Effectiveness of the botanical insecticide azadirachtin against Tirathaba rufivena (Lepidoptera: Pyralidae).
Currently, the control of T. rufivena is still focused on chemical pesticides, which not only causes environmental pollution but also affects human health. The tetranortriterpenoid azadirachtin is the most active insecticide component found in neem seeds and leaves (Butterworth & Morgan 1968), and has generated a great deal of interest because of its bioefficacy and biodegradability. Azadirachtin has a number of biological properties, including repellency, feeding and oviposition deterrence, growth-disrupting activity, and low mammalian toxicity (Schmutterer 1990). Most research has examined the effects of azadirachtin on insect development, including weight reduction (Schluter 1985; Isman 1993) and mortality (Rembold et al. 1982; Meisner et al. 1986; Zehnder & Warthen 1988; Wilps et al. 1992; Padmanaban et al. 1997; Raguraman & Singh 1999; Raman et al. 2000). However, there have been few studies on the effects of azadirachtin on T. rufivena. Here, we present research on the effects of azadirachtin on the development and mortality of T. rufivena.
Materials and Methods
A stock solution of 95% azadirachtin (Sigma-Aldrich.Corp, St. Louis, Missouri) was used for the bioassays. The insecticides were diluted with acetone (Guangzhou Chemical Reagent Factory, Guangzhou, China) to the desired concentrations of active ingredient (AI) (120, 60, 30, 15, and 7.5 mg AI/L).
Tirathaba rufivena larvae were collected from an areca field without any history of pesticide spraying, and were fed with areca leaves under controlled conditions (25 [+ or -] 1 [degrees]C, 70 [+ or -] 5% RH, and a 11:13 h L:D photoperiod) so that all development stages were available when necessary.
STOMACH TOXICITY OF AZADIRACHTIN TO LARVAE
Fresh areca leaves were immersed for 10 s in azadirachtin solution at the desired concentration, and the leaves were removed and placed under a chemical hood to dry for 2 h. Different instars of T. rufivena were selected and distributed to rearing containers (clean transparent plastic boxes covered with gauze, 10 x 5 x 8 cm). There were 20 larvae per box, and 3 boxes were used for each concentration (120, 60, 30, 15, and 7.5 mg AI/L). To ensure that larval feeding was consistent, the larvae were starved for 24 h and then allowed to feed on the treated leaves for 24 h. Thereafter, they were removed and placed in new rearing boxes containing fresh untreated areca leaves. Leaves immersed in acetone were used as controls. The percentage of mortality was calculated after 48 h and corrected according to Abbott (1925). The slope, [LC.sub.50], and 95% confidence limits were calculated according to the methods used by Finney (1964).
CONTACT TOXICITY OF AZADIRACHTIN TO LARVAE
The inner walls of the rearing containers were coated with a solution of azadirachtin (60, 30, 15, 7.5, and 3.75 mg AI/L), and boxes coated with acetone were used as controls. After the evaporation of the solvent, 20 larvae of T. rufivena were introduced into the rearing box for 12 h, followed by the addition of areca leaves. Each treatment was repeated 3 times. After 48 h, the survival rate was monitored, and the [LC.sub.50] was calculated.
EFFECTS OF AZADIRACHTIN ON HATCHING AND NEONATE LARVAE
Tirathaba rufivena eggs were treated 1, 2, or 3 d after deposition with an [LC.sub.25] (11.35 mg AI/L), [LC.sub.50] (28.79 mg AI/L), or [LC.sub.90] (169.00 mg AI/L) of azadirachtin solution based on stomach toxicity to 1st instars. Areca leaves with 20 eggs were dipped into the solution for 10 s and then removed and placed under a chemical hood to dry for 2 h. For each treatment, 3 replicates were conducted, and all replications were performed at the same time. The mortality was recorded until no additional hatch occurred. To detect the residual effect of azadirachtin on newly hatched larvae, the survival of neonate larvae was observed until the 1st stadium was completed.
EFFECTS ON DEVELOPMENT AND ADULT EMERGENCE
To assess the effects of azadirachtin on the development of T. rufivena, areca leaves were immersed in azadirachtin solution of [LC.sub.25], [LC.sub.50], and [LC.sub.90] (based on stomach toxicity, as noted for egg treatment) for 10 s, removed, and then placed under a chemical hood to dry for 2 h. Thirty 3-d-old larvae (2nd instar) were fed treated leaves for 24 h and were then kept individually in a separate rearing box and reared on untreated leaves. The development time of 20 surviving larvae that were treated with [LC.sub.25], [LC.sub.50], or [LC.sub.90] solution was recorded and averaged. Twenty larvae reared on leaves treated with acetone were used as controls.
