Parasitoids identification and its abundance

A total of 771 MB parasitoid specimens were obtained in the three evaluation plots, 136 from El Coyote, 270 from Campo Leyson and the largest number of 365 specimens was obtained in La Maroma (Figures 1, 2 and 3). In all cases the identified parasitoid species was Eretmocerus eremicus (Rose and Zolnerowich) (=Eretmocerus sp. nr. californicus, Arizona strain) (Hymenoptera: Aphelinidae), the MB most important parasitoid in northern Sinaloa due to its abundance, another one present in the region but not obtained in this study is Encarsia formosa Gahan (Hymenoptera: Aphelinidae) (^{Avilés et al., 2004}; ^{Cortez et al., 2005}). Unfortunately, the percentage of induced parasitism was not measured; however, it should have been interesting, as averages of almost 20 parasitoids were obtained on average/leaf/sampling date (Figures 2 and 3), despite insecticides applications against different pest insects and even against MB in the plot La Maroma; at El Coyote insecticides were sprayed against lepidoptera and cotton weevil, and at Campo Leyson only against lepidoptera.

Figure 1 Average number of parasitoids/MB leaf obtained from nymphs, in three cotton varieties at Coyote plot.  

Figure 2 Average number of parasitoids/MB leaf obtained from nymphs, in three cotton varieties at Campo Leyson plot.  

Figure 3 Average number of parasitoids/MB leaf obtained from nymphs, in three cotton varieties at La Maroma plot.  

No significant difference between treatments (varieties) transgenic and conventional (p> 0.05) were observed regarding to the number of parasitoids/cotton leaf of MB, which coincides with ^{Romeis et al. (2006)} in the sense that transgenic cultivars do not affect the presence and behavior of non-target insects; at El Coyote, the highest mean of parasitoids per sample was for the DP 393 conventional treatment with 6.3 little wasps/leaf, 4.9 and 4.4 little wasps/leaves for the treatments DP 0935 and DP 167RF (Figure 1), at Campo Field the highest average corresponded to the transgenic variety DP 167RF with 9.7 little wasps/leaf, 9.3 and 8.0 little wasps/leaves for DP 393 and DP 0935 varieties (Figure 2), at La Maroma the highest mean was for the DP 0935 variety with 11 eretmocerus/leaf, 10.5 and 6.6 parasitoids/sample for DP 393 and DP 167RF (Figure 3).

Meaning that the nature of the evaluated treatments had no effect on MB parasitism. This is possibly due to the Bt technology having no direct effect on MB, in this regard, ^{Bernal et al. (2010)} found that transgenic maize negatively affects the search for Cotesia marginiventris (Cresson) parasitoid of larvae of the Spodoptera frugiperda spider worm (JE Smith) fed with GM maize tissue; besides the Bt endotoxins present in spider worm larvae affect the C. marginiventris parasitoid (^{Ramírez et al., 2007}). On the other hand, although the real values of adult populations showed greater abundance in the conventional variety over transgenic in the three experimental plots (Figure 4), no significant difference between treatments (p> 0.05) was detected by the ANOVA.

Figure 4 Average number of MB adults leaf^-1 of the fifth apical node in three cotton varieties, in three comercial plots.  

It is not known the cause of the greater uniform increase of the parasitoid during the third sampling date of April and first week of May in the three plots and in the three varieties, it might be possibly related to the climatic conditions, especially temperature, in this sense ^{Weeden et al. (2012)} mention that the optimum temperatures for the rapid development and egg production of E. eremicus is 25-29 °C. In the municipality of Guasave, around the experimental plot, historical average temperature of April and May is 22.6 and 24.6 °C (^{Ruiz et al., 2005}), the temperatures of the end of May and even June are still conducive to the parasitoid, but in this research host populations declined earlier or at least were more abundant at earlier dates.

Abundance and behavior of the whitefly

The E. eremicus was positively related to the abundance of the host [positive numerical response (^{Rodríguez, 2007})] increasing between the first and second experimental plot as the host increased; while in the third date the parasitoids abundance in the population peaks was similar to the second planting date, possibly due to the insecticides spray for the control of the same MB; however, the total specimens of eremicus wasp was the largest (365) because the period of parasitoid increase was for one more week and by June 30^th they were no longer obtained. According to the above, the largest MB and parasitoid population was at the La Maroma plot, followed by the Campo Leyson and El Coyote showed the smallest population (Figure 4), corresponding to the population behavior of the insect pest to the effect of the planting date; as already indicated, cotton was planted on October 27^th in El Coyote, November 11^th in Campo Leyson and December 1^st in La Maroma, that is, population density was influenced by the sowing date (^{Cortez et al., 2005}), among other factors, which in turn was influenced by weather conditions, mainly temperature.

In the three evaluation plots the fluctuacional behavior of E. eremicus parasitoid was similar, with some exceptions, it was detected from the first sample and its population increased in the third week of April, showing the greatest increase in the last week of that month and in the first week of May, and by the third week of May the population decreased practically to the same population level observed in the first sampling. It should be noted that the population behavior of MB was different in each plot, in three samples made in the first stages (floral buds), 50% flowering and 50% buds, in El Coyote the largest number of adults were observed in the first sampling (PC) on January 27^th, in Campo Leyson the highest adult abundance of MB occurred at the second sampling (50% flowering) on May 16^th and in La Maroma the largest number of MB adults were recorded as in El Coyote, in the first sampling (PC).

On April 4^th, in each evaluation plot the MB abundance was homogeneous; that is, the three cotton varieties showed the highest insect population in the same sampling date, indicating that the development stage of the crop was determinant to the MB’s population growth, which is documented (^{Cortez et al., 2005}). However, the fluctuation behavior of the parasitoid was similar in the three plots (Figures 1, 2 and 3) and in the three evaluated cotton varieties, with different planting dates and different times of MB population increase.

Implications of results

The results allow to suggest biological control by conservation and use of E. eremicus on MB in cotton, establishing an early crop in the planting date authorized for northern Sinaloa by the Secretaría de Agricultura, Ganadería, Pesca y Alimentación (SAGARPA) (November 15 to December 15, ^{CEVAF, 2003}). The use of transgenic varieties, effective for the control of lepidopteran plague of cotton, as determined in different parts of the world, as well as in the north of the entity (^{Cortez-Mondaca and Martínez-Carrillo, 2007}), is subject to authorization from the Servicio Nacional de Sanidad, Inocuidad y Calidad Agroalimentaria (SENASICA). This paper is the first formal report on parasitism of E. eremicus on MB in cotton, in northern Sinaloa.

Conclusions

The parasitoid species of whitefly on cotton was Eretmocerus eremicus (Rose and Zolnerowich) (= Eretmocerus sp. nr. californicus, Arizona strain) (Hymenoptera: Aphelinidae). No significant difference was observed in whitefly parasitism among transgenic and conventional traditionally modified cotton varieties. As well as among adult populations of whitefly in transgenic and conventional cotton varieties, and its populations were significantly higher at the later planting date (December 1) at La Maroma plot and the lowest at the earlier planting date (October 27) at El Coyote plot.