS'

Low temperature (< 400C) alkaline thermal spring effluents in Yellowstone National Park support a benthic algal-bacterial mat fed upon by a single herbivorous brine fly (Diptera: Ephydridae), which in turn is consumed by a number of arthropod predators (water mites, spiders, and a predaceous fly). A census of an entire spring ecosystem provided a framework upon which to integrate present knowledge of this system. Growth of the algal-bacterial mat appears to be nutrient limited; measures of free CO2 concentrations are good predictors of the differences in productivity within and among springs. Paracoenia turbida, the herbivore, saturates all suitable oviposition sites, and its larvae destroy all algae available below them. The larvae apparently suffer substantial density-dependent mortality late in their development, when they consume algal dams protecting them from hot water flows. Thus, they compete for limited space of suitable temperature. Algal-bacterial biomass persists because, at any one time, most of the mat is too hot for larvae, or is covered by flowing water and consequently unsuitable for oviposition. The brine fly eggs which con- stitute the prey for most predators are also largely unavailable because the flies insert them into the algal-bacterial mat, where they are difficult to remove and feed upon. The demand- availability ratio for fly eggs is high; therefore reproduction of the major predatory water mite, Partnuniella thermalis (Hydrachnellae) is probably limited by the supply of available eggs. Despite the abundance of their prey species, then, both primary and secondary consumers seem to be resource limited because most of their prey are unavailable. The important role of refuges in defining the biomass structure of this ecosystem prompted us to define four condi- tions which tend to limit consumer influence on prey density: (1) evolutionary specialization by consumers, which narrows the spectrum of potential food; (2) poor food quality which does not allow population growth; (3) evolved defenses of the prey; (4) the degree of uncertainty in resource distribution in space or time. Restrictions of type (4) are difficult to document and may often go unappreciated, but an example from the thermal spring system shows the uncertainty in resource distribution may be evolutionarily insoluble, and it may be a powerful constraint on consumer impact. There is little reason to believe that these conditions are any less powerful in complex than in simple systems. By analogy then, the ability of consumers to influence prey density in complex terrestrial and aquatic systems may also be severely limited. The importance of refuges in complex systems might be inferred from measurements of community responses to low-level enrichment or harvesting. Even though their direct influence on prey densities is limited, consumers in the thermal spring ecosystem increase the net primary production of the system and increase the efficiency of energy transfer between the first and second trophic levels. Thus, they affect processes, not prey densities, in this system.