Spinosad resistance, esterase isoenzymes and temporal synergism in Frankliniella occidentalis (Pergande) in Australia.

Spinosad has been widely used in Australia to control western flower thrips Frankliniella occidentalis (Pergande) but spinosad usefulness is now compromised by resistance. Here we studied a highly spinosad resistant strain of F. occidentalis to explore if esterases had a role in spinosad resistance. Enhanced esterase activity in pressured spinosad-resistant F. occidentalis was confirmed via PAGE electrophoresis and estimated to be approximately three times higher than that in a susceptible strain. Spinosad-esterase inhibition data in the resistant strain, showed a concentration effect with significant esterase-spinosad binding occurring at spinosad concentrations from 6.2× 10(-7) to 1.5× 10(-5) M. Similarly, a spinosad-piperonyl butoxide (PBO) inhibition curve showed a concentration effect, with significant esterase-PBO binding occurring in the resistant strain at PBO concentrations between 3.3× 10(-5) M and 8.4× 10(-4) M. No binding of esterase to spinosad or PBO occurred in the susceptible strain. Results of bioassays in which spinosad resistant F. occidentalis were sprayed with a 4h delayed release formulation of cyclodextrin-complexed spinosad with immediately available PBO demonstrated that spinosad resistance was significantly reduced from 577 to 72-fold. With further development the PBO synergism of spinosad using a delayed release formulation, similar to that used here, may provide effective control for spinosad resistant F. occidentalis. Temporal synergism of spinosad may prove to be effective tactic for the control of spinosad resistant F. occidentalis where the main resistance mechanism involved has been confirmed to be esterase based.

The effects of thermal acclimation on lethal temperatures and critical thermal limits in the green vegetable bug, Nezara viridula (L.) (Hemiptera: Pentatomidae).

According to geographical distribution, Nezara viridula (Heteroptera: Pentatomidae) can be found across tropical, subtropical, and temperate regions and this pattern is assumed to reflect differences in thermal adaptation, particularly in cold tolerance. Here the lethal temperature (LT) and critical thermal limits (CTL) (thermal tolerance) are examined for N. viridula. The upper LT for N. viridula at two contrasting climate locations (Breeza and Grafton, New South Wales, Australia) was 40.3°C with 20% survival under the stress of high temperature. The lower LT did not differ between these two populations and was -8.0°C with 20% survival under low temperature stress. Survival of N. viridula increased after acclimation at high temperature for 7 days. In contrast, when acclimated at lower temperatures (10 and 15°C), survival of Breeza and Grafton N. viridula was lower than 20% at -8.0°C. Control-reared N. viridula adults (25°C) had a mean CT(MinOnset) (cold stupor) of 1.3 ± 2.1°C and a mean CT(Max) (heat coma) of 45.9 ± 0.9°C. After 7 days of acclimation at 10, 20, 30, or 35°C, N. viridula adults exhibited a 1°C change in CT(Max) and a ~1.5°C change in CT(MinOnset). CT(Max) and CT(MinOnset) of Breeza and Grafton N. viridula populations did not differ across acclimation temperatures. These results suggest that short-term temperature acclimation is more important than provenance for determining LTs and CTL in N. viridula.

Investigating the potential of selected natural compounds to increase the potency of pyrethrum against houseflies Musca domestica (Diptera: Muscidae).

