Abundance of Helicoverpa (Lepidoptera: Noctuidae) pupae under ...

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Abundance of Helicoverpa (Lepidoptera: Noctuidae) pupae under cotton and other crops in central Queensland: Implications for resistance management.
Australian Journal of Entomology (2001) 40, 264–269

Abundance of Helicoverpa (Lepidoptera: Noctuidae) pupae under cotton and other crops in central Queensland: Implications for resistance management Richard V Sequeira* and Christina L Playford Farming Systems Institute, Agency for Food and Fibre Sciences, Queensland Department of Primary Industries, Locked Bag 6, Emerald, Qld 4720, Australia.

Abstract

Planting of refuge crops and post-harvest cultivation of soil in winter are key elements of resistance management strategies (RMS) for Bt-transgenic (Ingard♦) and conventional cottons in southern Queensland and New South Wales. As part of a larger project to examine the feasibility of growing Ingard♦ in central Queensland (CQ), field assessments were conducted during the 1996–97 growing season to examine the adequacy of the southern Bt-cotton RMS under local environmental conditions. The suitability of cotton and a number of other field crops as refuges for Bt-cotton was assessed in terms or their relative Helicoverpa pupal productivity. The practicality and potential effectiveness of postharvest cultivation under CQ conditions were also assessed. Field assessments show that pigeon pea has the greatest potential as a refuge for Bt-cotton. Unsprayed cotton, sorghum and maize also produced substantially high pupal densities and hence are suitable refuge options, but they will require larger areas to be planted relative to pigeon pea. Post-harvest cultivation in cotton fields is largely ineffective for resistance management under CQ conditions. A Bt-cotton RMS for CQ is proposed. The CQ strategy includes refuge crop options contained in the southern strategy and the use of late season trap-crops of pigeon pea as an alternative to post-harvest cultivation.

Key words

post-harvest cultivation, refuge, resistance management, transgenic cotton, trap-crop.

INTRODUCTION Larvae of Helicoverpa armigera (Hübner) and H. punctigera (Wallengren) are responsible for significant economic losses in cotton (Gossypium hirsutum L.) and a wide variety of other field crops in Australia (Wardhaugh et al. 1980; Zalucki et al. 1986). Management and control of these species has traditionally been based on application of insecticides (Fitt 1994). The development of resistance in H. armigera to most commonly used insecticides and increasing field control failures (Forrester et al. 1993; Gunning et al. 1996) have highlighted the need to develop alternative, strategic integrated pest management (IPM) programs (Jallow et al. 1999). Within the Australian cotton industry few events have had a greater impact on insect pest management than the commercialisation of transgenic cottons (Ingard♦) expressing the CryIAc endotoxin gene from the bacterium Bacillus thuringiensis var. kurstaki (Bt) in the mid-90s (Fitt et al. 1994). Bt-cotton has paved the way for development of more robust IPM systems against key pests such as Helicoverpa spp. by facilitating the integration of its built-in protection with a wide range of non-chemical pest control techniques (Fitt et al. 1994; Roush 1997). One major concern, however, *Author to whom correspondence should be addressed (email: [email protected]).

for the new technology is that the key target pest, Helicoverpa, will eventually develop resistance to the bacterial toxins through constant selection pressure during the life of the crop over a number of cropping cycles (McGaughey & Whalon 1992; Forrester 1994; Roush 1997). Before Bt-cotton can be introduced into a new area or cropping system, performance indicators, potential impact on the environment, and risk associated with the product in the new area or system need to be thoroughly understood. A regulatory requirement for growing Bt-cotton is the implementation of a comprehensive Helicoverpa resistance management strategy (RMS). The RMS for Bt-cotton employed in southern Queensland and New South Wales has two major components (Fitt 1996). The first is a pre-emptive tactic requiring growers to plant and maintain a prescribed area of a refuge crop in close proximity to the Bt-cotton crop. The second component involves post-harvest cultivation of Btcotton fields during winter for control of Helicoverpa pupae to potentially eliminate resistant individuals. During the 1996–1997 growing season, field research trials were initiated in the Emerald and Dawson-Callide Valley irrigation areas as precursors to the introduction of commercial Bt-cotton into the Central Queensland (CQ) region. Specific research outcomes included the assessment of Helicoverpa pupae production under transgenic and conventional cottons, and other locally grown field crops. The objectives of the assessment were to identify potentially

