Keywords

biological control, egg parasitoid, Hymenopthers, insecticides, natural enemies, Trichogrammatidae

Introduction

Egg parasitoids are known to be very effective against a number of crop pests. Parasitoids belonging to the genus Trichogramma have worldwide distribution and play an important role as natural enemies of the lepidopteron pests on a wide range of agricultural crops (Hassan and Abdelgader, 2001). During the last decades, Trichogramma wasps have been used as biological control agents for pest suppression in several countries (Abdelgader and Hassan, 2002). The use of reared Trichogramma sp. in the biological control has gained widespread interest in many countries (Hassan et al., 1998) Due to the importance of the Trichogramma sp. as natural enemies of several pests, the study of insecticide selectivity to them is of major importance for their role as biological control agents. The sp. Trichogramma chilonis is an important natural enemie and one of the important biological control agent used to control Lepidopteron pests in several countries (Consoli et al., 1998). As the use of insecticides is the common practice among tomato producers, the use of selective insecticides is an important strategy for the maintenance of the Trichogramma populations within the crops. However, there are few published studies about the effect of insecticides used to control the tomato pests on this parasitoid. Therefore the effect of insecticides used on this crop on the adult survival of laboratory and field populations of T. chilonis, using the standard tests recommended by IOBC (International Organization for Biological Control) were evaluated.

Material and methods

Field Collection Of T. Chilonis

About 100 eggs of Helicoverpa armigera were collected from tomato fields of Malur, Karnataka. Four to six collections were made during crop season. Eggs were brought to the laboratory and observed for parasitism, and subsequently adult emergence. Identity of emerged Trichogramma was determined by Division of Biosystematics, NBAII, Bangalore. During the survey insecticide usage pattern was also recorded.

Maintenance Of Lab And Field T. Chilonis

Stock populations of lab (obtained from Mass Production Unit of NBAII, Bangalore) and field collected T. chilonis was maintained on the eggs of the Corcyra cephalonica (Stainton) under the controlled conditions at 27 ± 1ºC, RH of 70 ± 10% with 14 h of photophase. The host eggs about 0.2 cc (0.2 = 2000 eggs) were glued on a white paper cards (7 X 2 cm) impregnated with 50% diluted gum with water and UV killed before being offered to parasitism @ 50 eggs / female parasitoid. Approximately after 5 days of the parasitism period, parasitized eggs (indicated by black shiny appearance of the egg chorion) were transferred to glass tubes (8.5 cm height X 2.5 cm dia.) and kept in the incubator until adult emergence at same controlled conditions previously described. Fine streaks of honey and water (1: 1) was provided as adult food .

Insecticides Selected For The Study

In the present study following eight commonly used insecticides were selected (Table 1) in order to document insecticide resistance in lab and field populations of T. chilonis.

Preparation Of Insecticidal Concentrations

Different dilutions of the selected insecticides such as abamectin (1.9% EC), chlorpyriphos (20% EC), cypermethrin (25% EC), indoxacarb (14% EC), malathion (50% EC), quinalphos (25% EC), spinosad (2.5% EC) and triazophos (40% EC) were prepared out of the respective formulated insecticides. The top dose was at respective field recommended concentration by preparing stock solution in 1 litre of water. ctive insecticides.

Insecticidal Bioassay Studies On Lab And Field Populations Of T. Chilonis

The commercial insecticides were diluted in distilled water at concentrations recommended for selected insecticides. Seven serial dilutions were prepared starting from field recommended dose for lab population and the field populations (Table 2). Diluted insecticides according to its respective concentrations were sprayed with an atomizer to obtain a uniform layer of spray droplets on the inside of the glass tubes (20 x 4 cm) with both ends open. After spraying the tubes were shade dried. One end of the sprayed and dried tube was closed with double layered long black cloth of (32 µm pore size). About 100 Trichogramma adults were allowed to move into sprayed tubes (labeled with different concentrations of insecticides) through the open end and after 15 mins other end of the tube was also closed with double layered long black muslin cloth. Adult morality/survival after 1 h, 2 h, 4 h, 6 h and 24 h release were recorded. For calculating LC50, mortality at 4 h was considered as differential response was noticed for different insecticides at 1 h and 2 h mortality pattern. Data from the replicates were pooled and dose-mortality regressions were computed by probit analysis using SPSS 18.0 (SAS Institute Inc, Cary, NC) to obtain LC50 and fiducial limits for each insecticide. Non-overlapping method of the fiducial limits was used to differentiate variation in insecticide resistance. Resistance factor (RF) was calculated based on LC50 of the resistant strain / LC50 of the laboratory strain

Results

Insecticide Bioassay On Laboratory Lab And Field (Tomato) Populations Of T. Chilonis

