Preliminary blowfly survey report, August 1991

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.,

DL HIST 595.774 LOG 1991

PRELIMINARY BLOWFLY SURVEY REPORT

AUGUST 1991

D LOGAP /

MEDICAL ENTOMOLOGY BRANCH

NT DEPARTMENT OF HEALTH & COMMUNITY SERVICES

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SYNOPSIS

1.0 2.0 3.0

4.0 5.0

6.0 7.0

Background Area of Survey Aim of Survey

11111111111111111111111111111111111111111111111111111111111111111111111111111111

3 0820 00019001 2

TABLE OF coNrENTS

Method of Investigation Results

5.1 5.2

5.3

Discussion

Mean and Standard deviation

Linear regression analysis of fly numbers on time

Analysis of variance for site differences

Conclusjons

Tables Figures

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SYNOPSIS

The Darwin City Council (DCC) is currently conducting a blowfly trapping survey in response to public concern over possible

blowfly increase following the introduction of wheelie-bins. The Medical Entomology branch of the NT Department of Health and

Community Services is providing technical assistance to the DCC in this survey.

This report details results of the blowfly trapping program to date and the statistical analyses of those results.

Traps catch predominately Chrysomya megacephala. Analysis of traps catches over time reveal that to date numbers of this blowfly fluctuate with no significant trend to increase or

decrease. There are significant differences between trap sites.

Traps at sites at Karama and Alawa catch significantly more~- megacephala than others. The trap at Parap catches significantly less~- megacephala than most others. The trap at Palmerston lies in the median range of trap catches. Numbers of Lucilia cuprina are relatively low. There were no significant

differences between~- cuprina catches at different sites.

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1.0 BACKGROUND

There has been public concern over an increase in blowfly numbers associated with the introduction of wheelie-bins to Darwin. In response to this public concern the Darwin City Council (DCC) is currently conducting a blowfly trapping survey with technical support from the Medical Entomology branch, NT Department of Health and Community Services. The survey follows a previous survey undertaken in September 1990 in which the sheep blowfly, Lucilia cuprina was the major species trapped. This survey showed that numbers of

1·

cuprina were seven times higher in Palmerston, which does not have wheelie-bins, than in Darwin. Another common

blowfly, Chrysomya megacephala, was caught in equal numbers in Darwin and Palmerston.

2.0 AREA OF SURVEY

The survey is based on seven trap sites in Darwin. An eighth trap site is located in Palmerston and acts as a control. The location of each trap site is figured in fig.1.

3.0 AIM OF SURVEY

The survey aims to detect seasonal trends in blowfly numbers and is useful as an initial blowfly database for future

reference.

4.0 METHOD OF INVESTIGATION

An 'Efekto' fly trap is set at each of 7 Darwin locations and 1 Palmerston location weekly. Each trap is collected after two days by DCC staff and delivered to the Medical Entomology branch for processing. Processing involves estimating the volume of flies caught and determining the species composition by subsampling. A subsample of 50 flies is taken from each trap and identified to species level.

The total number of flies is estimated by measuring the total volume of flies caught in each trap. These results are recorded on computer files each week.

5.0 RESULTS

5.1 Mean and Standard Deviation

The mean and standard deviation of all blowflies caught per week and for~. megacephala caught per week is given in fig.2 and fig.3 respectively. (Standard deviation is a measure of the variability between trap catches. It is not

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equivalent to the range of the numbers of flies caught.) The average numbers of Q. megacephala and Lucilia cuprina caught per week can be compared in fig.4. The proportion of the catch which is Q. megacepti~la has remained consistently high since trapping began. In comparison the proportion of

1·

cuprina caught has remained low.

5.2 Linear Regression Analysis of Fly Numbers on Time

Linear regression analysis of mean numbers of Q. megacephala caught per week, from 9-11 April to 30 July - 1 August,

produced the linear equation y=1616.95 + 39.95x. The line is figured in fig.5. The equation indicates that there is an increase in Q. megacephala numbers caught over the time period of the analysis. The increase occurs at the rate of about 40 Q. megacephala caught per week or an increase of some 2.5% per week. The correlation co-efficient (r) is 0.41. This indicates that time is not a very good indicator of Q. megacephala numbers. The co-efficient of

determination (r 2 ) is 0.168. This indicates that the total variation in Q. megacephala numbers explained by a linear regression on time is small, ie 16.8%. The correlation between time and~. megacephala numbers is therefore poor.