Helicoverpa pupae production suitable Helicoverpa refuge options, to examine the potential effectiveness of post-harvest cultivation at cotton maturation under CQ conditions, and to identify other IPM options specifically suited to the development of a locally adapted RMS if necessary. Here, we present the results of the field assessment and discuss their implications for resistance management in transgenic and conventional cotton.

MATERIALS AND METHODS The assessment was made on five irrigated plots, each located on a commercial cotton farm in the irrigation area. Each plot contained a number of ‘treatments’ that varied in size depending on the cultivated area that farm owners/ managers were willing to set aside for trial purposes (Table 1). The treatments included: (1) Bt-cotton cv. CS50I; (2) unsprayed conventional cotton cv. CS50 (control); (3) sprayed conventional cotton cv. CS50; (4a) Adzuki bean (Vigna angularis (Willd.) Ohwi & Ohashi); (4b) Sorghum (Sorghum bicolor (L.)); (4c) Maize (Zea mays L.); (4d) Pigeon pea (Cajanus cajan L.); and (4e) Sunflower (Helianthus annuus L.). Locally grown commercial varieties of treatments 4a,b,c,e were used in the trial. Pigeon pea (4d) plots were planted to cv. Quest, a short-season determinate cultivar. Commercial planting rates recommended for CQ (QDPI 1996) were used for all crops. Treatments (1) and (3)

Table 1 Farm/Plot

were sprayed with insecticides for pest management whenever required, whereas (2) and (4a–e) were grown without insect control of any kind. All treatments were planted on 1-m row spacing. Soil sampling was carried out at regular intervals throughout the season to determine the abundance of Helicoverpa pupae under each treatment. At each sampling pupae were collected from 20 randomly selected 1-m2 areas of each treatment. The pupae were placed in individual 10-mL disposable lidded cups and maintained at room temperature (24–25°C) until moth or parasite emergence. The status (parasitised or healthy) of pupae and species identity of individual moths were recorded. To overcome the unbalanced design of the assessment (not all non-cotton crops were grown at each farm), the data were analysed using the method of residual maximum likelihood (REML; Patterson & Thompson 1971). Statistical analysis of the data was restricted to pupal densities recorded between 21 January and 24 February because pupae were found in all treatments only between these dates. Treatment effects were tested using a Wald test. Pair-wise comparisons of the means were made using a 5% least significant difference (LSD). The data were log-transformed in order to satisfy normality assumptions. Due to a high number of zero counts for five of the eight sampling dates, only data for three dates were included in the statistical analysis. Total counts for the experimental period were not

Plot descriptions for the Emerald field assessment conducted between September 1996 and March 1997 Treatment no.

Crop

Size (ha)*

1

1 2 3 4a

Bt-cotton Unsprayed Sprayed Adzuki bean

15.4 2.5 –– 6

23 Oct 23 Oct 6 Oct 23 Oct

2

1 2 3 4a

Bt-cotton Unsprayed Sprayed Adzuki bean

15.4 1.1 –– 6

14 Oct 14 Oct 5 Oct 14 Oct

3

1 2 3 4b 4c

Bt-cotton Unsprayed Sprayed Sorghum Maize

15.6 1.1 –– 3 3

18 Oct 20 Oct 11 Oct 20 Oct 20 Oct

4

1 2 3 4b 4c 4d 4e

Bt-cotton Unsprayed Sprayed Sorghum Maize Pigeon pea Sunflower

37.4 2.154 –– 3 3 3 3

15 Oct 15 Oct 20 Sep 15 Oct 15 Oct 15 Oct 15 Oct

5

1 2 3 4d 4e

Bt-cotton Unsprayed Sprayed Pigeon pea Sunflower

23.2 1.0 –– 3 3

24 Oct 24 Oct 29 Oct 24 Oct 24 Oct

*Size of sprayed treatment is remainder of cultivated area on the farm.

