Plant Protection Ouarterly Vol. 3(3) 1988 97

Relationships between the lucerne leafroller, was assessed to determine th e number of Merophyas divulsana (Walker) (: stems damaged per 10 stems. Experiments were terminated at harvest at which time ), and damage to lucerne yield estimates were made by harvesting 4 x 0.1 m2 quadrats per plot. Samples were bagged, oven dried at SO °C for 48 h A. L. Bishop and weighed. Horticultural Research and Advisory Station. P.O. Box 581. Gosford . New South An analys is of variance was used to test Wales 2250 for differences in yie ld between damage levels (P < 0.05). Once differences were M. A. O'Connell confirmed, a relationship was derived Scientific Services, National Parks and Wildlife . 189 Kent Street . Sydney, New South between the number of damaged stems per Wales 2000 10 stems (x) and yield (y). The equations were used to derive secondary relationships between yield reduction and damaged stems by assuming maximum yield at zero damage and then calcu lat ing damage levels Summary in confe rence proceedings (Bishop and corresponding with yield reductions (OJD) O'Connell t984). Thi s paper deals with from the maximum yield. Relationships were established between the these data in detail together with new Cross· validation of the yield reduction number of stems damaged by the lucerne information. learroUer, Merophyas divulsana (Walker) models was made later by using fie ld data (Lepidoptera: Tortricidae), and yields of from two seasons. Experimental areas were lucerne in the Hunter Valley, N.S.W. The Materials and methods located in fie ld s of Hunter River and relationships were used to derive damaged The study was designed after considering CUFIOI lu cerne. Treatments consisted of redudion to x 10 m plots of lucerne that were either stem/ yield models from which a the following previous observations: be protected from early regrowth with damage Ihreshold could proposed. A damage (defined as the rolling of a stem ter· methidathion or endosulfan (340 or yield reduction of 10070 was set as the toler­ minal) is genera lly cumulative because a 735 g a.i. ha- i respectively) or left un­ able limit and this corresponded to approx­ larva can damage more than one stem and protected. Each treatment was replicated imalely 23-25'70 of sIems damaged. larval numbers increase; there is usually four li mes and arranged in ra ndomized However, the threshold applies only when only one larva (and/ or roll) per stem; blocks. Counts were made of cumulative damage begins in the first 2 weeks of damage peaks are defined and are approx­ M. divu/sana damage and the maximum regrowth. Field data were used to cross­ imately 5 weeks apart (Bisho p 1984); major level of stems damaged was recorded as a validate the models. The data were also damage may seem to occur over a short percentage. considered in relation to hypothetical period when sampling happens to corres­ Yield differences were estimated from modes of attack and demonstrated some of pond wit h the peak in larval damage; any 4 X 0.1 m1 quadrats per plot, cut at har­ the variables which complicate the estab· number of stems may be damaged; and vest, bagged, oven dried at for 48 h lishment and interpretation or pest/ plant damage may occur at any stage of plant saoe and weighed. Eight points were derived and interacUons. growth. These observations can be repre­ these actual yield reduct ion data compared sented by three hypothetical modes of with model estimates by a X2 'goodness of Introduction allack (Bishop and O'Connell t984). fit' test. Complex relationships often exist between Damage could be either cumulative and an and the growth or yield of a plant continuous until harvest or could be Experiment 1 (Twine t 984). Consequenlly. although cumulative and cease at some point before An experimental area was demarcated there are several methods by which the harvest beyond wh ich no further yield loss within a I ha field of irrigated Pioneer 532 response of plants to insect attack can be occurs. A lternatively, the damage may lucerne at Tocal also in the Hunter Valley. estimated, each has its limitations and may occur over a short period at any time during Plots of lucerne (7 x 8 m) were cut such not apply in every circumstance (Poston plant growth. Experiments were designed that at any point in ti me there were five el al. 1983). Of these melhods one of Ihe to reveal and to quantify the relationships different growth stages present. Each more promising is the assessment of natural between M. divu/sana and yield of lu cerne growth stage represented a cutting treat­ and to propose a treatment threshold on infestations. This method is convenient to ment that was then allowed 10 regrow for use and has field reality. Other methods the basis of the data. 5 weeks before being rec ut and harvested. make use of artifi cial