Abstract: Empoasca decipiens Paoli (Hom.: Cicadellidae) has been causing damage in potato fields of Ardabil region in Iran. There has been an increasing interest in controlling of E. decipiens using resistant cultivars. The resistance of four commonly planted cultivars including Diamant, Agria, Casmos and Omidbakhsh to E. decipiens was compared using some life cycle parameters of this pest in greenhouse at 24±1°C, 50±5% RH and 16:8 h (L:D) photoperiod. Incubation period, development timeof1st, 2ndand 3rd instar larvae were not significantly different on the cultivars studied. Fourth and 5th instar larvae development time and female and male life span of E. decipiens decreased among the cultivars in the order of Diamant >Casmos >Omidbakhsh >Agria. The percentage of larval survival of E. decipiens on Diamant and Casmos were significantly lower than on Omidbakhsh and Agria. Sex ratio of E. decipiens on four cultivars was not significantly different. High correlation coefficients were observed between the density of simple and glandular trichomes with the percentage of larval survival, larval development time, female and male life span of E. decipiens. These results indicated that among the cultivars that were investigated, Diamant and Casmos were resistant and Omidbakhsh was tolerant to E. decipiens damage. The results of this study also confirmed that the density of glandular trichomes may have more affects on the life cycle of E. decipiens than the density of simple trichomes by restricting larvae and adult feeding. These results are useful in an integrated management program of E. decipiens in potato fields.
INTRODUCTION
Empoasca decipiens Paoli (Hom.: Cicadellidae) has been recorded on a wide variety of crops including cultivated and non-cultivated plants (Gunthart, 1971; Nielson et al., 1990; Schmidt and Rupp, 1997). E. decipiens is an economic pest of soybeans in Iowa and also potato and cotton fields in Turkey (Pedigo et al., 1986; Atlihan et al., 2003; Demirel and Yildirim, 2008). Biological parameters of Empoasca sp. has been studied under the field and greenhouse conditions on various plants (Habib et al., 1972; Radcliffe and Johnson, 1994; Medeiros et al., 2005; Naseri et al., 2007). These studies have revealed that the host plant trichomes has an important effects on the life cycle parameters of Empoasca sp. Raupach et al. (2002) also reported that temperature and host plants affected the life cycle parameters of E. decipiens .
In Ardabil plain at Northwest of Iran, more than 25000 ha of irrigated land is allocated for potato production and more than 20% of the annual potato production of the country is produced. Colorado potato beetle, Leptinotarsa decemlineata (Say), onion thrips, Thrips tabaci Lind., two-spotted spider mite, Tetranychus urticae Koch, green peach aphid, Myzus persicae (Sulz.) and Empoasca decipiens are injurious pests in Ardabil (Nouri-Ganbalani, 1989; Fathi and Nouri-Ganbalani, 2009). E. decipiens has been causing damage in potato fields of Ardabil region in Iran. Leafhoppers are capable of becoming resistant to insecticides. Management strategies aimed at reducing or preventing resistance will help conserve existing products for ongoing use (Charles, 2005). There has been an increasing interest in controlling leafhoppers using resistant cultivars (Medeiras et al., 2004, 2005). In this study, four potato cultivars were evaluated for their resistance to the E. decipiens by evaluating some life cycle parameters of this pest, such as incubation period, larval development time, larval survival, female and male life span and adult longevity in a greenhouse experiment.
MATERIALS AND METHODS
This study was conducted in greenhouse. Tubers of Diamant, Agria, Casmos and Omidbakhsh cultivars were obtained from Agriculture Research Station of Ardabil and planted in plastic pots (two tubers per pot) in a greenhouse at 24±1°C, 50±5% RH and a photoperiod of 16:8 h (L:D) during June 2008. At stem elongation stage of potato the number of simple and glandular trichomes in 1 cm-2 of the lower leaf surface of 10 randomly selected leaves from each potato cultivar were counted using micrometer under stereomicroscope (80 X) (Fig. 1). Leafhopper larvae prefer the lower leaf surface of the plant for feeding (DeLong, 1971). The lower leaf surface of potato is very pubescent, affecting the mobility of herbivores (Raupach et al., 2002).