265

Planting date

266

RV Sequeira and CL Playford

analysed because of missing values within the cotton treatments, giving rise to concerns of bias (i.e. under-estimation of these treatments). At several points during the season, access to some of the trial plots was restricted because of irrigation, rainfall or insecticide application to adjacent fields, thereby giving rise to seven missing data cells. The missing cells were: 21 January (Bt-cotton, Farm 1; unsprayed, Farms 1 and 2; sprayed, Farm 3); 24 February (Bt-cotton, unsprayed and sprayed treatments on Farm 3).

pea produced the largest number of pupae per unit area. Overall, pigeon pea was also the most productive treatment. Summaries of pupal parasitism levels and the species identity of moths emerging from the pupal stage are shown in Tables 2 and 3, respectively. Parasitism of pupae, mainly by ichneumonid wasps (Heteropelma scaposum (Morley) and Ichneumon promissorius Erichson) and tachinid flies (Carcelia spp. and Goniophthalmus spp.), was observed throughout the season. Levels of parasitism varied from 0 to more than 80% of pupae within each treatment (Table 2). Helicoverpa armigera constituted 77.4–89.5% of the population throughout the sampling period. The raw means of pupae found under various treatments over time are shown in Fig. 1. Among the cotton treatments, unsprayed conventional produced consistently greater numbers of pupae than the sprayed conventional or Bt treatment. Among the non-cotton treatments, sorghum and maize were attractive to moths for oviposition in the vegetative stage, considerably earlier than the other non-cotton crops. As a result pupae were recovered from these treatments earlier than from the other non-cotton treatments. Adzuki

RESULTS Helicoverpa pupae were recovered from all treatmen ts by mid-January. Table 2 shows seasonal totals of pupae recovered from each treatment. Abundance and distribution of pupae varied considerably among farms, fields within farms and even within fields. In general terms, among the cotton treatments more pupae were recovered from the unsprayed conventional cotton than from the Bt-cotton or sprayed conventional cotton. Among the non-cotton treatments, pigeon

Table 2 Density (seasonal totals) of Helicoverpa pupae, percentage parasitism and expected number of moths produced under various crops (treatments) in the Emerald field assessment conducted between September 1996 and March 1997* Farm

Variable m2

1

Pupae per 20 % Parasitism Moths ha–1 Pupae per 20 m2 % Parasitism Moths ha–1 Pupae per 20 m2 % Parasitism Moths ha–1 Pupae per 20 m2 % Parasitism Moths ha–1 Pupae per 20 m2 % Parasitism Moths ha–1

2

3

4

5

1

2

3

4a

72 49 18360 15 40 4500 8 20 3200 39 0 19500 24 20 9600

104 33 34840 21 83 1785 20 50 5000 108 37 34020 32 22 12480

75 24 28500 20 0 10000 3 0 1500 25 67 4125 12 50 3000

45 63 8550 10 20 4000

Treatment no. 4b

4c

55 79 5775 17 17 7055

83 34 27390 82 37 25830

4d

4e

342 67 56430 238 41 70210

42 45 11550 46 25 17250

*See Table 1 for explanation of treatment numbers.