The percentage of adult emergence in each treatment also was determined. The longevity and egg production of moths were determined with 10 pairs of moths used for each concentration, and then the hatching rate of eggs was recorded. The adults were fed honey water as supplemental nutrition.
For the stomach and contact toxicity of azadirachtin among T. rufivena larvae, Abbott's formula (Abbott 1925) was applied to correct the percentage of mortality if the control mortality was between 5 and 20%. Probit regression in the Statistical Package for Social Science (SPSS) Version 12 (http://www.stathome.cn/spss/) was used to determine the LC-P line (log concentration--probability regression line), the lethal concentration values, and the corresponding 95% fiducial limits of the upper and lower confidence limits. A significant difference was determined by the non-overlapping of the 95% confidence limit.
The effects of azadirachtin on egg hatch and development of T. rufivena were recorded as the mean + SE. The results were analyzed with ANOVA and significant differences determined by the Duncan new multiple range test in the Statistical Analysis System (SAS[R]) software version 8.1 (Hu 2010).
STOMACH TOXICITY OF AZADIRACHTIN TO LARVAE
Based on the [LC.sub.50] values, the 1st instars were 1.53, 2.01, 3.01, and 3.22 times more susceptible than the 2nd, 3rd, 4th, and 5th instars, respectively. A statistically significant difference between the [LC.sub.50] values of 1st instars and the other instars was obtained as a result of the non-overlapping of the 95% confidence limits. Similar differences were found between some other instars, as shown in Table 1, although the 95% confidence limits overlapped among some instars, indicating no significant differences.
CONTACT TOXICITY OF AZADIRACHTIN TO LARVAE
The contact toxicity of azadirachtin to T. rufivena larvae is shown in Table 2. No significant difference was evident in the toxicity of azadirachtin to 1st, 2nd, and 3rd instars based on the 95% confidence limits of [LC.sub.50]. However, some significant differences occurred (Table 2) when comparing early instars to late instars.
EFFECTS OF AZADIRACHTIN ON HATCHING AND NEONATE LARVAE
As shown in Table 3, [LC.sub.25] and [LC.sub.50] dosages of azadirachtin had no effect on the percentage of hatch from eggs, whereas significant differences were obtained when using the [LC.sub.90] dosage at 1 d (F = 32.39; df = 3; P < 0.001), 2 d (F = 15.16; df = 3; P < 0.001), or 3 d (F = 31.85; df = 3; P < 0.001) after oviposition. The greatest reduction in hatch (52.6 [+ or -] 3.91%) was obtained with treated 3-d-old eggs.
The percentage of survival of neonate larvae was inversely correlated with the concentration of azadirachtin and the age of the eggs (Table 3). Statistical analysis indicated that all tested concentrations affected the hatch of neonate larvae (except the [LC.sub.25] and [LC.sub.50] on 1-d-old eggs), particularly larvae that emerged from the treated 3-d-old eggs (F = 34.30; df = 3; P < 0.001). Additionally, the proportion of larvae surviving from treated 3-d-old eggs was only 29.3% compared with 92.6% in controls.
EFFECTS OF AZADIRACHTIN ON DEVELOPMENT AND ADULT EMERGENCE
Azadirachtin may significantly prolong larval development (F = 91.45; df = 3; P < 0.001) and pupal duration (F = 30.57; df = 3; P < 0.001) (Table 4). The duration of 2nd instars was 2.23 d in the control group. The development of 2nd instars fed leaves treated with an [LC.sub.25], [LC.sub.50], or [LC.sub.90] of azadirachtin was prolonged by 8.5, 11.2, and 18.4%, respectively. Similar results were obtained for 3rd, 4th, and 5th instars. Total larval development time was prolonged by 8.2, 10.2, and 13.9% after treatment with [LC.sub.25], [LC.sub.50], or [LC.sub.90] of azadirachtin, respectively.
The percentage of emerging moths decreased from 97.8% in the control to 75.6, 50.2, and 26.7% after 2nd instars were exposed for 1 d to [LC.sub.25], [LC.sub.25], and [LC.sub.90] azadirachtin treatments, and the percentage of decrease in emergence was 22.7, 48.7, and 72.7%, respectively (Table 5). Statistical analysis showed differences among the controls and different treatment concentrations (F = 69.57; df = 3; P < 0.001).
The longevity (Table 5) of the emerged adults was shortened (F = 21.98; df = 3; P < 0.001) by azadirachtin as compared with the mean longevity of the controls (11.2 d), but the azadirachtin dosages produced equivalent effects.