E. decipiens was collected by sweep net from unsprayed potato fields (cv. Agria) in the Ardabil plain during July 2007. This pest were reared in the same greenhouse on alfalfa (cv. Hamadan) planted in plastic pots which were placed inside a Plexiglas cage, 10 l size and the top of the cages were covered with fine mesh silk screen. Newly emerged adults of E. decipiens were collected from the colony and used in the experiment.
The life cycle parameters of E. decipiens were studied on a 4th
leaf, from the top of plant, in four potato cultivars separately at stem
elongation growth stage of potato in greenhouse. In these experiments,
a pair of one day old adult of E. decipiens (after being temporarily
anesthetized at -4°C for 5 min), were transferred with a fine hair
brush inside a transparent cylindrical plastic cage (10 cm diameter and
25 cm high with fine mesh silk screen that was stacked around the upper
part of the cage). The cage was tightened from silk screen portion around
the petiole of a 4th leaf in each experimental plant using a silk string.
The insectswerekept inside the cage (with one potato leaf) for three days
in order to feed and mate. Since, one day old eggs were needed to conduct
the experiments, the cage (with the same one adult pair) was removed from
the previous leaf and fastened from silk screen portion around the petiole
of a new 4th leaf in order to obtain one day old eggs for 24 h. After
24 h, the insects were removed from the cage. Then, the date of the egg
hatch and the number of 1st instar larvae emerged was recorded daily in
each cage. The status of the larvae (death or completion of their development
to the adult stage and their gender) was recorded in each cage. Also longevity
of male and female in each cage was recorded until their death. Each treatment
was replicated 25 times.
| Fig. 1: | Simple and glandular trichomes in the lower leaf surface of potato |
Data Analysis
The data was log transformed to meet the assumptions of normality
and homogeneity of variances. The data of the life cycle parameters of
E. decipiens on four potato cultivars were analyzed by one-way
ANOVA and the differences were compared by Student-Newman-Keals test (PROC
ANOVA, SAS). Also pearson correlation coefficients between the number
of simple and glandular trichomes on the lower leaf surface in four potato
cultivar with life cycle parameters of E. decipiens were calculated
by the correlation analysis (PROC CORR, SAS, 1999).
RESULTS AND DISCUSSION
According to data in Table 1, incubation period, development timeof 1st, 2ndand 3rd instar larvae of E. decipiens were not significantly different on the four potato cultivars studied. Fourth instar larvae development time was higher on Diamant and lower on Agria, while no significant difference was observed between Casmos and Omidbakhsh. Fifth instar larvae development time was higher on Diamant and lower on Agria and Omidbakhsh, while it was intermediate on Casmos. Total larval development time of E. decipiens was significantly higher on Diamant and Casmos in comparison to Agria and Omidbakhsh (Table 1).
Female and male longevity were not significantly different on four potato cultivars (Table 1). However, female life span was higher on Diamant and Cosmos and lower on Agria, while it was intermediate on Omidbakhsh. Male life span was highest and lowest on Diamant and Agria respectively, while it was intermediate on Cosmos and Omidbakhsh (Table 1).
The percentage of larval survival of E. decipiens on Diamant and Casmos were significantly lower than Omidbakhsh and Agria. Sex ratio of E. decipiens was not significantly different among four cultivars (Table 2).
The highest number of simple trichomes was observed on Diamant, Agria, Casmos and Omidbakhsh, respectively. The highest number of glandular trichomes was observed on Casmos, Diamant, Agria and Omidbakhsh, respectively. The highest number of leaf per plant was observed on Omidbakhsh, Diamant, Casmos and Agria, respectively. The highest yield was observed on Omidbakhsh, Diamant, Agria and Casmos, respectively (Table 3).