Table 3 Adjusted mean Helicoverpa pupal densities under various crops (treatments) in the Emerald field assessment in January and February 1997 Treatment n 1 2 3 4a 4b 4c 4d 4e

4 3 4 2 2 2 2 2

21 January Pupae per 20 m2 1.5 b 2.2 b 2.2 b 1.0 b 2.3 ab 3.3 ab 4.4 a 2.2 ab

(4) (8) (8) (2) (9) (26) (79) (8)

n 5 5 5 2 2 2 2 2

3 February Pupae per 20 m2 2.0 bc 3.1 ab 1.3 c 2.1 bc 2.7 abc 3.3 ab 4.1 a 3.1 ab

(6) (22) (2) (7) (14) (26) (57) (21)

n 4 4 4 2 2 2 2 2

24 February Pupae per 20 m2 3.0 b 3.5 b 2.7 bc 1.9 cd 1.5 d 2.7 b 5.5 a 3.6 b

(19) (34) (14) (6) (3) (14) (238) (35)

Data were log (x + 1) transformed prior to the analysis. Within a column, differences between means that share a common letter are not statistically significant (least significant difference (LSD) test, P > 0.05). Numbers in parentheses are back-transformed means.

Helicoverpa pupae production

267

Fig. 1. Changes in raw mean log (x + 1) Helicoverpa pupal density between 07 January and 11 March 1997 under various treatments in the Emerald trial. X, no pupae were found. Treatments: 1, Bt-cotton; 2, unsprayed cotton; 3, sprayed cotton; 4a, adzuki bean; 4b, sorghum; 4c, maize; 4d, pigeon pea; 4e, sunflower. ( ), 7 January; ( ▫), 21 January; ( ), 3 February; ( ), 24 February; ( ), 11 March.

bean was productive for the shortest period. Pigeon pea consistently produced the largest number of pupae per unit area over an extended period of time (Fig. 1). Pupae production under pigeon pea began later than in other crops but continued to the end of the sampling period (11 March; Fig. 1). Adjusted treatment means and the statistical significance of differences in mean pupal density among treatments recorded between 21 January and 24 February are shown in Table 3. The adjusted means differ slightly from corresponding raw means (Fig. 1) because of the unbalanced experimental design. Although differences between adjusted treatment means are apparent on all three sampling dates, the power of the statistical test to differentiate between treatment means across all sampling dates is limited by the small number of degrees of freedom. Pigeon pea clearly stands out as the treatment with the highest pupal production estimate on all three sampling dates, but the apparent differences between treatments can be statistically confirmed only for the sample dated 24 February (Table 3). Unsprayed and Btcotton can be loosely grouped with sunflower and maize as treatments that generally produced substantial numbers of pupae. Conventional cotton, Adzuki bean and sorghum generally produced fewer pupae compared to the other cotton and non-cotton treatments.

DISCUSSION The role of the refuge crop is to delay the build-up of Bt resistance in Helicoverpa by diluting the frequency of resistance conferring genes (Roush 1997). This is achieved by mating between moths that have survived a discriminating dose of the bacterial endotoxin and successfully developed

on Bt-cotton, and those that have developed free of selection pressure on the refuge crop (Roush 1997). Thus, Helicoverpa refuges should ideally produce moths in synchrony with and in greater numbers than Bt-cotton. Unsprayed cotton provided the best fit to Bt-cotton in terms of the timing of pupae production (Fig. 1). Although the non-cotton crops produced pupae over a shorter period than Bt-cotton there was temporal synchrony in pupae production throughout January and February between Bt-cotton and the non-cotton crops (with the exception of Adzuki bean). Our data show clearly that pigeon pea is potentially the most effective refuge crop for Bt-cotton in CQ both in terms of pupal abundance and distribution (Table 3; Fig. 1), and production of moths (Table 2). Unsprayed cotton, maize, sunflower and sorghum all produced substantial numbers of pupae during the season but not significantly more than Bt-cotton (Table 3). This result warrants cautious interpretation because of the large error variance associated with the treatment means, partly as a result of the unbalanced design. Our results are broadly consistent with those of Fitt and Tann (1996), who assessed the refuge value of a range of commercial crops during the 1995/1996 growing season. Their study also included smallplot evaluations of the grain legumes pigeon pea and adzuki bean. They found that pigeon pea was potentially the most productive refuge option followed by unsprayed cotton, sorghum and maize, and these in turn were more productive per unit area than Bt-cotton. Pigeon pea, unsprayed cotton, sorghum or maize could be used as Bt-cotton refuges but their effectiveness would depend on the abundance (area) of each and its pupal productivity in relation to the area of Bt-cotton. A smaller area of pigeon pea refuge crop would be required relative to