BACKGROUND: A study was undertaken to determine the efficacy of seven natural compounds compared with piperonyl butoxide (PBO) in synergising pyrethrum, with the intention of formulating an effective natural synergist with pyrethrum for use in the organic crop market. RESULTS: Discriminating dose bioassays showed PBO to be significantly more effective at synergising pyrethrum in houseflies than the seven natural compounds tested, causing 100% mortality in insecticide-susceptible WHO and resistant 381zb strains of housefly. The most effective natural synergists against WHO houseflies were dillapiole oil, grapefruit oil and parsley seed oil, with 59, 50 and 41% mortality respectively, compared with 18% mortality with unsynergised pyrethrum. Against 381zb houseflies, the most effective natural synergists were parsley seed oil and dillapiole oil. Esterase inhibition by the natural compounds and PBO in vitro showed no correlation with pyrethrum synergism in vivo, whereas the inhibition of oxidases in vitro more closely correlated with pyrethrum synergism in vivo. CONCLUSION: Dillapiole oil and parsley seed oil showed the greatest potential as pyrethrum synergists. PBO remained the most effective synergist, possibly owing to its surfactant properties, enhancing penetration of pyrethrins. The results suggest the involvement of oxidases in pyrethroid resistance in houseflies, with the efficacy of synergists showing a high correlation with inhibition of oxidases.

Temporal synergism can enhance carbamate and neonicotinoid insecticidal activity against resistant crop pests.

BACKGROUND: Piperonyl butoxide (PBO) effectively synergises synthetic pyrethroids, rendering even very resistant insect pests susceptible, provided a temporal element is included between exposure to synergist and insecticide. This concept is now applied to carbamates and neonicotinoids. RESULTS: A microencapsulated formulation of PBO and pirimicarb reduced the resistance factor in a clone of Myzus persicae (Sulzer) from >19 000- to 100-fold and in Aphis gossypii (Glover) from >48 000- to 30-fold. Similar results were obtained for a strain of Bemisia tabaci Gennadius resistant to imidacloprid and acetamiprid, although a second resistant strain did not exhibit such a dramatic reduction, presumably owing to the presence of target-site insensitivity and the absence of metabolic resistance. Synergism was also observed in laboratory susceptible insects, suggesting that, even when detoxification is not enhanced, there is degradation of insecticides by the background enzymes. Use of an analogue of PBO, which inhibits esterases but has reduced potency against microsomal oxidases, suggests that acetamiprid resistance in whiteflies is largely oxidase based. CONCLUSION: Temporal synergism can effectively enhance the activity of carbamates and neonicotinoids against resistant insect pests. Although the extent of this enhancement is dependent upon the resistance mechanisms present, inhibition of background enzymes can confer increased sensitivity against target-site resistance as well as increased metabolism. .

Use of pyrethroid analogues to identify key structural features for enhanced esterase resistance in Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae).

It has been reported previously that the major resistance mechanism to pyrethroid insecticides by the cotton bollworm Helicoverpa armigera (Hübner) in Australia is a consequence of overproduction of esterase isoenzymes. This paper reports structure-activity relationships that support such a view, based on in vivo bioassays conducted with a range of pyrethroid structures containing a variety of acid and alcohol moieties and the correlation with in vitro esterase inhibition assays against the same structures, and identifies the critical regions of the molecule with regard to esterase inhibition, and hence resistance. The implications of this work in terms of possible resistance management are evaluated and discussed.

Temporal synergism by microencapsulation of piperonyl butoxide and alpha-cypermethrin overcomes insecticide resistance in crop pests.

A microencapsulated formulation that gives a burst release of piperonyl butoxide (PBO) several hours before a burst release of a conventional pyrethroid can effectively overcome metabolic resistance in Bemisia tabaci Gennadius, Helicoverpa armigera (Hübner), Aphis gossypii Glover and Myzus persicae Sulzer. This increase in efficacy against resistant pests was reflected in a field trial against B. tabaci on cotton, eliminating the need for two treatments. The ratio between the active insecticide and the synergist was found to be crucial in reducing resistance factors.

Inhibition of carbamate-insensitive acetylcholinesterase by piperonyl butoxide in Helicoverpa armigera.

The cotton bollworm, Helicoverpa armigera, is a cosmopolitan, insecticide-resistant insect pest of food and fiber (Gunning et al., 1992). Acetylcholinesterase (AChE) is the insect target site for carbamate insecticides, and H. armigera has developed an insensitive form of AChE as a resistance mechanism (Gunning et al., 1996). Insensitive AChE is normally considered an intractable resistance mechanism in pests. The methenedioxphenyl compound, piperonyl butoxide (PBO), has a long history as an insecticide synergist in the control of resistant arthropod pests; it is known to inhibit mono-oxygenases and nonspecific esterases (Gunning et al., 1998). This work discusses PBO inhibition of AChE in H. armigera and explores synergism.

