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

Abstract

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.

Appendices

Appendix I

Below are shown blocks of five-by-five cells and their equilibrium values of wild adults per hectare. Each modeled square kilometre has four identical such five-by-five blocks.

 

Regular spatial distribution					Slightly aggregated
48.2	48.2	48.2	48.2	48.2	33.7	33.7	33.7	33.7	33.7
48.2	48.2	48.2	48.2	48.2	33.7	56.3	78.9	56.3	33.7
48.2	48.2	48.2	48.2	48.2	33.7	78.9	123.9	78.9	33.7
48.2	48.2	48.2	48.2	48.2	33.7	56.3	78.9	56.3	33.7
48.2	48.2	48.2	48.2	48.2	33.7	33.7	33.7	33.7	33.7
Moderately aggregated					Highly aggregated
24.1	24.1	24.1	24.1	24.1	19.3	19.3	19.3	19.3	19.3
24.1	61.7	99.3	61.7	24.1	19.3	45.1	70.9	45.1	19.3
24.1	99.3	174.3	99.3	24.1	19.3	70.9	429.2	70.9	19.3
24.1	61.7	99.3	61.7	24.1	19.3	45.1	70.9	45.1	19.3
24.1	24.1	24.1	24.1	24.1	19.3	19.3	19.3	19.3	19.3

Appendix II

List of subroutines, variables and parameters (with default values) in the model.

Subroutines:

Input:

reads a file allowing user to choose initial conditions and controls

Init:

initializes most of the population variables

Delfts:

initializes user-requested aggregation patterns

Contrl:

puts chosen control measures (except SIT) into effect

Mating:

performs mating and SIT each time step

Disperse:

performs dispersal among adjacent hectares each time step

Output1:

outputs tabular results to a file every 3 months

Output2:

outputs results to a file for graphing every month

List of variables¹:

The population components shown as (Nos./ha/age class) are triply subscripted, taking account of age and spatial specificity.

pup:

total number of pupae alive today (Nos./ha)

crtmax:

the current developmental time for fly larvae (days)

fe:

number of adult flies emerging today (Nos./ha)

ftot:

total number of wild adult flies alive today (Nos./km²)

fm:

total adult fertile males alive (Nos./ha/age class)

ff:

total adult fertile females alive (Nos./ha/age class)

vm:

total virgin fertile males alive (Nos./ha/age class)

vf:

total virgin fertile females alive (Nos./ha/age class)

gm:

total adult sterile males alive (Nos./ha/age class)

gf:

total adult sterile females alive (Nos./ha/age class)

wm:

total virgin sterile males alive (Nos./ha/age class)

wf:

total virgin sterile females alive (Nos./ha/age class)

ferfem:

fertile females saved for output (Nos./km²)

ffqx, fmqx, vfqx, vmqx, gfqx, gmqx, wfqx, wmqx:

density-independent mortality of fertile females, fertile males, virgin females, virgin males, sterile or sterile-mated females, sterile males, virgin sterile females, and virgin sterile males respectively (Nos./ha/age class)

ffdqx, fmdqx, vfdqx, vmdqx, gfdqx, gmdqx, wfdqx, wmdqx:

density-dependent mortality of fertile females, fertile males, virgin females, virgin males, sterile or sterile mated females, sterile males, virgin sterile females, and virgin sterile males respectively (Nos./ha/age class)

pupdd:

proportion of density-dependent pupal mortality (/ha)

adltdd:

proportion of density-dependent adult mortality (/ha)

pqm:

mean proportion of density-independent pupal mortality

pqdm:

mean proportion of density-dependent pupal mortality

qxm:

mean proportion of density-independent adult mortality

qxdm:

mean proportion of density-dependent adult mortality

List of parameters:

Natural parameters  Default value

fet:

total initial adult pests (carrying capacity)  50000

pup0:

initial number of pupae  20000

lxft:

d-i daily female temperature-dependent survivorship

lxmt:

d-i daily female temperature-dependent survivorship

lxst:

d-i daily female temperature-dependent survivorship

lxf:

d-i basic daily survivorship of adult females  0.99

lxm:

d-i basic daily survivorship of adult males  0.98

lxs:

d-i basic daily survivorship of sterile males  0.92

lxp:

d-i basic daily survivorship of pupae  0.98

di:

modifier for changing density independent mortality  1.0

mx:

maximum daily oviposition of adult pests (larvae/day)  0.1

ag:

aggregation coefficient (a nominal measure)  1.0

attr:

matrix of attractions for dispersing adults  0,1

maxt:

maximum daily temperature (°C)  40

mint:

minimum daily temperature (°C)  20

tmin:

minimum temperature for survival of pupae (°C)  15

tmax:

maximum temperature for survival of pupae (°C)  45

thr:

threshold temperature for pupal development (°C)  18

tmp:

array containing daily temperatures sin(jd)

yrs:

total years for simulation to run  20

spec:

glossina palpalis (1) or pallidipes (2) (flag)  1,2

fpm:

propensity of the females to mate (by age; %)

mpm:

propensity of the males to mate (by age; %)

tlx:

additional lx reduction specific to tenerals  0.98

tmpqx:

multiplier for temperature effect on mortality  >1

mdisp:

flag for choosing the mode of dispersal  1,2

Control parameters

kdisp:

flag to enable the dispersal subroutine  0,1

pdisp:

percent of flies dispersing from each cell  50

sp:

percent kill from one cover spray  (1–100%) 0.0

xn:

number of odor traps  0

x:

mortality from odor trapping (proportion)  0.0

po:

number of insecticide-treated cattle  0

y:

mortality from cattle (proportion)  0.0

ipo:

counter to determine cells for cattle  1–10

igroup:

number of tens of groups of treated cattle  1,2,3

oflr:

overflooding ratio of sterile releases  0

r:

rate of release of sterile males (number/5 days/ha)  0.0

c:

competitive ability of sterile males  1.0

q:

residual fertility of irradiated males (percent)  0.0

jdate:

julian date to begin control  5

jdate-as:

julian date to begin aerial spraying  5

jdate-tt:

julian date to begin trapping  5

jdate-il:

julian date to begin livestock catches  5

jdate-sr:

julian date to begin sterile releases  5

count:

flag for each control method (1)  0

rmf:

relative mating frequency of steriles with wild palpalis  1.0,0.9