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unsprayed cotton, sorghum or maize. A surprising result is that sprayed cotton produced a substantial number of pupae, not dissimilar to Bt-cotton. From an operational standpoint sprayed cotton is a practical refuge option but the least suitable in terms of potential efficacy, possibly requiring larger areas than Bt-cotton. Under southern Queensland and New South Wales environmental conditions, Helicoverpa activity comes to a virtual standstill through winter. Large numbers of diapausing pupae are frequently found under cotton stubble, indicating that a substantial proportion of the population is in diapause before the crops are harvested (Wilson et al. 1979; Fitt & Daly 1988, 1990). The practice of post-harvest cultivation directly targets resistance-carrying individuals in their most vulnerable stage. It is therefore a key component of the southern Bt-cotton RMS and a tactic for managing Helicoverpa resistance to conventional insecticides (Fitt & Forrester 1987; Fitt & Daly 1990; Murray & Titmarsh 1990; Forrester et al. 1993; Forrester & Bird 1996). The usefulness of cultivating cotton stubble as a resistance management technique in CQ is questionable for two reasons. First, diapause behaviour of Helicoverpa in the area would make post-harvest cultivation ineffective. Pupae collections over several years under summer crop residues and winter crops indicate that diapause seems to occur every year but the proportion of the population in diapause varies greatly between years, ranging from less than 10% to approximately 90% (R. Sequeira, unpubl. data 2000). Diapausing pupae are found generally from late April onwards but substantial larval populations are nevertheless found every year on winter chickpea crops (Sequeira 2001). These findings suggest that post-harvest cultivation of cotton fields in winter would target only a fraction of the population in any given year. Second, the majority of cotton crops in CQ are harvested in March and early April. Post-harvest surveys of pupae under cotton fields indicate the absence of significant numbers of unemerged pupae in most fields in most years (Sequeira 2001). This is consistent with the results of simulation modelling of diapause initiation in Helicoverpa spp., which also suggests that post-harvest cultivation of cotton fields in CQ is likely to be ineffective (Dillon & Fitt 1997; Dillon 1998). But post-harvest cultivation in fields of late-planted summer crops may be of considerable value in resistance management. Based on our results and the findings of Fitt and Tann (1996) on the value of various refuge options, a ‘best-bet’ Bt-cotton RMS for CQ is proposed to include: (i) planting of refuge crops in common with the southern RMS (unsprayed conventional cotton, sprayed conventional cotton, sorghum or maize); and (ii) an end-of-season trap-crop of pigeon pea in lieu of post-harvest cultivation. Continuing evaluation of Helicoverpa refuges over a number of years (Fitt & Tann 1998; Fitt et al. 1999, 2000) has provided considerable evidence in support of the refuge crop specifications of the southern Bt-cotton RMS. These studies show that pigeon pea consistently produces higher pupal densities (and implicitly moths) than unsprayed cotton,

