Each experimenter tested half of the days. We used high quality rewards that would be easily visible to the subjects. Blueberries were not part of their daily diet but were sometimes presented as enrichment in puzzle feeders and were highly desirable for all gibbons housed at the GCC. The apparaThus was composed of a plastic folding table with a square wooden plank clamped to the top. At one end of the plank a transparent plastic bin was taped so that it could be lifitted up or hang down. Te bin, at rest, would hang down and remain unmoved on the top of a wooden ramp. A hole big enough to ft blueberries was drilled on the back side of the bin so that when at rest on the ramp, the experimenter could place five blueberries into the bin. A thin purple rope was tied to the far end of the plastic bin and was routed back to the opposite end of the wooden plank. This was set up so that pulling on the purple rope would reliably lift the plastic bin, so blueberries could fall down the wooden ramp and be easily accessible for subjects to obtain. Te extreme end of the rope was attached to the mesh of the enclosure. To allow reaching and pulling the rope, we attached a small, handheld, opaque white handle . At the right tension, pulling on the handle would reliably lift the plastic bin. Te handle could contain a single blueberry inside depending on the condition presented. We used two handles of the same dimensions and appearance to avoid contamination of blueberry leftovers after the trial. Te table with wooden plank would be set up at a distance so that it could not be grabbed by subjects and the ramp was placed underneath so that blueberries would roll down and land in front of the enclosure gate.
E2 would then distract the two subjects to an opposite or adjacent side of the subjects’ enclosure with a handful of cereal pieces while E1 tied the end of the purple rope with the handle onto the mesh gate of the enclosure, roughly at the experimenter height, blueberry packing boxes approximately 2 m to the right or left. Te distance and location of the rope was kept constant for all trials of each dyad; however, because the enclosures differed in layout, the rope would go to the most convenient side. This way, we ensured that the rope had proper tension to be pulled by gibbons and lift the plastic bin as well as be distant enough from the ramp so that a subject could not easily pull on the rope and obtain food from the ramp at the same time.Individual solo pre-testing of the mechanism of the apparaThus was not possible because the separation of the days was prohibited. However, gibbons had had experience with ropes before as part of their enrichment and several individuals had participated in pilot sessions where they had to pull from different ropes and handles. Tree conditions were tested: direct food test condition, indirect food test condition and no food control condition. In the direct food test condition, the following procedure was performed. E1 would place five blueberries in the plastic bin on the apparaThus. To gain the attention of the subjects, E1 would call the subjects names and show the food, if they were not already focused on the food/experimenter. Once both subjects had observed the five blueberries placed in the plastic bin, E1 would squeeze a single blueberry on top of the handle, so that the blueberry would be clearly visible. Te rope and handle would be set up so that the handle was just far enough from the enclosure in order for subjects to need to pull on the rope to obtain access to the handle and blueberry. Consequently, pulling the rope would also lift the plastic bin and drop five blueberries down the ramp, accessible to subjects. Te experimenter would also call the names of the subjects when placing the single blueberry in the handle.
A choice was recorded when one of the subjects pulled the rope. If no subject pulled the rope within 90 s, the trial ended and was recorded as no pull. If an experimenter error was made , up to 3 repetitions of the trial would be completed. Environmental conditions such as rain would also end test sessions to be continued the next day. In the indirect food test condition, there was no single blueberry placed in the handle. To compare conditions, we followed the same procedure as in the direct food test condition. Instead of inserting a blueberry inside the handle, we approached it with the first close and then we touched it with the fingers. In the no food control condition, no blueberries were used in the trial. In order to control for time and actions, we used the same procedure of calling the subjects and touching both the box and the handle.Two cameras on tripods recorded footage concurrently. One was placed to the side of the experimenter in order to capture a wide view of the trials, specifically to show the positions of the subjects, their choices and if they obtained blueberries. Te other was placed close to the ramp to accurately count the quantity of blueberries obtained by each subject. For all trials we coded the act of pulling or not pulling and the ID of the puller and non-puller . We also coded the number of blueberries each subject ate and whether the actor subject ate the blueberry from the handle. Next, we coded whether a passive subject was present in front of the ramp or within one meter from it at the moment the plastic bin was lifted and at the moment the actor arrived at the release location. Additionally, we coded instances of cofeeding and displacements. Cofeeding was coded when individuals feed within a distance of 1 m of one another. Displacements occurred when an individual left her spot due to the partners’ arrival. Additionally, we calculated the latency to pull from the start of the trial until the individual releases . All analyses were conducted with R statistics . We used Generalized Linear Mixed Models to investigate gibbons’ choices . Covariates were z-transformed. Every full model was compared to a null model excluding the test variables. We controlled for session and trial number in all our models. We controlled for the length of the dyad in models 1 to 3 given the larger dataset compared to models 4 to 6. In addition, in model 3 we included individuals’ age and sex as control predictors.