Since the x-values (time of trapping) are not distributed in a normal probability curve an analysis of variance was

completed for the regression (Table 1). The F-value

produced is not significant. This means that we can accept the hypothesis that there is no relationship between time and Q. megacephala numbers for the period analyzed. As a consequence there should be no significance attached to the increase of 40 flies per week. This is merely an artifact of the analysis.

5.3 Analysis of Variance for Site Differences

A two-way analysis of variance of Q. megacephala numbers {Table 2) shows that there are highly significant

differences between sites but no significant differences between dates of collection. Further analysis (calculation of Least Significant Differences) (fig.6) indicates that

traps at site 2, Karama, and site 3, Alawa, catch

significantly more on average than other traps. The trap at site 7, Parap, catches significantly less than most other trap sites. The mean catch in the trap at site 8,

Palmerston, lies in the median range for trap catches. It does not differ significantly from mean catches at four other trap sites.

One-way analysis of variance of

g.

cuprina numbers (Table 3) failed to find any significant difference between sites.

This was probably a result of the high variability between trap catches at different sites and over time.

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6.0 DISCUSSION

Statistical analysis indicatei that some trap sites catch significantly more~- megacephala on average than others but that catches of

g.

cuprina are too variable for any

differences between sites to be significant. Analysis of the trend of~- megacephala trap catches indicates that catches remain at a relatively constant level over time.

Both analyses over time and on a per site basis are most meaningful with regard to trap characteristics. They indicate that the trap is consistently good at catching C.

megacephala but less efficient at catching~- cuprina. This may or may not reflect the composition of the blowfly

population in the vicinity of those traps.

Trap catches at Palmerston were not found to significantly differ from most other sites. Since this site acts as a control site i t can be inferred that Wheelie-bins are not a necessary factor in~- megacephala generation. Assuming that bins do produce blowflies, a better explanation is that the type of bin in use as a household refuse receptacle has little influence on the volume of~- megacephala production.

The trap results of the survey to date do not agree with the results of the survey of September 1990. In that survey L.

cuprina was the most common blowfly trapped. A second species~- Varipes was also caught in high numbers. These two speceis are poorly represented in 'Efekto' trap catches to date. One explanation is that the difference can be attributed to different trap attraction qualities.

Alternatively the composition of the fly population may have changed since September 1990.

7.0 CONCLUSIONS

Conclusions that can be reached by analysis of the survey data are limited because of the limited nature in which data is collected.

Numbers of C. megacephala caught in the traps remain high and are relatively constant over time. Numbers of

g.

cuprina are relatively low and their catch pattern is

erratic. Trap catches of~- megacephala at Palmerston are not significantly different to trap catches at most other sites. Trap catches of~- cuprina failed to show any significant differences between. sites.

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I •

FIGURES

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...

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- - . . - , - - 1 -

8 Palmerston ^Metres^IOO^~o^'.'.1⁵ ~ 1 ; ; ; - -1;;00~0~ ~ ~2~0"';;0;;0- -3;;;1000 Moires

Fig.

Darwin;

1 Location of eight trapping sites (indicated as nos. 1-7 in 8 1n Pal~erston) used in the current blowfly survey undertaken

by the DCC and lle~,ical Ento111logy.

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5000 N 4500 u

m 4000 b

e 3500 r ₃₀₀₀

0 2500

f

2000 F 1500

I

i 1000 e

s 500 0

Fig.2 Mean nuMbers and standard deviation for all flies collected at the 8 saMpling sites. plus SD= nean plus standard deviation. Minus SD

= Mean ninus Standard deviation.

9-11Apr 16-18Apr 2J-25Apr JOAor-2 7-91~ 14-16M{l'f 21-2Hloy 28-30!.lay 4-6.Nne 11-13),;ne 18-20June 25-27J1.11e 2-4.AJly 9-11Jt1~ 16-18Jl1~ 23-25Jt~Y 30J-1At1g 6-8~.JJg l.!<Jy

Date of collection

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nean Rinus standard deviation.

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Date of collection

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N u m b e r s

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Fig.4 Hean nuMbers of all flies, C.negacephala and L.cuprina trapped at 8 sanpling sites.

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2000

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Fig.5 Linear regression of ~ean nuMbers of c.~egacephala fro~ 8 sa~pling sites on tiMe.