sorghum or maize which, in turn, produce substantially more pupae than Bt-cotton. These results form the basis of current refuge option specifications of the southern Bt-cotton RMS that includes the use of pigeon pea as a refuge option (Shaw 2000). The proposed CQ Bt-cotton RMS places greater importance on the use of pigeon pea for end-of-season resistance management than as a refuge option. The rationale for using pigeon pea as a trap crop in the strategy is the assumption that its pupal productivity reflects, in part, attractiveness to ovipositing Helicoverpa moths. In practice this means that flowering pigeon pea can be used to attract the last generation of Helicoverpa moths emerging from cotton and concentrate their progeny in small areas (Sequeira 1998). The larvae potentially carrying Bt resistance could then be eliminated mechanically by destruction of the trap-crop. Although both Helicoverpa species are attracted to pigeon pea (Zalucki et al. 1986), the trap-crops are expected to target mainly H. armigera because of its predominance as the principal late season pest of cotton (Fitt 1989). Pupal production on pigeon pea relative to the other crops (Table 3; Fig. 1) may be viewed as a rough indication of its relative attractiveness to Helicoverpa. Pupal productivity cannot be directly equated to attractiveness for oviposition if there are large differences in mortality of immature stages among the different host crops being evaluated. The limited data on parasitism levels in Table 2 do not reveal any obvious trends indicative of differential mortality on any of the host crops tested. Other studies (R Sequeira, unpubl. data 2001) also show that flowering pigeon pea is clearly preferred as a host for oviposition compared to cotton and a number of other crops. Growers in CQ have been planting late-season pigeon pea trap-crops with every cotton crop since 1997 as part of a regional Helicoverpa management program (Sequeira 2001). The practice of late-season trap cropping is conceptually attractive as a resistance management tool, especially in areas where post-harvest cultivation is unlikely to be effective. Even in areas where post-harvest cultivation is deemed to be effective for resistance management purposes, lateseason trap cropping may enhance the practicality and effectiveness of post-harvest cultivation by concentrating pupae in small, more manageable areas of the farm. Wider adoption of this technique, however, needs to be underpinned by substantive data on its efficacy and potential impact on Bt and conventional insecticide resistance.

ACKNOWLEDGEMENTS We are grateful to Michael McCosker and Renee Schmidt (Department of Primary Industries, Emerald), staff of Emerald Cotton Grower Services, staff of Emerald Seed and Grain, and staff of Queensland Cotton Merchandising for assistance in data collection. Financial support for this work was provided by the Cotton Research and Development Corporation.

Helicoverpa pupae production REFERENCES Dillon M. 1998. Predicting autumn diapause induction in Helicoverpa using long term average temperatures. In: Proceedings of the Ninth Australian Cotton Conference. August, 1998, Broadbeach, Queensland pp. 475–479. Australian Cotton Growers Research Association, Brisbane. Dillon ML & Fitt GP. 1997. A spatial simulation model of the regional population dynamics of Helicoverpa moths. Agricultural Systems and Information Technology 7, 32–34. Fitt GP. 1989. The ecology of Heliothis species in relation to agroecosystems. Annual Review of Entomology 34, 17–52. Fitt GP. 1994. Cotton pest management: Part 3. An Australian perspective. Annual Review of Entomology 39, 543–562. Fitt GP. 1996. Transgenic cotton resistance strategy. Australian Cottongrower 17, 30–31. Fitt GP & Daly JC. 1988. The overwintering foe: Winter populations of Heliothis in cotton growing areas and the importance of stubble cultivation. In: Proceedings of the Australian Cotton Conference. August, 1988, Surfers Paradise, Queensland pp. 13–24. Australian Cotton Growers Research Association, Brisbane. Fitt GP & Daly JC. 1990. Abundance of overwintering pupae and the spring generation of Helicoverpa spp. (Lepidoptera: Noctuidae) in northern New South Wales, Australia: Implications for pest management. Journal of Economic Entomology 83, 1827–1836. Fitt GP, Dillon M & Tann C. 1999. Entomological research: Cotton season 1998/99. In: Variety Trial Results 1999 pp. 85–86. Cotton Seed Distributors, Wee Waa NSW. Fitt GP, Dillon M & Tann C. 2000. Entomological research: Cotton season 1999–2000. In: Variety Trial Results 2000 pp. 84–85. Cotton Seed Distributors, Wee Waa NSW. Fitt GP & Forrester NW. 1987. Overwintering of heliothis: The importance of stubble cultivation. Australian Cottongrower 8, 7–8. Fitt GP, Mares CL & Llewellyn DJ. 1994. Field evaluation and potential ecological impact of transgenic cottons (Gossypium hirsutum) in Australia. Biocontrol Science and Technology 4, 535–548. Fitt GP & Tann C. 1996. Quantifying the value of refuges for resistance management of transgenic Bt cotton. In: Proceedings of the Eighth Australian Cotton Conference, August, 1996, Broadbeach, Queensland pp. 77–83. Australian Cotton Growers Research Association, Brisbane. Fitt GP & Tann C. 1998. Helicoverpa research: 1997/98 cotton season. In: Variety Trial Results 1998 pp. 80–81. Cotton Seed Distributors, Wee Waa NSW. Forrester NW. 1994. Resistance management options for conventional Bacillus thuringiensis and transgenic plants in Australian summer field crops. Biocontrol Science and Technology 4, 549–553. Forrester NW & Bird LJ. 1996. Conventional insecticide and Bt transgenic resistance management in Australian cotton. In: Proceedings of the Eighth Australian Cotton Conference, August, 1996,