Buprofezin inhibits acetylcholinesterase activity in B-biotype Bemisia tabaci.

B-biotype Bemisia tabaci is a severe insect pest worldwide in many ornamental, agricultural, and horticultural industries. Control of this insect is hampered by resistance to many acetylcholinesterase (AChE)-inhibiting insecticides, such as organophosphates and carbamates. Consequently, insect growth regulators such as buprofezin, which act by inhibiting chitin synthesis, are being investigated for use against B-biotype B. tabaci in Australia. This study discusses the effects of buprofezin on B. tabaciAChE.

Effect of pretreatment with piperonyl butoxide on pyrethroid efficacy against insecticide-resistant Helicoverpa armigera (Lepidoptera: Noctuidae) and Bemisia tabaci (Sternorrhyncha: Aleyrodidae).

Pyrethroid resistance in B-type Bemisia tabaci Gennadius and Australian Helicoverpa armigera Hübner field populations is primarily conferred by esterase isoenzymes which metabolise and sequester pyrethroid insecticides. It has been shown previously that pyrethroid resistance-associated esterases in H. armigera are inhibited by the insecticide synergist piperonyl butoxide (PBO) over a 22-h period. It is demonstrated here that similar inhibition can be obtained against B-type B. tabaci. Small-scale field trials showed excellent levels of pyrethroid control when insects were pretreated with PBO and then dosed with pyrethroid during the time of maximum esterase inhibition. These results demonstrate that PBO can restore pyrethroid efficacy in the field against both B-type B. tabaci and resistant H. armigera.

New resistance mechanism in Helicoverpa armigera threatens transgenic crops expressing Bacillus thuringiensis Cry1Ac toxin.

In Australia, the cotton bollworm, Helicoverpa armigera, has a long history of resistance to conventional insecticides. Transgenic cotton (expressing the Bacillus thuringiensis toxin Cry1Ac) has been grown for H. armigera control since 1996. It is demonstrated here that a population of Australian H. armigera has developed resistance to Cry1Ac toxin (275-fold). Some 70% of resistant H. armigera larvae were able to survive on Cry1Ac transgenic cotton (Ingard) The resistance phenotype is inherited as an autosomal semidominant trait. Resistance was associated with elevated esterase levels, which cosegregated with resistance. In vitro studies employing surface plasmon resonance technology and other biochemical techniques demonstrated that resistant strain esterase could bind to Cry1Ac protoxin and activated toxin. In vivo studies showed that Cry1Ac-resistant larvae fed Cy1Ac transgenic cotton or Cry1Ac-treated artificial diet had lower esterase activity than non-Cry1Ac-fed larvae. A resistance mechanism in which esterase sequesters Cry1Ac is proposed.

The effect of piperonyl butoxide on pyrethroid-resistance-associated esterases in Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae).

Pyrethroid resistance in field populations of Australian Helicoverpa armigera (Hübner) is primarily a consequence of the overproduction of esterase isoenzymes which metabolise and sequester pyrethroid insecticides. Biochemical studies have shown that pyrethroid-resistance-associated esterases in H armigera are inhibited by the insecticide synergist piperonyl butoxide (PBO). Esterase inhibition by PBO did not occur immediately after dosing, but exhibited maximum inhibition 3-4 h after dosage. Esterase activity subsequently recovered until full activity was restored by 24 h. Topical bioassays using a pre-treatment of PBO showed that maximum H armigera mortality was achieved with pre-treatment times corresponding to maximum esterase inhibition. These results demonstrated that, with correct temporal application, PBO can restore pyrethroid efficacy against H armigera. It would also be expected that restoration of efficacy with other conventional insecticides, currently compromised by esterase-based resistance mechanisms, would occur.