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Date of collection

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Kararna Alawa T

r Milner

a

p Nakara

s Wagaman i

t Palmerston e

Anula Parap

0

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Fig. 6 Representation of Least significant difference (LSD) between neans of C.negacephala collected in traps over 17 weeks. LSD analysis

defines the significant differences between sites indicated in the regression ANOUA.

LSD = 684.2

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mean number of C.megacephala

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TABLES

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Table 1. Analysis of Variance for the linear regression of C.megacephala numbers from 8 different trap sites and times of collection.

Source of variation Regression

Error Total

DF 1 15 16

ss

MS

491760.5 491760.5 2435533.5 162386.9 2927294.1

F

3.03 NS

Table 2. Two-way analysis of Variance for numbers of C.megacephala trapped weekly at 8 different sites over a period of 17 weeks.

Source of variation Trap site

Date Error Total

DF

7 16 112 135

ss

MS

70115556.5 10016508 25137112.5 1571069.5 113670366 1014913.9 208923035

F 9.87 1.55

**

NS

Table 3. One-way analysis of Variance for numbers of L.cuprina

trapped weekly at 8 different sites over a period of 1 7 weeks.

Source of variation Trap site

Error

DF

7 126

ss

108716.6 1383099

MS 15530.9

10976

F 1.41

...

Total 133 1491816

Note:

DF degrees of freedom

ss

sum of squares MS mean square

F F-value

NS not significant

significant at the 5% level

** significant at the 1% level (highly significant) For F values see Appendix A.

NS

I --

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APPENDIX A

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,

F-Distribution (F._{95 )}

The numbers given in this table are the values of F - for which the area to the left equals 0.95

F

=

^1- ^Fgs

Degrees of freedom for numerator

2 3 4 5 6 7 8 9 10

1 161 200 216 225 230 234 237 239 241 242 2 18.5 19.0 19.2 19.2 19.3 19.3 19.4 19.4 19.4 19.4 3 10.1 9.55 9.28 9.12 9.01 8.94 . 8.89 8.85 8.81 8.79 4 7.71 6.94 6.59 6.39 6.26 6.16 6.09 6.04 6.00 5.96 5 6.61 5.79 5.41 5.19 5.05 4.95 4.88 4.82 4.77 4.74 6 5.99 5.14. 4.76 4.53 4.39 4.28 4.21 4.15 4.10 4.06 7 5.59 4.74 4.35 4.12 3.97 3.87 3.79 3.73 3.68 3.64 8 5.32 4.46 4.07 3.84 3.69 3.58 3.50 3.44 3.39 3.35

.s

_cu ₁₀⁹ ^5.12_4.96 ⁴_4.10^.26 ^3.86_3.71 ^3.63_3.48 ^3.48_3.33 ³_3.22^.37 ^3.29_3.14 ^3.23_3.07 ^3.18_3.02 ^3.14_2.98

C

E ⁰ 11 4.84 3.98 3.59 3.36 3.20 3.09 3.01 2.95 2.90 2.85

C 12 4.75 3.89 3.49 3.26 3.11 3.00 2.91 2.85 2.80 2.75

Q)

"O

13 4.67 3.81 3.41 3.18· 3.03 2.92 2.83 2.77 2.71 2.67 -2 14 4.60 3.74 3.34 3.11 2.96 2.85 2.76 2.70 2.65 2.60 E 15 4.54 3.68 3.29 3.06 2.90 2.79 2.71 2.64 2.59 2.54

0

"O

Q) 16 4.49 3.63 3.24 3.01 2.85 2.74 2.66 2.59 2.54 2.49

1 ₁₇ _4.45 _3.59 _3.20 _2.96 _2.81 _2.70 _2.61 _2.55 _2.49 _2.45

0 ₁₈ ₄_.41 _3.55 _3.16 _'.2.93 _2.77 _2.66 _2.58 _2.51 _2.46 _2.41

U)

19 4.38 3.52 3.13 2.90 2.74 2.63 2.54 2.48 2.42 2.38

Q)

~ 20 4.35 3.49 3.10 2.87 2.71 2.60 2.51 2.45 2.39 2.35

OJ Q)

