Plant Protection Ouarterly Vol. 3(3) 1988 97 Relationships between the lucerne leafroller, was assessed to determine th e number of Merophyas divulsana (Walker) (Lepidoptera: stems damaged per 10 stems. Experiments were terminated at harvest at which time Tortricidae), 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 insect 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).