269

Broadbeach, Queensland pp. 159–172. Australian Cotton Growers Research Association, Brisbane. Forrester NW, Cahill C, Bird LJ & Layland JK. 1993. Management of pyrethroid and endosulfan resistance in Helicoverpa armigera (Lepidoptera: Noctuidae) in Australia. Bulletin of Entomological Research, Supplement Series 1, 1–132. Gunning RV, Graham DM & Devonshire AL. 1996. Insensitive acetylcholinesterase and resistance to thiodicarb in Australian Helicoverpa armigera Hubner (Lepidoptera: Noctuidae). Pesticide Biochemistry and Physiology 55, 21–28. Jallow MFA, Zalucki MP & Rogers J. 1999. Host selection and management of Helicoverpa (Lepidoptera: Noctuidae) on cotton. In: CRDC Occasional Papers: Insects. Cotton Research & Development Corporation, Narrabri NSW. McGaughey WH & Whalon ME. 1992. Managing insect resistance to Bacillus thuringiensis toxins. Science 258, 1451–1455. Murray D & Titmarsh I. 1990. Winter control of Heliothis: Cultural, not chemical. Australian Cottongrower 11, 60–62. Patterson HD & Thompson R. 1971. Recovery of inter-block information when block sizes are unequal. Biometrika 58, 545–554. QDPI. 1996. Crop Management Notes: Central Queensland 1996. Farming Systems Institute, Queensland Department of Primary Industries, Brisbane. Roush RT. 1997. Bt-transgenic crops: Just another pretty insecticide or a chance for a new start in resistance management? Pesticide Science 51, 328–334. Sequeira R. 1998. Trap-cropping: A way of managing heliothis. In: Proceedings of the Ninth Australian Cotton Conference. August, 1998, Broadbeach, Queensland pp. 263–269. Australian Cotton Growers Research Association, Brisbane. Sequeira R. 2001. Inter-seasonal population dydnamics and cultural management of Helicoverpa spp. in a central Queensland cropping system. Australian Journal of Experimental Agriculture 41, 249–259. Shaw AJ. 2000. Cotton Pest Management Guide 2000/2001. Australian Cooperative Research Centre, New South Wales Agriculture, Orange. Wardhaugh KG, Room PM & Greenup LR. 1980. The incidence of Heliothis armigera and H. punctigera (Lepidoptera: Noctuidae) on cotton and other host plants in the Namoi Valley of New South Wales, Australia. Bulletin of Entomological Research 70, 113–132. Wilson AGL, Lewis T & Cunningham RB. 1979. Overwintering and spring emergence of Heliothis armigera (Lepidoptera: Noctuidae) in the Namoi Valley, New South Wales, Australia. Bulletin of Entomological Research 69, 97–110. Zalucki MP, Daglish G, Firempong S & Twine PH. 1986. The biology and ecology of Helicoverpa armigera (Hübner) and H. punctigera Wallengren (Lepidoptera: Noctuidae) in Australia: What do we know? Australian Journal of Zoology 34, 779–814. Accepted for publication 27 February 2001.