Comparing three Methods of Calculation Economic Threshold and Injury Levels for Land Snail Monacha cantiana (Montagu) on Sugarbeet in Nubaryia, Egypt Ebieda1,a,

This investigation is the first to determine Monacha cantiana's EIL and ETL. This study was accomplished during two seasons to identify EIL and ETL of M. cantiana in West Nubaryia region in Egypt. Four infested techniques (SnP, SnM, ILnP and ILnP %) and three methods (i.e. 1WRB, Chi-square and the equation of Pedigo et al. 1986) were applied to calculate EIL and ETL. Generally, at SnP, SnM, ILnP and ILnP%, the EIL values for RWL and RYL ranged from 5.0 to 6.4, 35.1 to 44.7, 14.9 to 18.3 and 48.4 to 55.0, respectively. The EIT values for SYL at the techniques infested above ranged from 5.9 to 8.7 SnP, 41.4 to 58.7 SnM, 16.7 to 26.7 ILnP and 53.1 to 73.1 ILnP%. For RWL and RYL, range ETL values were 3.8 to 5.3, 22.4 to 38.4, 8 to 14.6 and 32.5 to 44.0 at SnP, SnM, ILnP and ILnP%, respectively. Nonetheless the corresponding values for RYL were 4.4-5.1, 22.4-35.8, 8-11.9 and 32.5-38.7. On the other hand, the aforementioned SYL treatment values were 4.0 to 7.0, 26.8 to 46.9, 10.3 to 21.4 and 37.0 to 58.0, respectively. Chi-square were only significant for root weight and their losses, therefore, it was neglected. The ranges in SnP expression were very closely. So, It could be inferred that SnP with Pedigo et al. 1 or 1WRB is an effective technique for EIL or ETL determination.

Land snails are dangerous agricultural pests which classified under phylum Mollusca, Class Gastropoda. These snails have increased rapidly causing economic damage to the field crops, vegetables as well as horticultural crops3. In Egypt, land snails have been increased and distributed rapidly in most Governorates. They caused considerable damage especially in most areas where they found the optimum conditions for survival and dispersion4. In addition, they have a role as intermediate hosts for many parasitic diseases which infected human, animals, and birds. Some of them have been identified as disseminators of the spores of phytopathogenic fungi3.
In recent times, the snails invade sugarbeet fields in Nubaryia5, Kafr El-Sheikh6 and Sharkia7. Increasing invasions of this pest gave attention to control it. The recent method was used to control M. cartusiana in Egypt by applying Zingiber officinale extract as an ecofriendly molluscicide8, 9 . Thus to harness maximum benefits from sugarbeet, there is a need to determine the economic threshold and economic injury levels of insect pests such as snails. There are several methods to calculate these parameters such as Chi-square (χ2) 10,11,12,13 the equation of Pedigo et al. 1 and defoliation 14 who modified the equation of ETL percentages by adding natural moralities and parasitism percentages.
Entomologists and economists have defined thresholds differently. Entomologists revealed that Economic Injury Level (EIL) is lowest pest population density that will cause economic damage. Economic Threshold (ETL) is pest population density at which control measures should be initiated to prevent an increasing pest population from reaching the EIL15. While the economic terminology coincides with that of Headley16, who mentioned that, ETL is the population that produces incremental damage equal to the cost of preventing that damage. Edwards and Heath17 reported that Action Threshold (ETL) is pest population large enough to cause damages valued at the cost of practical control.
The aim of the study is to calculate EIL and ETL for land snail M. cantiana (Montagu) by different methods to select the suitable one under Egyptian conditions. The profits of the results of this study are selection the best expression to such snail for controlling it.

Material & Methods
In West Nubaryia area (30 ° 42′51′′N30 ° 44′32′′E), the field with high land snails infested, to determine the EIL. The snails were named by Department of Zoology, Faculty of Agriculture, University of Mansoura, Egypt. Three species (Monacha cantiana, Theba pisana, and Helicell vestalis) were found, the most dominated snail in this region was M. cantiana, therefore, such species was chosen (an experimental area of one feddan (= acre)) in the 1st season. In the same region, another field was chosen in the 2nd season with the same conditions. The studied plots could be grouped according to the previous natural infestation with land snail M. cantiana into 12 levels as shown in Table (1) according to fore information5 and our observations. Each level was divided into three replicates. Each replicate was 2 m2. The snail numbers were recorded to each replicate from Jan till Apr (harvest time) half-month. The average of each plot was calculated at harvest date. The control was selected in the same region; this control field was planted by the same sugarbeet cultivar, the same soil texture, similar infection/infestation levels with other pests and free infestation with such snail. The soil textures of experimental and control areas were sand soil in both seasons.
For determination quantitative studies of land slug populations, South18 suggested three categories: absolute (number of individuals per unit area), relative (the number of individuals per unit effort or the relation to non-standardized traps) and indirect methods (sizes of population in terms of traces left or the effects produced by land snails). These methods were also used for snails (http://www.usc.es/export9/sites/webinstitucional/gl/investigacion/grupos/malaterra/publicaciones/ Proyect_UE_FP7/FP7_Proyecto_UE_Castillejo.pdf Page 11).
Four infested techniques for expressing the infestation with snails, which were, snail numbers/ m219 and snail numbers/ plant20, and two other methods were added, the number of infested leaves/ plant and % number of infested leaves/plant. Where, the actual area which was planted = 4000 m 2 Sugar yields = Root Yield × % sucrose Percentage of sucrose was determined using Saccharimeter according to the method described in A.O.A.C. 21 The losses of root weights/yields and sugar yield were calculated as follow: For evaluation, the losses in sugar yield, the average % of sucrose in sugarbeet was 18.2% 2 . Therefore, the estimated price of sucrose ton was LE (= Where the price of root ton was 379.37 LE.

