Susan B. Opp

Sympatric host-race formation and speciation in Rhagoletis (Diptera: Tephritidae): a tale of two species for Charles D

The question of why there are so many host plant-specific phytophagous insects has long perplexed entomologists, ecologists, and evolutionary biologists alike. In this chapter, we will argue that part of the answer resides in the relationship between host-plant specialization and reproductive isolation. Plants (either different parts, varieties, or species) represent different niches to phytophagous insects. Traits adapting an insect to one species or variety of plant may prevent an insect from efficiently utilizing alternative hosts. Do such host-associated traits also result in reproductive isolation (Walsh 1864; Thorpe 1930; Bush 1966; Futuyma and Keese 1992)? Can isolation evolve as an inadvertent, pleiotropic byproduct of a phytophagous insect adapting to a new host plant (Rice 1987; Berlocher 1989; Rice and Hostert 1993)? If so, then the plethora of host specialists is due, at least in part, to numerous plant niches that have imposed divergent selection pressures on phytophagous insects (Hutchinson 1968; Rosenzweig 1978).

Intra‐tree foraging behavior of Ceratitis capitata flies in relation to host fruit density and quality

We examined the intra‐tree foraging behavior of individually‐released, wild‐population Mediterranean fruit flies (medflies), Ceratitis capitata (Wiedemann), on field‐caged host trees bearing each of three different densities (0, 3, or 12 per tree) of non‐infested host fruit (kumquat) or each of two levels of fruit quality (12 non‐infested fruit or 12 fruit infested with eggs and covered with host marking pheromone). With increasing den: ity of non‐infested fruit, medflies tended to remain longer in trees, visit more fruit before leaving, oviposit more often, accept a proportionately smaller number of fruit visited, and emigrate sooner after the last egg was laid (i.e. have a shorter Giving‐Up‐Time). Medflies spent much less time, oviposited much less often, and exhibited a longer Giving‐Up‐Time on trees harboring pheromone‐marked fruit than non‐infested fruit. Variation in temperature within the range at which experiments were conducted (25–36 °C) had little detectable influence on foraging behavior. We compare our findings with published findings on the intra‐tree foraging behavior of another tephritid fly, Rhagoletis pomonella (Walsh), and with current foraging behavior theory. We discuss implications of our findings with respect to medfly management strategies, particularly fruit stripping in eradication programs and use of synthetic marking pheromone for control.

Effect of marking pheromone on clutch size in the Mediterranean fruit fly

Abstract Using acridine orange to selectively stain eggs, we showed that wild‐collected female Mediterranean fruit flies (Ceratitis capitata Wiedemann) laid fewer eggs per clutch in fruit previously infested with eggs than in uninfested fruit. This effect is apparently attributable to marking pheromone deposited by females after oviposition: clutch size on fruit infested with eggs but free of marking pheromone was not statistically different from that on uninfested fruit. Clutch size on uninfested fruit on which marking pheromone was artificially transferred was significantly lower than that on uninfested and untreated fruit. Marking pheromone had a comparable though not statistically significant effect on the clutch size of females originating from a strain maintained in the laboratory for several hundred generations.

Cross-induction of fruit acceptance by the medfly Ceratitis capitata: the role of fruit size and chemistry

Groups of female Mediterranean fruit flies, Ceratitis capitata(Wiedemann), were exposed for several days to one of three host fruit species. Oviposition-site acceptance behavior was subsequently assayed on five fruit species. Females accepted most often the fruit to which they were exposed. Females exposed to a small fruit, mock orange, accepted other fruit species less often as the size of the fruit increased; females exposed to a large fruit, sweet orange, accepted other fruit species more often as the size of the fruit increased. This tendency for experience with one host fruit species to alter differentially behavioral responses to alternative host fruit species has been defined as cross-induction. In contrast, females exposed to a medium fruit, kumquat, were not cross-induced: females accepted the medium fruit very often and rejected all other fruit species to approximately the same degree regardless of size. When females were exposed to small, medium, or large fruit and tested on spherical wax fruit models of a variety of sizes, patterns similar to those with real fruit were observed. Whereas naive females generally accepted a given model as frequently as real fruit of a similar size, experienced females generally accepted models much less frequently than real fruit. In a final experiment, females were exposed to different fruits and tested on spherical wax models treated with fruit chemicals. Experienced females generally accepted fruit-treated spheres more often than untreated spheres. In addition, females usually accepted most often models treated with chemicals from the fruit to which they were exposed. Two hypotheses about the mechanism by which experience alters fruit acceptance— termed the “sliding template” and “closing window” hypotheses— are presented. Results of fruit and model acceptance by naive and experienced females support the latter hypothesis.