populations or i.e. treatment I was cut at week 0 and har­ damage simulation. When applied to an Experiment I vested at week 5, treatment 2 was cut at agricultural crop, the plant/insect relation· Experimental areas were set up in a field week I and harvested at week 6. and so on. ship may be interpreted in terms of its effect of irrigated Hunter River lu cerne at Treatments were a rra nged in three on crop yield and/ or economics. A damage Mt Thorley in Ihe Hunter Valley, N.S.W. randomized blocks. threshold for pest numbers can be deter· Levels of damage we re manipulated by T his experiment was designed (0 inves­ mined and a treatment level derived for the allowing an increasing larval population to tigate damage 10 lucerne at different stages initiation of control measures against a damage plots of lucerne progressively for of growth at the same point in time and pest. varying periods before spraying with used the stem as th e unit expressing yield. In the Hunter Valley, N.S. W., numer­ methidathion (340 g a.i. ha- I ) to prevent Ten stems, naturally and newly damaged ous varieties of lucerne are grown with a further damage. This experiment was by M. divu/sana larvae. were randomly range of known tolerances to aphids and carried out on two occasions (Experiments selected, tagged and measured on the same disease. Damage to lu cerne by the lucerne 10 and lb) in which the following number day in each re pli cate of the five growth leafroll er, Merophyas divulsana (Walker) of damage levels, replicated four times, stages. The stems were collectect. together (Lepidoptera: Tortricidae), is common in were achieved by spraying to x 10 m plots with 10 undamaged stems, when each treat· lale summer and autumn (Bishop t984). in randomized blocks starting with the ment was due for recutting. Measurements The larvae attack lucerne stems by rolling earliest regrowth. were made of individual stem lengt hs and and uniting the terminal buds into sheltered Experiment la. Continuous damage up to dry weights and associated differences due feeding sites. subsequently reducing both 95070 of stems (five damage levels) to M. divu/sana damage at each of the the yield and quality of Ihe crop. The ques· Experiment lb. Continuous damage up to growth stages were assessed. Relationships tion of establishing the relationship of M . 51'70 of stems (four damage levels) were derived expressing the effect of M . divulsana to the lucerne plant and thereby Each week damage to five lots of 10 divu/sana damage at each growth stage on quantifying treatment levels was outlined adjacent stems in each experiment al plot stem length and stem dry weight. 98 Plant Protection Quarterly Vol. 3(3) 1988

Experiment 3 Table 1 Mean yields of lucerne (g dry wt 0.1 m -') at different levels of damage by the There are many lucerne varieties with a lucerne leafroller Merophyas divuisana range of growth characteristics which could (Yield means in columns identified by different letters are significantly possibly affect the M . divu/sana/ lucerne different using the Student-Newman-Keuls mUltiple range test) damage relationship. This experiment con­ E.perlment la Experiment Ib sidered possible variations in infestation levels and subsequent larval and pupal Dama.'" ..ems! Yield Dam,.ed stems/ Yield development due to variety. The varieties 10 stems (SO) 10 "ems (SO) used covered the range of varietal charac­ 0.1(0.12) 24.Sa 0.S(0.43) 26.0a teristics available to Australian lucerne 0.6(0.30) 24.0a 1.1(0.44) 24.4a growers (Anon. 1996). 2.4(0.46) 20.3b 3.3(0.61) 22.0b (a) Natural infestation levels were 7.1(0.90) 17.2c 5.1(0.95) 22.4b assessed on 18 varieties in the field. These 9.5(0.50) 15.3c varieties were grown in 6 x 3 m plots arranged in four randomized blocks. The number of stems damaged per 10 adjacent 30 stems was assessed in each plot on two • occasions. Data were tested for differences due to variety (P < 0.05). (b) Artificial population levels were assessed on 30 varieties of lucerne grown ;/ at 25 ± 2°C under glass house conditions. z 1 Each variety was grown individually in 0 ~ 20 12-cm pots arranged in four randomized u blocks. Thirty pairs of M. divulsana adults :::> 0 were randomly released at day 1 and the w number of stems damaged per 10 stems in ~ each pot assessed 2 weeks later. The experi­ 0 • ~ ment was carried out once and the data w • • tested for differences due to variety >- (P < 0.05). ~ (e) Larval and pupal development on each of the 30 varieties (Experiment 3b) were investigated at 20 ±2°C on two occa­ sions. Five newly hatched first instar larvae were transferred singularly on to a lucerne terminal of a designated variety and held in plastic containers arranged at random on 0 ~--~--~2~'0~;~~--~--~~--~----~--~--~ N I/') 40 60 80 trays. Terminals were changed as required NN until the larvae pupated. Larval and pupal