Egg production by T. rufivena was also reduced (F = 6.80; df = 3; P < 0.001) after treatment with azadirachtin, although there were no detectable differences among the azadirachtin treatments (Table 5). Hatch from eggs of the emerged adults was similarly affected (F = 48.71; df = 3; P<0.001)
The use of plant-based insecticides has been recommended as an alternative for plant protection with minimal negative risks (Isman 2006; Pavela 2007). Botanical insecticides have long been a subject of research in an effort to develop alternatives to conventional insecticides. Currently, several insecticides based on various plant extracts are used around the world. Azadirachtin is the insecticidal ingredient found in the neem tree and is a naturally occurring substance that belongs to an organic molecule class called tetranortriterpenoids. Azadirachtin is used to control whiteflies, aphids, thrips, fungus gnats, lepidopteran larvae, beetles, mushroom flies, mealybugs, leafminers, gypsy moths, and other insects in food, greenhouse crops, ornamental plants, and turf (Thomson 1992). Our results indicated that azadirachtin had a strong stomach and contact toxicity to T. rufivena larvae, and that the contact toxicity was greater than the stomach toxicity.
In this study, azadirachtin affected larval hatch, larval development, pupal duration, adult longevity, and egg production in T. rufivena. Azadirachtin produced a significant reduction in the percentage of hatch when it was applied directly to the eggs 1, 2, or 3 d after they had been deposited. Survival of neonate larvae that had hatched from treated eggs diminished, especially when eggs had been treated with a high concentration just before hatch. The ovicidal activity of some plant extracts on other insects such as Spilosoma obliqua (Walker) (Lepidoptera: Arctiidae), Spodoptera litura F. (Lepidoptera: Noctuidae), and Dysdercus koenigii (F.) (Hemiptera: Pyrrhocoridae) was reported by Ghatak & Bhusan (1995) and Suryakala et al. (1995). They suggested that high concentration levels of many plant extracts may inhibit the hatching from insect eggs. Our results confirmed that azadirachtin was toxic to eggs and also affected the neonate larvae from treated eggs.
Azadirachtin isstructurally similartothe insect ecdysone hormones, which control the process of metamorphosis as the insects pass from larva to pupa to adult. Metamorphosis requires the careful synchrony of many hormones and other physiological changes to be successful, and azadirachtin seems to be an ecdysone blocker. It blocks the production and release of these vital hormones in insects, and when they are exposed to azadirachtin, insects will not molt, which breaks their life cycle (National Research Council 1992; AgriDyne Technologies, Inc. 1994). The results of this study showed that there was a significant reduction in the development of T. rufivena among 2nd instar larvae that survived azadirachtin treatment. The longevity of moths that grew from treated larvae was significantly shorter compared with untreated moths. Additionally, there was a reduction in egg production among females, and hatch from deposited eggs decreased. These findings suggest that toxicity may persist through all life stages from larva to adult, although only 2nd instar larvae were treated with azadirachtin. Therefore, it appears that azadirachtin could effectively suppress T. rufivena populations either directly through acute toxic effects on the larvae or indirectly through delayed effects on development.
We gratefully acknowledge grants from the Key Research and Development Project of Hainan Province, China (Grant No. ZDYF2016059), the Major Planned Science & Technology Project of Hainan Province, China (Grant No. ZDXM 20120029), and the project for the Special Foundation for Scientific Research in the Public (Agricultural) Industry of China (Grant No. 200903026).
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Baozhu Zhong (1), Chaojun Lv (1,*), and Weiquan Qui (1,*)
(1) Chinese Academy of Tropical Agricultural Sciences, Coconut Research Institute, Wenchang, Hainan 571339, China; E-mail: email@example.com (B. Z.), Icj5783@126.com (C. L), firstname.lastname@example.org (W. Q.)
(*) Corresponding authors; E-mail: Icj5783@126.com (C. L.), email@example.com (W. Q.)
Please Note: Illustration(s) are not available due to copyright restrictions.