The correlation analysis indicated significant positive correlation coefficients
between density of simple and glandular trichomes on the lower leaf surface
of four cultivars with larval development time and female and male life
span of E. decipiens. The significant negative correlation coefficients
were observed between density of simple and glandular trichomes on the
lower leaf side of four cultivars with the percentage of larval survival
(Table 4). Correlation coefficients between density
of glandular trichomes with the percentage of larval survival, larval
development time, female and male life span were higher than correlation
coefficients between density of simple trichomes with the percentage of
larval survival, larval development time, female and male life span separately
(Table 4). These results indicated that density of glandular
trichomes on the lower leaf side of four cultivars probably had higher
effect on life cycle of E. decipiens than the density of simple
trichomes. Correlation coefficient between the density of simple and glandular
trichomes with incubation period was very low (Table 4).
This result revealed that the density of simple and glandular trichomes
had no effect in incubation period of E. decipiens.
| Table 1: | Mean comparison of life cycle parameters of E. decipiens in greenhouse condition |
| Different letters indicate significant differences within rows at p=0.05 | |
| Table 2: | The percentage (±SE) of larval survival and sex ratio of E. decipiens on four potato cultivars |
| Table 3: | The mean number of simple and glandular trichomes on lower surface of four potato cultivars and the mean number of leaf and yield/plant (±SE) |
| Table 4: | Pearson correlation coefficients between mean number of simple and glandular trichomes and life cycle parameters of E. decipiens |
| The numbers inside the parenthesis showed the p-value for each correlation coefficient | |
Potato cultivars which were investigated in this study were significantly different on their effects on the life cycle parameters of E. decipiens. There was a high negative correlation between trichome (simple and glandular) density on the lower leaf surface of the cultivars with larval survival. Also in the more pubescent cultivars larval development time was significantly longer compared to the less pubescent cultivars. Meanwhile, the adult life span was longer in less pubescent cultivars. It appeared that the high densities of trichome affect the mobility of E. decipiens and impair its feeding activities, hence decreases the survival rate and increases the larval development time. Leafhopper larvae generally prefer the lower surface of the leaves for feeding (DeLong, 1971), but when the trichome densities of lower leaf surface is very high, they prefer to feed on the upper leaf surface to avoid the deterrent effects of the pubescence. Habib et al. (1972) observed a prolonged larval development time on tomato compared to broad bean. The significantly longer larval development time was also reported on tomato compared to broad bean, sweet pepper and cucumber (Stamp and Yang, 1996), due to more pubescent nature of the tomato leaf surface. The hairy glands on leaves and stems of tomato possibly affect the mobility and feeding activities of E. decipiens larvae. Medeiras et al. (2005) concluded that survival rates of larvae, males and females of potato leafhopper, Empoasca fabae (Harris), on six potato (Solanum sp.) genotypes with varied levels of resistance was related to type, density and droplet/head size of glandular trichomes. Survival rate was significantly lower on Solanum berthaultii Hawkes accessions, because the dense glandular trichomes of these accessions affect the survival of the leafhopper through restricting its feeding and mobility. Most of the larvae died from starvation and dehydration on glandular genotypes. Also it was reported that trichome densities and trichome types play the most important role in resistance of Solanum sp. to the leafhoppers (Avé and Tingey, 1986; Raupach et al., 2002). It has been found that the main component of glandular trichomes that affect the leafhoppers is phenolic oxidase. For example, Avé and Tingey (1986) concluded that phenolic oxidase activities in glandular trichomes of S. berthaultii influences the survival and feeding behavior of the potato leafhopper. Present findings on four potato cultivars also revealed that the larval development time is significantly longer on more pubescent cultivars compared with glabrous or less pubescent cultivars. We also found that the survival rate of larvae was higher on less pubescent than more pubescent cultivars. Therefore, it can be concluded that pubescent cultivars of potatoes are more resistant to E. decipiens and should be included in the IPM program of this pest.
ACKNOWLEDGMENT
The Research Council of Mohaghegh Ardabili University (Iran) is gratefully acknowledged for their financial support of this research with grant number 4191.