developmental times were recorded and the %STEMS DAMAGED results compared. Figure 1 Yield reduction/damaged stem relationships when damage was cumulative and continuous up to 94% of stems damaged (line 1) and cumulative and continuous up to 51% of stems damaged (line 2) ; together with eight yield reduction points derived from Hunter River R•• ulte (e ) and CUF1 01 (_ ) lucernes used for cross·validatlon. Experiment 1 The dilferent levels of damage by M . yield reduction. At the 10070 tolerable level. assigned to one of three categories (Table divulsana corresponded with significant the damage threshold was 22 .5"1, of stems 2). The impact of damage on the develop­ differences in yield of Hunter River lucerne damaged. The cross-validation data ment of stems decreased with time and on both occasions (P < 0.05) (Table I). showed no significant departure, when the seemed to be related to the degree of Data from Experiment la were best lowest six of the eight data points were used damage to the terminal and the ability of expressed as a linear relationship by the (Figure I) (X' = 1.6, ef X ' = 11.07 at the plant to co mpensate for this damage. equation: 5 d. f., P < 0.05). The upper two points were neglected in this analysis as they were Experiment J y-24.02 - O.96x(r' =O.SI) ... .. (a) above the limits of the data used to derive There were no differences in infestation Data from Experiment I b were best the model. levels on each of th e varieties tested under expressed as a logarithmic relationship by either field or glasshouse conditions (P the equation: Experiment 1 > 0.05). M. divulsana completed its life y - 24.7 - 1.631n x (r' = 0.52) .... . (b) Reductions in stem len gth and stem dry cycle on each of the 30 varieties tested. Equation (a) was used to construct a weight due to M. divuisana damage at Larval developmental times ranged from linear relationship between damaged stems different growth stages were described by 17.0-20.7 days (X±SE = IS.S± I.S) on and yield reduction (Figure I, line I). A non-linear asymptotic models (Figure 2). the first occasion and IS.5-22.3 days maximum yield reduction of 10'10 was set The comparison of damaged and undam­ (l9.S ± 1.4) on the seco nd . Corresponding as the tolerable level of damage and gave aged stems in each of these growth stages pupal times were 7.0-8.5 days (7.6±0.S) a damage threshold of 25'10 of stems dam­ indicated that differences (P < 0.05) and 7.3-9.6 (8.0±0.6). These data were aged. The eight data points used to cross­ occurred only in the first 2 weeks of consistent with previously established validate this relationship (the points given regrowth, although stem dry weight reduc­ developmental times at 20 ± 2°C (Bishop in Figure I) showed no significant depar­ tions > 10% were recorded for up to and McKenzie 19S6). ture (X' = 7.5, ef Xl - 14.07 at 7 d.f., 3 weeks. P < 0.05). The initial stem length of each damaged DlecuuIon Equation (b) was used to construct the stem was measured and its stem length at curve (Figure I, line 2) also expressing the harvest was compared with this and stem All three hypothetical modes of attack relationship between damaged stems and lengths of undamaged stems and the effect by M. divulsana larvae were observed. Plan1 Pro1ec llon Quarlerly Vol 3(3) 1988 99

. 0 1 2 plete lack o f growth and even death of some stems attacked during the first weeks , of regrowth would support this proposaL ~ The experiment would probably over estim­ z 30 z \\ ate the effect of damage. Quadrat sampling o • for yie lds as used in Experiment I would u'" u-'" \ ~ \ ~ alleviate this problem. 0 0 ~ Data also suggested that the modification 20 ~ • X X of nalUral infestations was the best way to '(. 4 0 · 2 6 10 · S J J 16(0 , '" 1 ' 60J ~ approach this study. Artificial populations, 2 .. 0 9 8 > ,2 0: 0 9 1 "~ whether plant of insect, have questionable , • 0 ·, 0 • field reality. Damage simulation would , 10 , • have proven- unrealisti c as there was no

~ uniform effect of damage on the subse­ • ~ quent growth and yield of individual stems, especially at the critical early stages of 0 0 regrowth. 2 , 2 3 , In this study, the models and the derived GRO WTH STAGE (WEEK S fR O M HUVE S T) damage thresholds were mutually suppor­ tive and were validated by field data. It Figure 2 Damage 10 lucerne by M. divulsana at different stages (weeks of regrowth) of growth. would appear, by assuming that a 10070 yield reduction can be tolerated, that a damage threshold of approximately 23- Table 2 A comparative assessment of stems damaged by M. divulsana larvae at differ­ 25070 of stems damaged could be proposed ent stages of growth (weeks from last harvest or at different stem lengths) with for testing as a treatment threshold. The undamaged stems when the crop was due for harvest threshold would apply only when damage begins in the first 2 weeks of regrowth. Week Mean stem Dead! Stunted Full no. length (SD) no growth (%) develo pment (em) ('I.) (%) Acknowledgments