When the comparison between the full and the null model was significant, we further investigated the significance of the test variables and/or their interactions. We used the “drop1” function of the lme4 package to test each variable significance including interactions between test predictors. Non-significant interactions were removed and a new reduced model was produced when necessary. A likelihood ratio test with significance set at p<0.05 was used to compare models and to test the significance of the individual fixed effects. We ruled out collinearity by checking Variance Infation Factors . All VIF values were close to 1 except for age and length of dyad in model 3. Te two variables were slightly collinear . For every model we assessed its stability by comparing the estimates derived by a model based on all data with those obtained from models with the levels of the random effects excluded one at a time. All models were stable. We also fitted a mixed-effects Cox proportional hazards model to analyze gibbons’ latencies to act. For this purpose, we used the “coxme” function from the coxme package. Te results of Model 2 are reported as hazard ratios . An HR greater than one indicates an increased likelihood of acting and an HR smaller than 1 indicated a decreased hazard of acting. In addition, to obtain the p-values for the individual fixed effects we conducted likelihood-ratio tests.Drosophila suzukii Matsumura is an economic pest of small and stone fruit in major production areas including North America, Asia and Europe . Female D. suzukii oviposit into suitable ripening fruits using a serrated ovipositor. This is unique compared to other drosophilids, including the common fruit fly, D. melanogaster, package of blueberries which oviposit into overripe or previously damaged fruit. Developing fruit fly larvae render infested fruit unmarketable for fresh consumption and may reduce processed fruit quality and cause downgrading or rejection at processing facilities. In Western US production areas, D. suzukii damage may cause up to $500 million in annual losses assuming 30% damage levels, and $207 million in Eastern US production regions [9]. Worldwide, the potential economic impacts of this pest are staggering. Pesticide applications have been the primary control tactic against D. suzukii both in North America and in Europe. The most effective materials are those that target gravid females, including pyrethoids, carbamates, and spinosyns. These applications are timed to prevent oviposition in susceptible ripening host crops. In the Pacific Northwest, many growers have adopted scheduled spray intervals of 4–7 days. This prophylactic use of insecticide is unsustainable as growers have a limited selection of products and modes of action. This could ultimately lead to D. suzukii becoming resistant and may cause secondary pest problems because of negative effects on beneficial organisms. Furthermore, production costs have increased substantially in crops where D. suzukii must be managed. Effective sampling methodology for D. suzukii is lacking despite extensive efforts to improve trap technology or determine effective fruit infestation sampling protocols. Theoretically, traps to capture adult flies should aid growers in the timing of spray applications so that insecticides could be used more judiciously. Traps baited with apple cider vinegar or a combination of sugar-water and yeast are currently used to monitor adult D. suzukii flight patterns. However, without standard methods for trapping or management thresholds based on trap count data, it is questionable how much is gained by establishing and monitoring traps in crops.
Establishing, monitoring, and maintaining traps is very labor intensive and the costs do not justify the benefits for many growers. Historically, trap data has not provided a reliable warning against D. suzukii attack, especially for susceptible crops in high-density population areas where considerable oviposition can occur in short time periods. Currently, no significant differences are found in any traps used for monitoring D. suzukii given differences between crops and environments where traps have been tested. Monitoring fruit infestation levels to guide management may also be impractical. Furthermore, by the time larvae are detected in the fruit, it is too late for management action and damage has already occurred. No detailed studies could be found using monitoring for fruit infestation for this pest, and precision of sampling methodology is currently unavailable. Degree-day , or phenology models, are standard tools for integrated pest management in temperate regions and are used to predict the lifte stages of pests in order to time management activities and increase the effectiveness of control measures. Degree-day models work best for pests with a high level of synchronicity and few generations. Our data suggest that D. suzukii has short generation times, high reproductive levels, and high generational overlap compared to other dipteran fruit pests. Given this lift history, stage-specific population models represent an alternative and potentially more applicable tool for modeling pest pressure. Pest population estimates may be greatly improved by employing additional tools such as mark recapture and analytical or individual-based models. The ability to describe and forecast damaging pest populations is highly advantageous for fruit producers, policy makers, and stakeholder groups. Many such studies have been directed at forecasting populations of medically important insect species. The major factors affecting survival, fecundity and population dynamics of drosophilids include temperature, humidity, and the availability of essential food resources. Therefore, an improved understanding of the role of temperature on D. suzukii may provide for a better understanding its seasonal population dynamics. In this paper, we present a population model for D. suzukii that represents a novel modification of the classic Leslie projection matrix, which has proven to be one of the most useful age structured population models in ecology, with applications for diverse organisms including plants, animals, and diseases.