Action of a novel nonsteroidal ecdysteroid mimic ... - Insects.ugent.be

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88 Guy Smagghe, Danny Degheele Fig. 1. Cross-section through the thorax of a last-instar S. exigua larva fed for 24 h on leaves previously dipped in an aqueous solution of 3 mg litre- tebufenozide. Arrows indicate the formation of a double cuticle. concentrations (S. exempta: 0.5-5 ng larva- ', 0.01-0.05 mg litre- '; S. exigua: 10-40 ng larva- ', 1-10 mg litre- '; S. littoralis: 5-30 ng larva-'; M. brassicae: 3-30 ng larva-'; G. mellonella: 100-750 ng larva- '), many lastinstar larvae that did not appear to be affected underwent an abnormal and lethal pupation. They were unable to ecdyse successfully or to produce a normal pupal cuticle, resulting frequently in larva-pupa intermediates. The lowest concentrations (S. exempta: 60.01 ng larva- ', 60.001 mg litre-'; S. exigua: 60.1 ng larva-', d0.05 mg litre- '; S. littoralis: 60.3 ng larva- I; M. brassicae: 60.3 ng larva- '; G. mellonella: 6 10 ng larva-'), had no effect on larval growth or development. When tebufenozide, diluted in DMSO, was topically applied at 100-200 pg per larva to last-instar larvae of S. exernpta and S. exigua, nc signs of neurotoxicity were noted. All larvae underwent premature head capsule apolysis and died in their old cuticle within 24 h of treatment. It was also found that third- to fifth-instar larvae of S. exempta and first- to fourth-instar larvae of S. exigua, fed tebufenozide at 3 0.02-0.05 and 3 1-2 mg litre- ', respectively, showed symptoms of a prematurely induced and lethal moult within 24 h of treatment. Likewise, food intake and weight gain ceased dramatically (P 6 0.05) as compared to controls. Other symptoms of toxicity included extrusion of the hindgut, loss of hemolymph, and no, or an incomplete, shedding of the old cuticle. Treated larvae died shortly afterwards. No visible effects were observed at doses of 60.001 and d0.05 mg litre-' for the various larval instars of S. exempta and S. exiyua, respectively. The effect on cumulative fecundity per female in S. exigua is presented in Fig. 3. Tebufenozide rapidly halted egg-laying in a dose-dependent manner. At 3 10 mg litre- ', oviposition was totally suppressed after one or two days of continuous treatment. S. exigua femaies treated with 1 mg litre- ' stopped egg-laying after three or four days of treatment. Dissection of femaies which had stopped oviposition suggested that the formation of new oocytes was inhibited. Moreover, the ovaries showed signs of degeneration and were therefore frail. In the adults treated with 3 10 mg litre ~ ', death appeared 4( k 1) days after treatment, vs. 9( f 1) days in the control. In spite of a disastrous effect on fecundity, all egs of treated S. exigua adults were equally viable (P > 0.05). Mean percentages of egg hatching were 81 ( f 6), 75( f S), 79( f 8) and 76( k 8)% following application of 0, 1, 10 and 100 mg litre-', respectively. 3.2 Toxicity and effects of tebufenozide in Coleoptera Growth and ecdysis were not affected in L. decemlineata larvae of all different instars (L,-L,) treated orally with concentrations up to 50 mg litre-'. No significant differences (P > 0.05) between weight gain of control and
Tebufenozide in different orders of insects 89 300 -m- Control 250 0.005 = h f '' 200 c 0 - 150 m 0 15 2 I c 100 I 50 4 ----.__ 0 24 48 12 96 120 144 Time (h into last-larval instar) 200 -c Control S 150 -A- E Y E 0 1 I00 - E m c I 50 250 200 B v 150 'E - m - ii 100 c I 50 0 0 24 4a 12 96 Time (h into last-larval instar) -0- Contra\ ..+. 50 0 24 4a 72 96 120 144 Time (h into last-larval instar) Fig. 2. Effect of various concentrations of tebufenozide (mg litre-') on weight gain of last-instar larvae of (A) S. exernpta, (B) S. exigua and (C) L. decernlineata. Data represent the means of three to four groups of 10 larvae. At various intervals, * and O indicate a significant difference (P < 0.05) from the untreated control.29 I 700 600 $500 c c 2 400 P) > - p 300 5 i 200 100 0 1 2 3 4 5 6 7 Time (days after first oviposition) Fig. 3. Effect of various concentrations of tebufenozide (mg litre-l) on the cumulative fecundity per female of S. exigua adults by continuous feeding on treated honey-water. The symbol * indicates that all adults were dead. treated larvae (< 50 mg litre-I) were noted, as is shown in Fig. 2C for the last-instar larvae. However, although no toxicity was noted at concentrations up to 50 mg litre- ', tremors and paralysis were induced at 100 mg litre-' within 1 h of treatment in some cases. Tebufenozide possessed no significant activity against second- and last-instar larvae of D. virgifera virgifera, since larval growth and development were unaffected when freshly cut maize seedlings previously dipped in an aqueous solution of 50 or lOOmglitre-' of the compound were continuously supplied. On the other hand, tebufenozide inhibited egg-laying within a short time of uptake in L. decemlineata. In six-day ovipositing females, oviposition was significantly decreased (P < 0.05) within two days of continuous feeding with potato leaves treated with 100 mg litre- ' (Fig. 4). Males and females also stopped feeding and movement, further copulation was not observed and the insects died shortly afterwards. On dissecting such females, there was an absence of developing oocytes. At 30 mg litre- ', the cumulative number of eggs was only significantly (P < 0.05) reduced relative to controls after about 10 days of treatment. Up to this time, the average fecundity of treated insects was lower than that of the control insects, although this difference was not significant at P = 0.05 level. At the end of the test, 243( f26) eggs per female were deposited by these adults vs. 297( f 24) in the control (Fig. 4). In adults topically treated with 20 pg per adult, oviposition was suppressed within two or three days of treatment, resulting in 103(+9), 133( 1 1) and 154( _+ 13) eggs per female beetle after 2,4 and 15 days of treatment vs. 143(+12), 174(f13) and 297( f24) in the control (Fig. 4). On the other hand, egg viability was similar (P < 0.05) in all treatments. Average hatching percentages reached 73( f 8), 70( f 8) and 69( i 7)% for the groups fed continuously with leaves dipped in 0, 30 and 100mglitre-', respectively, and
Page 1 and 2: Pestic. Sci. 1994, 42, 85-92 Action
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Action of a novel nonsteroidal ecdysteroid mimic ... - Insects.ugent.be

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