Hugh J. Barclay

Modeling of Preferential Mating in Areawide Control Programs That Integrate the Release of Strains of Sterile Males Only or Both Sexes

Abstract Understanding the principles that govern the mating behavior of insects that are the target of areawide integrated pest management (AW-IPM) programs by using the sterile insect technique (SIT) is a prerequisite to ensure optimal efficiency of such programs. Models were constructed to assess the effect of mating preference of insects, which display a female- or male-choice mating system, on the efficiency of SIT programs that release males only or programs that release both sexes. The model on preferential mating indicated that in a male choice mating system [e.g., screwworm, Cochliomyia hominivorax (Coquerel)], overcoming the discrimination of wild males against mating with sterile females would require a doubling of the number of sterile males compared with male-only releases. The model on female choice was incapable of distinguishing between reduced sterile male competitiveness and female preference for wild males and implied, in addition, that the release of both sexes and male-only releases required the same sterile to wild male overflooding ratio. Operational SIT projects have, however, shown a significant benefit with male-only releases against insects which have a female-choice mating system [e.g., Mediterranean fruit fly, Ceratitis capitata (Wiedemann)], and models were constructed to assess the potential effect of sterile female presence or absence on some parameters, i.e., reduced sterile sperm quantity with remating, reduced sterile sperm quality with aging and incomplete redistribution of the sterile males with the wild insects. The model suggests that in all three cases, male-only releases result in relatively more efficient sterile insects compared with programs releasing both sexes. The results of the models are discussed in relation to data available from operational screwworm and Mediterranean fruit fly AW-IPM programs with an SIT component.

A Conceptual Model for Assessing the Minimum Size Area for an Area-Wide Integrated Pest Management Program

A conceptual model was developed based on the two basic spatial elements of area-wide integrated pest management (AW-IPM), a core area and a buffer zone, to determine the minimum size of the protected area for the program to be technically feasible and economically justifiable. The model consisted of a biological part (insect dispersal) and an economic part. The biological part used random walks and diffusion equations to describe insect dispersal and to determine the minimum width of the buffer zone required to protect the core area from immigration of pests from outside. In the economic part, the size of the core area was calculated to determine the point at which the revenues from the core area equal the control costs. This model will need to be calibrated and validated for each species and geographic location. Tsetse flies and the Mediterranean fruit fly are used as case studies to illustrate the model.

Modeling the Area-Wide Integration of Male Annihilation and the Simultaneous Release of Methyl Eugenol-Exposed Bactrocera spp. Sterile Males

ABSTRACT We present simple models that describe the factors influencing application of the Sterile Insect Technique (SIT) alone, followed by the factors affecting the integration of the Male Annihilation Technique (MAT) and SIT simultaneously to annihilate wild males and replace them with methyl eugenol (ME)-exposed sterile males that are less responsive to the toxic MAT baits. The removal by MAT of large numbers of males to deprive females of mates is rather ineffective to suppress populations unless a proportion of females are also attracted to baits. When simultaneously implementing SIT and MAT, much less stringent requirements for the annihilation of the wild male population apply and the sterile fly release rate can be reduced to only ≈5% of that when SIT is applied alone. The main determinants of control efficiency are whether a proportion of wild females is attracted to MAT baits, whether most of the wild males respond to the MAT baits before mating, and whether a majority of ME-exposed sterile males do not respond to the baits. However, a high remating frequency in mated females, and the release of only sterile males or both sterile males and females, do not greatly affect the ease of control. We conclude that the simultaneous MAT and SIT application is synergistic and the combination appears to be an extremely powerful control strategy for tephritid fruit fly pests that are attracted to ME.