The results on the toxicity of the test insecticides of field and laboratory populations of T. chilonis on tomato are presented in Figure. 1 (abamectin, chlorpyriphos, cypermethrin and indoxacarb) and Figure. 2 (malathion, quinalphos, spinosad and triazophos) on laboratory populations and Figure. 3 (abamectin, chlorpyriphos, cypermethrin, indoxacarb, malathion, quinalphos, spinosad and triazophos) on field populations of T. chilonis (tomato), The statistical comparison of toxicity and resistant factor is shown in Table 2. The data revealed very minute variations in the responses of laboratory and field populations of T. chilonis to the insecticides applied. In field populations, the median lethal concentration (LC50) ranged from 1.03 to 271.13 ppm for the eight different insecticides, with the adults exhibiting the highest LC50 to quinalphos and the lowest to malathion (Table 2). In lab populations median lethal concentration LC50) ranged from 1.05 to 322.47 ppm for the eight different insecticides, with highest LC50 values obtained for quinalphos and the lowest for malathion (Table 2).

The LC50 of field populations to organophosphate compounds was found to be highest for quinalphos (271.23) followed by chlorpyriphos (12.93), triazophos (1.33) and least in malathion (1.03) and in laboratory strain the LC50 organophosphate compounds was found to be more in quinophos (322 .47) followed by chlorpyriphos (11.34), triazophos (1.29) and least was in malathion (1.05). The LC50 of indoxacarb was 176.05 and 288.09 in field and laboratory strain populations; similarly LC50 of abamectin was 1.62 and 1.72 in field and laboratory strain. The LC50 of pyrethroid (cypermethrin) was 1.69 and 1.30 in field and laboratory strain. Comparing the LC50 values of all eight insecticides of field and laboratory populations, it shows that field populations of T. chilonis have no resistance to the insecticides tested.

Table 1

Details of different insecticides used for the calculation of the LC50 for T. Chilonis

Sl. No Trade Name Formulation Chemical Name Group T. chilonis
Conc. Range*
S F Laboratory Field
1 Tagmec 1.9% EC Abamectin Avermectin 7 3 0.3-18.96 0.3-2.37
2 Lethal TC 20% EC Chlorpyriphos Organophosphate 7 3 6.25-400 6.25-25
3 Mokard 25% EC Cypermethrin Pyrethroid 7 3 0.625-40 0.625-2.5
4 King Doxa 14% EC Indoxacarb Oxadiazine 7 3 4.37-280 70-280
5 Malamal 50% EC Malathion Organophosphate 7 3 0.77-49.6 0.77-3.1
6 Quinalphos 25% EC Quinalphos Organophosphate 7 3 7.81-500 125-500
7 Success 2.5% EC Spinosad Spinosyn 7 3 0.152- 10 1.25-5
8 Hostathion 40% EC Triazophos Organophosphate 7 3 0.67-39.68 0.67-2.48

Figure 1: Toxicities of abamectin, chlorpyriphos, cypermethrin and indoxacarb on lab adult populations of T. Chilonis after 24 h of exposure

Figure 2:Toxicities of malathion, quinalphos, spinosad and triazophos on lab adult populations of T. Chilonis after 24 h of exposure

Figure 3: Toxicities of abamectin, chlorpyriphos, cypermethrin, indoxacarb, malathion, quinalphos, spinosad and triazophos on field adult populations of T. Chilonis from tomato crop after 24 h of exposure

Table 2

Statistical comparison of toxicity of eight commonly used insecticides on field (tomato) and laboratory populations of T. Chilonis after 24 h of exposure

Test (Population of adult T. chilonis) Insecticide LC50 (ppm) 95% FL of LC50 RF Slope ± SE Chi square
Lower Upper
Field (Tomato) Abamectin 1.62 0.92 0.0708 ± 0.1481 9.9
Chlorpyriphos 12.93 1.14 0.0914 ± 0.0164 8.77
Cypermethrin 1.69 1.31 0.6050 ± 0.1428 11.11
Indoxacarb 288.05 0.61 0.0050 ± 0.0012 13.5
Malathion 1.03 0.91 0.9386 ± 0.1567 20.55
Quinalphos 322.23 0.94 0.5020 ± 0.0006 22.08
Spinosad 2.75 0.96 0.4564 ± 0.0758 7.98
Triazophos 1.33 1.03 0.7521 ± 0.1584 14.13
Laboratory strain Abamectin 1.72 0.6454 ± 0.1442 10.92
Chlorpyriphos 11.34 0.0845 ± 0.0158 12.52
Cypermethrin 1.3 0.8 1.95 0.3719 ± 0.1414 3.9
Indoxacarb 176.09 0.0049 ± 0.0013 8.47
Malathion 1.05 0.7309 ± 0.1345 26.84
Quinalphos 271.47 0.0017 ± 0.0005 25.41
Spinosad 2.86 0.4522 ± 0.0825 4.44
Triazophos 1.29 0.7245 ± 0.1604 13.14
LC50 : Concentration of insecticide that killed 50% of the adult population in the observation period of 4 h. FL: Fiducial Limit. SE: Pooled binomial standard error.

k

Discusion

Insecticide Bioassay On Field (Tomato) And Laboratory Lab Populations Of Trichogramma Chilonis.