Table 1. Stomach toxicity of azadirachtin to Tirathaba rufivena larvae. [LC.sub.50] 95% confidence limits Instar LC-P line (a) (mg/L) (lower-upper) First y = 2.57 + 1.67x 28.79 24.37-34.00 Second y = 2.24 + 1.68x 44.19 36.89-52.94 Third y = 2.27 + 1.55x 57.75 46.31-72.03 Fourth y = 2.01 + 1.54x 86.73 65.13-115.49 Fifth y = 1.61 + 1.72x 92.70 70.34-122.16 (a) Log concentration-probability regression line. Table 2. Contact toxicity of azadirachtin to Tirathaba rufivena larvae. 95% confidence limits Instar LC-P line (a) (mg/L) (lower-upper) First y = 3.40 + 1.44x 12.85 10.62-15.55 Second y = 3.51 + 1.35x 12.82 10.47-15.71 Third y = 3.58 + 1.15x 17.07 13.54-21.51 Fourth y = 3.54 + 1.09x 21.97 16.91-28.55 Fifth y = 3.17 + 1.21x 32.34 24.19-43.24 (a) Log concentration-probability regression line. Table 3. Effects of azadirachtin on hatch from eggs treated at different ages and on survival of hatched larvae through instar 1 of Tirathaba rufivena. Egg age (d) Treatment Hatch (%) Larval survival (%) 1 [LC.sub.25] 88.5 [+ or -] 1.75a 85.3 [+ or -] 1.51a [LC.sub.50] 88.3 [+ or -] 1.67a 83.0 [+ or -] 3.22a [LC.sub.90] 62.6 [+ or -] 3.73b 54.3 [+ or -] 4.06c Control 91.7 [+ or -] 1.67a 92.8 [+ or -] 3.61a 2 [LC.sub.25] 85.3 [+ or -] 2.62a 77.0 [+ or -] 2.94b [LC.sub.50] 85.0 [+ or -] 2.89a 70.8 [+ or -] 2.41b [LC.sub.90] 61.7 [+ or -] 4.41b 52.2 [+ or -] 8.06c Control 90.0 [+ or -] 2.89a 94.4 [+ or -] 0.18a 3 [LC.sub.25] 82.0 [+ or -] 1.53a 64.2 [+ or -] 6.37b [LC.sub.50] 88.3 [+ or -] 4.41a 47.4 [+ or -] 2.63c [LC.sub.90] 52.6 [+ or -] 3.91b 29.3 [+ or -] 5.62d Control 88.3 [+ or -] 1.67a 92.6 [+ or -] 3.70a Means ([+ or -] SE) in the same column followed by the same letter are not significantly different at the probability level of 0.05 determined by the Duncan new multiple range test. Table 4. Effects of azadirachtin on Tirathaba rufivena larval and pupal development. Development time (d; mean [+ or -] SE) Treatment First instar Second instar Third instar Fourth instar [LC.sub.25] 2.09 [+ or -] 2.42 [+ or -] 3.00 [+ or -] 3.41 [+ or -] 0.03a 0.02b 0.04b 0.04b [LC.sub.50] 2.11 [+ or -] 2.48 [+ or -] 3.12 [+ or -] 3.51[+ or -] 0.03a 0.02b 0.03a 0.03ab [LC.sub.90] 2.12 [+ or -] 2.64 [+ or -] 3.15 [+ or -] 3.61 [+ or -] 0.02a 0.02a 0.02a 0.02a Control 2.05 [+ or -] 2.23 [+ or -] 2.75 [+ or -] 3.08 [+ or -] 0.04a 0.04c 0.04c 0.10c Development time (d; mean [+ or -] SE) Treatment Fifth instar Total larval Pupal [LC.sub.25] 4.52 [+ or -] 15.44 [+ or -] 10.70 [+ or -] 0.05c 0.07b 0.02b [LC.sub.50] 4.61 [+ or -] 15.81 [+ or -] 10.71 [+ or -] 0.03b 0.06b 0.02b [LC.sub.90] 4.72 [+ or -] 16.25 [+ or -] 10.80 [+ or -] 0.03a 0.05a 0.01a Control 4.15 [+ or -] 14.27 [+ or -] 10.39 [+ or -] 0.06d 0.15c 0.06c Development time (d; mean [+ or -] SE) Treatment Total larval-pupal [LC.sub.25] 26.14 [+ or -] 0.07c [LC.sub.50] 26.53 [+ or -] 0.06b [LC.sub.90] 27.05 [+ or -] 0.05a Control 24.66 [+ or -] 0.16d Means ([+ or -] SE) in the same column followed by the same letter are not significantly different at the probability level of 0.05 determined by the Duncan new multiple range test. Table 5. Effects of azadirachtin on Tirathaba rufivena adult biology. Emergence Longevity No. of eggs Treatment (%) (d) per female [LC.sub.25] 75.6 [+ or -] 8.65 [+ or -] 71.90 [+ or -] 2.22b 0.30b 2.18b [LC.sub.50] 50.2 [+ or -] 9.00 [+ or -] 66.80 [+ or -] 5.20c 0.24b 2.98b [LC.sub.90] 26.7 [+ or -] 8.30 [+ or -] 67.60 [+ or -] 3.85d 0.25b 3.28b Control 97.8 [+ or -] 11.25 [+ or -] 83.90 [+ or -] 2.22a 0.33a 3.50a Hatch Treatment (%) [LC.sub.25] 70.0 [+ or -] 1.38b [LC.sub.50] 71.8 [+ or -] 1.40b [LC.sub.90] 72.4 [+ or -] 2.13b Control 92.4 [+ or -] 0.86a Means ([+ or -] SE) in the same column followed by the same letter are not significantly different at the probability level of 0.05 determined by the Duncan new multiple range test.
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|Author:||Zhong, Baozhu; Lv, Chaojun; Qui, Weiquan|
|Date:||Jun 1, 2017|
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