Abundance and diversity of soil nematodes in chronosequences of coastal Douglas-fir forests on Vancouver Island, British Columbia

Summary Soil nematodes were used as indicators of possible differences in biodiversity in seral stages of Douglas-fir forests at three chronosequence sites on southern Vancouver Island, British Columbia. At each site, the seral stages were represented by regeneration, immature, mature and old-growth stands. Average ages of these stands were 8, 42, 93 and 286 years, respectively. The surveys to obtain soil and nematode data were conducted in summer and winter of 1993. Old-growth stands served as control forests. The nematode parameters used were their abundance, taxonomic composition and trophic structure. Abundance was lowest in the regeneration stands and highest in old-growth stands. Intermediate densities were found in the immature and the mature stands, but were generally higher in the former than in the latter stands. This trend is interpreted as indicating that nematode population levels in the three earlier seral stages would be restored to old-growth levels as they attain old-growth features with time. Forty nematode genera/species were found across these sites. Percent distributions of these taxa were generally similar across the seral stages. Measures of diversity and maturity indices, based on relative abundances of the individual genera, were also very similar for all seral stages. Correspondence analysis separated the genera clearly by layer of soil horizons (organic versus mineral), but less clearly by seral stage. Abundance of the genera by trophic habit ranked as: bacteriovores (38%) > omnivores (30%) > predators (25%) > fungivores (5% > plant-parasites (2%), but their relative distributions across seral stages were very similar and closely matched these percentages. These results pertaining to nematode taxa and their trophic habit indicated that the soil nematode community was essentially similar across the four seral stages.

A dynamic population model for tsetse (Diptera: Glossinidae) area-wide integrated pest management

A spatial model of tsetse (Glossina palpalis ssp. and G. pallidipes) life cycle was created in FORTRAN, and four control measures [aerial spraying of non-residual insecticides, traps and targets, insecticide-treated livestock (ITL) and the sterile insect technique] were programmed into the model to assess how much of each of various combinations of these control tactics would be necessary to eradicate the population. The model included density-independent and -dependent mortality rates, temperature-dependent mortality, an age-dependent mortality, two mechanisms of dispersal and a component of aggregation. Sensitivity analyses assessed the importance of various life history features and indicated that female fertility and factors affecting survivorship had the greatest impact on the equilibrium of the female population. The female equilibrium was likewise reduced when dispersal and aggregation were acting together. Sensitivity analyses showed that basic female survivorship, age-dependent and temperature-dependent survivorship of adults, teneral-specific survivorship, daily female fertility, and mean temperature had the greatest effect on the four applied control measures. Time to eradication was reduced by initial knockdown of the population and due to the synergism of certain combinations of methods [e.g., traps-targets and sterile insect technique (SIT); ITL and SIT]. Competitive ability of the sterile males was an important parameter when sterile to wild male overflooding ratios were small. An aggregated wild population reduced the efficiency of the SIT, but increased it with increased dispersal. The model can be used interactively to facilitate decision making during the planning and implementation of operational area-wide integrated pest management programs against tsetse.

Scaling-up an autoregressive time-series model (of spruce budworm population dynamics) changes its qualitative behavior

The emergence of issues such as climate change has motivated the development of time-dependent models to forecast how plant and animal populations will react over large spatial extents. Usually the best data available for constructing such models comes from intensive, detailed field studies. Models, thus implicitly developed at the fine spatial resolution of experimental studies, are then scaled-up to coarser resolution for management decision-making. Typically, this process of scaling-up involves merely adapting the models computer code for data input so that it will accept the large scale spatial averages (often derived from relatively remote (e.g. aerial) sensing) that form the basis for management planning. Unfortunately, such scaling-up can inadvertently affect model predictions and dynamical behavior. Improper incorporation of data collected at multiple resolutions during model development and use, and misinterpretation of model output can result. The consequences of scaling-up a linear, second-order, autoregressive, time series model of spruce budworm population dynamics on the models predictions and on the interpretation of the models output are considered. Such time series models have been proposed as templates for incorporating outbreak dynamics in the decision systems supporting forest insect management that are currently being adapted to climatic change problems. Analysis of the underlying deterministic component of the time series model showed that: (1) parameter estimates changed with the spatial resolution-parameter values estimated from time series data consisting of large area averages were negatively correlated (r=−0.931, P<0.0005) and as much as 40 or 50 times greater in absolute value than the parameters generating the fine resolution data from sampling sites 1600 times smaller in extent. (2) Even the qualitative nature of the dynamics appeared to change in response to scaling-up. The long cycle, converging oscillations generated at fine resolutions gave way to five additional types of qualitative behavior at coarser resolutions including various types of divergent behavior and non-oscillating behavior. (3) The amount of distortion involved in scaling-up depends on the models degree of non-linearity and on the fine scale spatial variation in population densities. An approach to correcting for such distortion is outlined. The potential consequences of scaling-up deserve consideration whenever data measured at different spatial resolutions are integrated during model development, as often happens in climate change research.