Estimation of economic injury level (EIL) and the economic threshold level (ETL)
Both measurement methods, EIL and ETL, have been implemented according to previous definitions:-

1) Entomologist definition (Statistical variations) 1.1)
One way randomized blocks (1WRB). In this analysis, the first significant difference was used as EIL 22

1.2)
Chi-square (χ 2 ) 10,11,12,13 2) Economist definition: 2.1) The equation of Pedigo et al. 1 for determination EIL which was modified by Goebel 23 was applied. The management control cost was calculated using the cost of molluscicide (metaldehyde 5 kg/ha), labour charge for the application which accounted to 1680 LE/fed. The gain threshold (GT) was calculated by using the formula, Gain threshold (GT )= = From the linear regression equation parameters between infestation technique and root weights/yields, sugar yields or their losses, a (intercept) and b (slope) were used.

Calculation of ETL
ETL was set at 75% of ETL 24 or 80% 25

Statistical analysis
One way randomized blocks (1WRB), correlation coefficients and least significant difference (LSD) at p<0.05 were done according to the techniques outlined by Steel and Torrie 26 using Costat under window version 6.311. Arcsin (angular) transformation was done for % number of infested leaves/ plant. Chi-square was calculated as mentioned by Zaghloul et al. 13 .

Results and Discussion
Infested Techniques with the snail, M. cantiana (Montagu), Table (  To select the best expression for this snail, correlation coefficients were calculated for all tested techniques and RWL, RYL and SYL (Table 2).
It is interesting to mention that the correlation coefficients of each parameter and its losses have obtained the same values. Nonetheless, each parameter's correlation coefficients (RWL, TYL or SYL) were negative and were positive for their losses.
All correlation coefficients of the tested infested techniques had significant differences in the two seasons except for SYL in the 2 nd season. Besides, all correlation coefficients were slightly higher in the 1 st season than the 2 nd one, except the correlation coefficient between RYL and Snp or SnM. In comparison among the tested techniques, the highest correlation coefficient was recorded for ILnP% and RYL (0.949) in the 1 st season. In addition, the correlation coefficients for arcsin (angular) transformation of ILnP% were slightly lower than not translated values, therefore, in this study, ILnP% without transformation was used.  Note: Coefficients of correlation of root yields, root weights and yields of sugar were negative and were positive for losses for previous parameters. **, *** Significant differences in probability, respectively, at 1 and 0.1 percent.

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In general, the variations among the four infestation techniques and RWL or RYL correlation coefficients were not clearly observed. However, the SYL were varied from the 1 st season to the 2 nd one. Therefore, all tested techniques were used in this study and they were written in all tables. SYL may be sensitive to change meteorological parameters 27 , they mentioned that the sugarcane is very sensitive to temperature, rainfall, solar radiations etc. Consequently, the findings of SYL became less attention in this study. The arbitrary economic threshold usually reflects schedules of the farmer and applicator, weather, equipment, farm size, the farmer's intuition and any number of such factors that are difficult to model 28 .

1) 1WRB
The results of EIL values were similar for both RWL and RYL, so these results were combined in Table (3). In comparison with the control, control treatments values of RWL or RYL were not statistically significant than up to 4.3 and 6.2 Snp in the 1 st 2 nd seasons, respectively. However, in both tested seasons, SYL were not statistically significant than up to 5.0 SnP in the 1 st season and 6.2 SnP in the 2 nd one. In this investigate, RWL, RYL and SYL were higher (> 10 percent) than those reported by Shalaby et al. 6 in the governorate of Kafr El-Sheik they found that the overall mean of root weight and sugar yield reduction (10-50 SnP) was 0.128-3.46 and 1.06-12.23%, respectively. Additionally, the examined sugarbeet cultivar was more susceptible to land snail Mcantiana than the sugarbeet cultivar recorded by Shalaby et al. 6 and its soil texture was clay and our soil is sand. It is also possible to restore these changes to locations, meteorological parameters or other parameters.

2) Chi-Square Test
Only root weight and root weight losses were applied for chi-square tests because there were significant differences between those results. Conversely, there were no significant differences in the data for RYL and SYL (Table 5), so it could not evaluate EIL or DTL.
In 1 st season EIL values for root weight were higher than 2 nd one. Nevertheless for root weight losses in the 1 st season, EIL values for SnP, SnM, ILnP and were lower than the second vice versa for ILnP%. The EIL range values for root weight were 6.2-6.  The EIL values for RWL and SYL were higher in the 2 nd season as well as for RYL at SnP and SnM (Table 6) The least values of EIT or ETL were recorded for RYL and the highest one was observed for SYL. Furthermore, the range between the least and the highest values of EIT/ETL was higher for SYL than SWL or RYL.  7 reported that land snail M. cartusiana (Mullar) has the relatively highest economic threshold rates. was 3,12 snail / lettuce, 3,58 snail/100 pea, 3,19 clover snail / m2 and 3,224 wheat snails.

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Results revealed a discrepancy between the ranges of the techniques tested for infestation. The ranges in SnP expression were very closely followed by ILnP for RWL RYL, ILnP% techniques and the highest ranges were recorded for SnM.
The same sequence was recorded for SYL. The ranges of EIL or ETL for RWL/RYL lower than for SYL.

Conclusion
The joint results of the testes of EIL lead to the conclusion that Chi-square tested was not valid in this study because this test gained significantly differences for only RWL, also it gave insignificantly differences for RYL and SYL. In addition, these results were varied and lower than other previous methods. Therefore, their results were neglected. 1WRB and Pedigo et al. 1 equation could be used for determination EIL or ETL.
According to the present investigations, assessment of EIL on the basis of SnP is an appropriate idea than the rest of tested techniques.