0

21 4.32 3.47 3.07 2.84 2.68 2.57 2.49 2.42 2.37 2.32 22 4.30 3.44 3.05 2.82 2.66 2.55 2.46 2.40 2.34 2.30 23 4.28 3.42 3.03 2.80 2.64 2.53 2.44 2.37 2.32 2.27

cJ 24 4.26 3.40 3.01 2.78 2.62 2.51 2.42 2.36 2.30 2.25

25 4.24 3.39 2.99 2.76 2.60 2.49 2.40 2.34 2.28 2.24 30 4.17 3.32 2.92 2.69, 2.53 2.42 2.33 2.27 2.21 2.16

\ 40 4.08 3.23 2.84 2.61 2.45 2.34 2.25 2.18 2.12 2.08

60 4.00 3.1 5 2.76 2.53 2.37 2.25 2.17 2.10 2.04 1.99 120 3.92 3.07 2.68 2.45 2.29 2.18 2.09 2.02 1.96 1.91

co 3.84 3.00 2.60 2.37 2.21 2.10 2.01 1.94 1.88 1.83

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F-Distribution (F._{99 )}

F= F.99

Degrees of freedom for numerator

2 3 4 5 6 7 8 9 10

1 4,052 5,000 5,403 5,625 5,764 5,859 5,928 5,982 6,023 6,056 2 98.5 99.0 99.2 99.2 99.3 99.3 99.4 99.4 99.4 99.4 3 34.1 30.8 29.5 28.7 28.2 27.9 27.7 27.5 27.3 27.2 4 21.2 18:0 16.7 16.0 15.5 15.2 15.0 14.8 14.7 14.5 5 16.3 13.3 12.1 11.4 11.0 10.7 10.5 10.3 10.2 10.1 6 13.7 10.9 9.78 9.15 8.75 8.47 8.26 8.10 7.98 7.87 7 12.2 9.55 8.45 7.85 7.46 7.19 6.99 6.84 6.72 6.62 8 11.3 8.65 7.59 7.01 6.63 6.37 6.18 6.03 5.91 5.81 9 10.6 8.02 6.99 6.42 6.06 5.80 5.61 5.47 5.35 5.26 1 o. 10.0 7.56 6.55 5.99 5.64 5.39 5.20 5.06 4.94 4.85

m 0

C 11 9.65 7.21 6.22 5.67 5.32 5.07 4.89 4.74 4.63 4.54 E 1 2 9.33 6.93 5.95 5.41 5.06 4.82 4.64 4.50 4.39 4.30

0

C 13 9.07 6.70 5.74 5.21 4.86 4.62 4.44 4.30 4.19 4.10

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-0 14 8.86 6.51 5.56 5.04 4.70 4.46 4.28 4.14 4.03 3.94 .2 ¹⁵ ^8.68 ^6.36 ^5.42 ^4.89 ^4.56 ^4.32 ^4.14 ^4.00 ^3.89 ^3.80

E

0 16 8.53 6.23 5.29 4.77 4.44 4.20 4.03 3.89 3.78 3.69

-0

(1) 17 8.40 6.11 5.19 4.67 4.34 4.10 3.93 3.79 3.68 3.59

! 18 8.29 6.01 5.09 4.58 4.25 4.01 3.84 3.71 3.60 3.51 0 19 8.19 5.93 5.01 4.50 4.17 3.94 3.77 3.63 3.52 3.43

(/) 20 8.10 5.85 4.94 4.43 4.10 3.87 3.70 3.56 3.46 3.37

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~

Ol 21 8.02 5.78 4.87 4.37 4.04 3.81 3.64 3.51 3.40 3.31

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o · 22 7.95 5.72 4.82 4.31 3.99 3.76 3.59 3.45 3.35 3.26 23 7.88 5.66 4.76 4.26 3.94 3.71 3.54 3.41 3.30 3.21 24 7.82 5.61 4.72 4.22 3.90 3.67 3.50 3.36 3.26 3.17 25 7.77 5.57 4.68 4.18 3.86 3.63 3.46 3.32 3.22 3.13 30 7.56 5.39 4.51 4.02 3.70 3.47 3.30 3.17 3.07 2.98 40 7.31 5.18. 4.31 3.83 3.51 3.29 3.12 2.99 2.89 2.80 60 7.08 4.98'· 4.13 3.65 3.34 3.12 2.95 2.82 2.72 2.63 120 6.85 4.79 3.95 3.48 3.17 2.96 2.79 2.66 2.56 2.47 co 6.63 4.61 3.78 3.32 3.02 2.80 2.64 2.51 2.41 2.32

.,