Models of sterile insect releases for populations under attack by parasitoids

Abstract Five host-parasitoid models are presented to determine the effects of: (a) host stage parasitized, and (b) position of density-dependent population regulation on the efficiency and dynamics of the sterile insect release method for pest control. Previous models indicate a catastrophic decline of the pest population when sterile releases are above a certain threshold. The five models incorporating parasitoids presented here indicate that this catastrophic decline will usually not occur, but rather the parasitoids will be driven extinct whereupon the system reverts to the one-species dynamics. In this situation, either a population explosion or extinction of the pest species will generally follow, depending on the level of sterile releases and on chance events. The models predict that the behavior of the system is similar regardless of whether the parasitoids parasitize host larvae or host adults. The system dynamics do depend, however, on whether density-dependent population regulation is effective in the hosts or the parasitoids. If population regulation is in the hosts, then the parasitoids may or may not reduce the sterile release rate required for eradication; usually the existence of the parasitoids will substantially reduce this sterile release rate. If population regulation is via the parasitoids, then the sterile release rate required for eradication is greatly reduced over that with no parasitoids; however, the possibility of a major population explosion during the release program is more likely than if population regulation is in the hosts. Model stability was much greater with population regulation in the parasitoids than in the hosts.

Models for Assessing the Male Annihilation of Bactrocera spp. with Methyl Eugenol Baits

ABSTRACT Models were presented that describe the attraction and killing at toxic baits or traps of adult males and females of an insect pest species. These models were used to test the effects of the following factors on ease of control: female monogamy versus polygamy, attraction of only males versus both sexes, initiating mating before versus after responding to baits/traps, the ability of males to mate many times each day versus only once per day, and the existence of a time lag of several days before females can mate versus mating immediately after emergence. The models indicated that mating before trapping or the ability of males to mate many times each day will probably render this control method ineffective. The other factors tested (mating habit and specifics of attraction) had little effect on the efÞciency of trapping males as a control method. We then included age structure and a refractory period for virgin females before they can mate. The models were then made specific to Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) by using parameter values derived from the literature. The results of the models imply that the attraction and killing of large numbers of males is rather ineffective to suppress populations. However, the combination of attracting both males and females can be more effective than attracting either sex alone. The increased attraction of females to methyl eugenol baits that has been observed with the declining presence of males during Male Annihilation Technique campaigns may explain the reported effectiveness against invasive Bactrocera pest species.

Is there an optimal sex ratio for insect mass rearing

Abstract The optimal sex ratio ( sr ) for insect mass rearing depends upon the use to which the insects will be put. Mass rearing for release in control programs and for producing insects for experimental purposes may require sr values in the breeding population that optimize the production of male offspring, female offspring, or some combination of both. We have developed a number of models to guide those in mass rearing in their choice of strains to be used in the rearing facility. We have considered the cases of both prezygotic and postzygotic determination of sr , as well as whether or not there is a limit to the resources and time available to raise the insects. In general the number of females or males and females together produced is maximized for a sr in which there is a maximum possible number of females in the breeding population. When males alone are required the optimal sr in the rearing facility lies between 0.1 and 0.5 its actual value being dependent upon a variety of parameters such as the number of generations available and the number of eggs laid per female.

Determination of the sterile release rate for stopping growing age-structured populations

ABSTRACT A freely-growing age-structured population was modeled for growth and control by sterile male releases. Equilibrium populations yield critical sterile male release rates that would hold the population at equilibrium. It is shown here that these rates may be different from the release rates required to stop a growing population and bring it to an equilibrium. A computer simulation was constructed of this population and a parameter sensitivity analysis graphed the effects on the required sterile male release rate of fertility, mating delay in adult females, net juvenile survivorship, three adult survivorship curves, the time spent in the juvenile stages, and total life span. The adult survivorship curves had the greatest effect on the required sterile release rate for population elimination. The required release rate was also determined for Ceratitis capitata (Wiedemann) using survivorship and fertility data from a laboratory strain. The concepts of over-flooding ratio and release ratio were discussed and quantified for the cases above.