This study with avermectin (abamectin) and some organophosphates (chlorpyriphos, malathion, quinalphos, triazophos) and oxadiazine (indoxacarb), spinosyn (spinosad) as well as with pyrethroids (cypermethrin), clearly demonstrated that there was no such observable resistance found in field populations of T. chilonis when compared to that of laboratory lab populations. From the current results it can be seen that field and lab populations of adult T. chilonis are highly laboratory to the insecticides tested and this is due to compounds belonging to chemical groups of the lactones, carbamates, neonicotinoids and organophosphates and other chemicals are related in the scientific literature as highly harmful to parasitoids, important biological control agents of the arthropod pest (Croft, 1990; Yamamoto et al., 1995; Hassan et al., 1998a; Takahashi et al., 1998; Abdelgader and Hassan, 2002; Carvalho et al., 2003; Moura et al., 2004, 2005). High mortality was observed for the organophosphate insecticides even at the lowest concentrations. Among organophosphates malathion was found to be highly toxic to the adults followed by triazophos, chlorpyriphos and quinalphos . The high mortality of adults observed in the emergence tubes caused by these insecticides is not clear until this moment. A similar effect was related by Abdelgader and Hassan (2002). Similarly cypermethrin was found to be highly toxic to both field and lab adult population of T. chilonis. Findings of Consoli et al. (1998) and Hassan (1998b) also suggested that organophosphates and pyrethroids adversely affected the survivability of the adult populations of T. chilonis. Though abamectin had moderate effect on survivability of adult T. chilonis, complete mortality was observed after 24 h of exposure and this is in accordance with the studies conducted by Moura et al. (2006) for the effect of chlorpyriphos , abamectin on T. pretiosum. Findings of Smith et al. (1995), Consoli et al. (1998) and Hussain et al. (2010) also showed that abamectin had detrimental effect on adult survival of T. chilonis.

At the field recommended doses spinosad had moderate effect on the survivability of adult T. chilonis but at higher concentration 100 per cent mortality was seen after 24 h of exposure. These results are in agreement with William’s et al. (2003) who reported that spinosad residues degrade quickly in the field, with little residual toxicity to 25 species of hymenopteran parasitoids at 3–7 days after application. While on the other hand indoxacarb had very less effect on the survivability of adult T. chilonis even at higher insecticide dose. Indoxacarb has been tested having no or little effects on hymenopteran parasitoids (Aphidius, Cotesia, Bracon, Mocroplitis, and Trichogramma), spiders and predacious mites, and has no or moderate effects on lacewings and coccinellids (Studebaker and Kring 2003; Bostanian and Akalach 2004; Villanueva and Walgenbach 2005; Gradish et al., 2011; Biondi et al., 2012). Hewa-Kapuge et al. (2003) bioassayed seven insecticides for effects on T. brassicae, and found that indoxacarb was not toxic to T. brassicae in any assay when applied at field rates, and they consider indoxacarb as potentially suitable for inclusion in integrated pest management strategies because it does not influence adult survival or development of immature stages. When using indoxacarb against H. armigera on populations of T. pretiosum on sweet corn in Queensland, Australia, Scholz and Zalucki (2000) found that indoxacarb appeared to be very safe for T. pretiosum, with only minor mortality due to residues on leaves. Therefore, indoxacarb should be applied with caution if the sensitive natural enemies are dominant at the time of application. In contrast, Williams et al. (2003) reviewed the effects of spinosad on seven species of Trichogramma, and summarized that hymenopteran parasitoids are more laboratory to spinosad than predatory insects in laboratory and field studies. Nevertheless, spinosad has consistently been reported to be more harmful to parasitoids than indoxacarb (Nowack et al., 2001).

Earlier studies have reported that insect group Trichogrammatidae is highly susceptible to most of the insecticide groups and other toxic chemicals. However, there are few published studies about the effect of insecticides used to control tomato pests and its parasitoids. The results revealed high susceptibility of wasp to insecticides particularly adults, but demonstrated that some insecticides may be more compatible or suitable for conserving natural or released populations of Trichogramma wasps and also indicating that these insecticides could be used in conjunction with T. chilonis to control lepidopterous pests