Multiple Mating and Reproductive Success of Male and Female Apple Maggot Flies, Rhagoletis pomonella (Diptera: Tephritidae)

Mature male and female apple maggot flies mated frequently on a field-caged host tree during a 14-day study. Each sex averaged one mating per day (mean of 1.0 ± 0.1), but some females mated up to eight times per day and some males up to six times per day. Reproductive success was estimated based both on observed numbers of matings in the field cage and on previous work relating fecundity and fertility to female mating status. Male and female flies did not differ in mean or variance in reproductive success, indicating that this is a polygamous mating system consisting of both polygyny and polyandry. We discuss the significance of this with regard to the framework of insect mating control and frequency proposed by others. We also discuss behaviors (such as movements, agonistic encounters, occurrences on fruit, and ovipositions) that do and do not show correlations with mating success in apple maggot flies.

Prior experience influences the fruit residence of male apple maggot flies,Rhagoletis pomonella

Male apple maggot flies spend considerable time residing on individual host fruit as territories on which they force-copulate arriving females in search of oviposition sites. Here, we present evidence from investigations in nature and the laboratory that shows the propensity of males to reside on a hawthorn or apple fruit as a territory is significantly modifiable through prior experience with fruit and, hence, involves learning. Previous studies revealed that after a female apple maggot fly, Rhagoletis pomonella, arrived on a host hawthorn or apple fruit, its propensity to accept or reject that fruit for egg-laying was similarly modifiable through prior fruit-exposure experience and also involved learning. We discuss how host fruit learning in males and females, in concert with genetic-based differences in host fruit residence and acceptance behavior between populations of flies originating from hawthorn and apple, could give rise to a reduction in gene flow between populations of flies on these two host types.

Effect of Adult Diet on Longevity of Sterile Mediterranean Fruit Flies (Diptera: Tephritidae)

Abstract Fly longevity is critical to sterile release programs for Mediterranean fruit flies (medflies) because the longer sterile flies are present after a release, the greater the probability of mating. Current release programs provide sterile, adult medflies with sucrose in an agar matrix for 2-3 d before release. We used cages to compare the effects of different diets on the longevity of medfly, Ceratitis capitata (Wiedemann). A diet of dry hydrolyzed yeast + sucrose supplied during the pre-release interval did not significantly affect field survivorship of medfly adult males relative to the standard sucrose diet. Post-release diets, simulating nitrogen and sugar sources that released medflies may find after release, had significant effects on medfly survivorship. Hydrolyzed yeast + sucrose resulted in the highest medfly survivorship, followed by sucrose, and then water alone. Finally, diets containing hydrolyzed yeast were not found to have significant amounts of protein and thus are more likely nitrogen or amino acid sources for flies, rather than sources of protein.

Seasonal Changes in Resightings of Marked, Wild Rhagoletis Pomonella (Diptera: Tephritidae) Flies in Nature

We observed individually marked, wild male and female apple maggot flies (AMF) on a host apple tree for 24 days. Although we resighted a fairly high proportion (30.6%) of 599 marked flies, we observed most flies (74.9%) only during the first week after marking and release. Following the onset of reproductive maturity, as evidenced by mating and oviposition, we saw males over more consecutive days than females. This presumably occurred due to male arrestment following contact with female marking pheromone deposited on fruit. Peak diel time of mating by unmarked flies corresponded more closely with peak time of observation of marked males rather than marked females. We estimate some adult AMF may live up to 4 weeks in nature.

Approaches and Methods for Direct Behavioral Observation and Analysis of Plant-Insect Interactions

Perhaps the greatest contribution of Konrad Lorenz to the discipline of ethology was to advocate use of biological investigatory methods for behavioral studies of animals (Tinbergen, 1963). Observation conducted as a systematic biological investigation differs from merely watching the behavior of an animal in that observation utilizes the uniquely human capacity for thought. Thought gives rise to a framework of important questions to consider during observation. What, when, how, why, and where are all highly pertinent aspects of studying the activity of an animal (Lehner, 1979). Other behaviorists have suggested consideration of proximate and ultimate factors affecting behavior as an advisable approach to behavioral investigation (e.g., Drickamer and Vessey, 1982). Yet another way of visualizing the study of animal behavior focuses on the function, causation, ontogeny, and evolution of observed behaviors (Tinbergen, 1963). But no matter how the study of behavior is conceptualized, the necessity, when conducting observations, for continual thought and reflection, particularly within the context of behavioral-ecological theory, is an indispensible consideration.