I 8.2 (2.0) 34.5 44.8 20.7 We thank Dr A. Clift, Mr R. Holtkamp 2 20.5 (4 .0) 3.3 53.3 43 .3 and Mrs L. Spohr for their helpful 3 32.4 (3.9) 0 34.5 65 .5 comments and suggestions during the pre­ 4 49.9 (3 .7) 0 16.6 83 .3 paration of this manuscript. 5 54.2 (3 .9) 0 0 100.0 References

Cumulative and continuous damage until less importance. The commencement of an Anon. (1986) Lucerne Varieties: 1986. harvest were represented by the yield reduc­ infestation relative to the growth stage of Department of Agriculture, New South tion/ damaged stem model give n as Figure the crop should there fo re be a conditional Wales. Agfact P2.5 .13. I, line I. C umulative and continuous factor for the usc of any threshold. A Bishop. A. L. (1984). Heliothis spp. and damage ceasing al some point before har­ number of treatment thresholds are pos­ Merophyas divulsana (Walker) in the vest were represented by the second yield sible from the derived models and a single seasonal damage of luce rne in the Hun­ reduction/ damaged stems model (Figure I, treatment threshold obviously does not ter VaHey, New South Wales. General line 2). The 'goodness of IW of the cross­ reflect the dynamic nature of the inter­ and Applied Entomology 16, 36-44. validation points suggested that both would action. It may, however, be adequately Bishop, A. L. , and McKenzie, H. J. (1986). be an adequate assessment of the damage representative if the models represent the Development of Merophyas divulsana relationship. The cross-validation points normal cumulative increase in damage by (Walker) (Lepidoptera: Tortricidae) in ranged between 10070 and 65070 of stems a population in phase with the lucerne relation to lucerne harvesting as a con­ damaged and these were actual maximum growth cycle. Infestations beginning later trol strategy. Journal oj the Australian damage levels, suggesting that most infesta­ than the first 2 weeks of regrowth should Entomological Society 25, 229-33. tions cease before a ll stems are damaged. not require treatment and the population Bishop, A. L., and O'Connell, M. A. Therefore, although the data from experi­ would be regulated by harvesting. (1984). Damage levels for th e lucerne ment la gave a beller tit than that from There was no evidence to suggest that the lea froller, Merophyas divulsana (Wal­ experiment 1b, it would appear that the interaction, no r the threshold, would vary ker). Proceedings of the Fourth Austra­ second of these models could be the more significantly with the variety being grown. lian Applied Entomological Research biologically meaningful. However, at a Similar damage to stem terminals was Conference, pp.170-4. Eds P. Bailey 10% tolerable level of yield reduction both observed and there were no differences in and D. Swincer. models give similar damage thresholds. infestation levels or the ability of M. LeClerg, E. L. (197 1). Field experiments The third hypothetical mode of anack was divulsana to complete its life cycle on any for assessment of crop losses. In 'Crop demonstrated in Figure 2. of the varieties tested. Loss Assessment Methods. F.A .O. LeClerg (1971) proposed that insectsl Experiment 2 primarily demonstrated the Manual on the Evaluation and Preven­ plant interaction ex periments need to be susceptibility of lucerne at different growth tion o f Loss by Pests, Diseases and replicated because of the numerous biolog­ stages as it was realized that the quantita­ Weeds'. (A lden and Mowbrey Ltd: ical and environmental variables that affect tive value of the data could be limited by Great Britain.) fie ld relationships. Correspondingly, these the experimental design. Data were derived Poston, L. F .• Pedigo, L. P. , and Welch, data demonstrate a degree of variability from single stem samples. However, a S.M. (1983). Economic injury levels: that necessitates the careful interpretation lucerne plant comprises several stems aris­ reality and practicality. Bulletin 0/ the of the M. divulsana/ lucerne relationship. ing from a sin gle crown that is commonly Entomological Society oj America 29, While damage was actually cumulative, it indiscernible from other crowns. Different 49- 53. was also seen that the effects of damage numbers of stems per crown may be dam­ Twine, P. H. (1984) . Economic th resholds decreased with stem size. Most yield loss aged. Accordingly, energy and nutrients of pests. Proceedings of the would result from damage in the first 2-3 could be diverted to non-damaged parts of Fourth Australian Applied Entomolog­ weeks of what is normally a 5--6 week the plant resulting in compensatory growth ical Research Conference, pp. 197-203. regrowth cycle. Later damage would be of not recorded by stem sampling. The com- Eds P . Bailey and D. Swincer.