Ann. appl. Biol. (1984). 105, 529-538 Printed in Great Britain

529

Characteristics of resistance to the grain aphid Sitobion avenae in winter wheat

BY H. J . B. LOWE Plant Breeding Institute, Maris Lane, Trumpington, Cambridge, CB2 2LQ

(Accepted 23 July 1984)

S U M M A R Y

Five stocks of winter wheat were resistant to S. avenue in glasshouse screening tests, where resistance was more evident in older than younger plants but different types of resistance were not separated.

Antibiosis was measured from the growth and reproduction of caged individual aphids and antixenosis from the settling of adult aphids on detached leaf portions. Cvs Bounty, Rapier and Virtue were resistant due to antibiosis, whilst cv. Kador and line A4501-4E showed antixenosis. These and other differences indicated that these five wheat stocks probably include at least four distinct sources of resistance to S. avenue. The existence of different types of resistance could account for variability in the results of screening tests.

I N T R O D U C T I O N

The numbers of cereal aphids attacking wheat and barley in W. Europe in summertime fluctuate widely from year to year (Carter, Dixon & Rabbinge, 1982). The amount and nature of damage directly attributable to these aphids is also variable (George & Gair, 1979; Rabbinge et al., 1981; Vereijken, 1979; Wratten, 1978), but the grain aphid, Sitobion avenue (F) (= Macrosiphum avenue) does more damage pro rata than the other common species, Metopolophium dirhodum, because S. avenue feeds on ears whilst M . dirhodum feeds on leaves (Vereijken, 1979; Wratten, 1975). During the development of screening methods used in breeding wheat for resistance to aphids (Lowe, 1984a, b), a number of different cultivars and breeding lines were found to be resistant to S. avenue to varying degrees (Lowe, 1981a, 1982a, 1984a, c ) . It is important under these circumstances to identify homologies and dissimilarities among these potential resistance sources so that breeding can be rationalised.

Investigation of the basis of resistance in the various stocks represents one step toward such identification and can also aid interpretation of screening test observations. Different types of resistance, especially antibiosis (Painter, 195 1) and antixenosis (Kogan & Ortman, 1978) (= non-preference, Painter, 195 l) , may be revealed by differing experimental techniques. This paper describes experiments carried out to characterise the resistance of some winter- wheat stocks.

M A T E R I A L S

Plants Winter wheat, Triticum aestivum L., cvs Armada, Joss Cambier, Maris Huntsman (suscep-

tible standard, Lowe, 1982a), Flanders, Bounty, Kador (resistant standard, Lowe, 1982a), Rapier and Virtue were used, together with a breeding line A4501-4E that is a non-glaucous variant of cv. Avalon. Both plant growth-stage and growing conditions affect the performance of cereal aphids (Carter, McClean, Watt & Dixon, 1980; Dewar, 1977; Lowe, 19816). 0 1984 Association of Applied Biologists

530 H . J. B . L O W E

Therefore, for experiments on antibiosis and antixenosis, plant culture was adapted to produce vigorous plants with a spread of ages to allow plants of different stocks to be matched closely for growth stage and condition.

Plants from successional sowings 3 wk apart in cellular paper pots (Lowe, 1984a) were vernalised after germination for 9, 10, 1 1 , or 12 wk in a controlled environment room at about 5°C with an 8 h photoperiod. From vernalisation, plants were grown singly in 10.5 cm diameter pots of John Innes No. 2 compost in a glasshouse at temperature ranges varying from 8-25°C in December and January to 15-34°C in July and August with, in winter, an 18 h photoperiod maintained by high pressure sodium lamps (Austin & Edrich, 1974). Aqueous fertilisers were supplied every 10-1 4 days commencing 7-1 0 days after potting. Typically, the initial treatment of compound fertiliser (yielding 0.0 14 g N, 0.004 g P, 0.01 8 g K per pot) was followed by a solution of NH4NO3 (0.3 g per pot), then a treatment combining the two, followed by two further treatments of nitrate. In these conditions and after 10 or 11 wk vernalisation, average numbers of well-filled, fertile ears ranged from 5 . 8 on Armada to 8 . 5 on Virtue. Plants with 9 or 12 wk vernalisation tended to produce respectively about one ear more or less than this.

Aphids The clone CASS of S. avenae (Lowe, 19848) was cultured continuously in the glasshouse

at a standard density in aphid-proof cages kept under fluorescent lights giving a 20 h photoperiod. Ten young apterous adults were released on to each 10.5 cm plant pot containing 10 seedlings of oats, cv. Tabard, at the 2-leaf stage. The adult aphids were removed after 24 h and the progeny allowed to develop, providing apterous aphids for use either when half- grown for antibiosis experiments or when newly adult for antixenosis experiments. About half of the aphids reared in each generation were kept to be the parents of the next, reared i n the same way.

M E T H O D S

Antihiosis These experiments were conducted in the glasshouse under conditions similar to those used

for growing the plants, at 10-20°C in February and March and 12-25°C in April and May 1983. Plants with at least three suitable strong shoots, normally tillers 1 to 3, were chosen using different batches (potted at 7 day intervals) as necessary to give eight plants of each stock, all at the same stage of development in any one experiment.

Separate experiments were used for different plant parts, with two experiments on the penultimate leaf of shoots at about growth stage (G.S.) 37 (Zadoks scale, Tottman & Makepeace, 1979). Four experiments on the flag leaf of shoots at booting lasted from about G.S. 43 to G.S. 68 and four others on ears lasted from near anthesis (G.S. 68) to early in the milk-ripe stage, about G.S. 73. Within an experiment, small differences in plant development were confounded with the eight replicates that each included one plant of each stock. On every plant, three similar shoots had one aphid caged on the appropriate part, leaf or ear, with 192 aphids in the whole experiment.

The aphids were caged as instar I1 or 111 larvae in cylindrical bags of cellophane, 18.5 cm Long. Each bag, approximately 2.5 cm in diameter, was closed and held firm on the plant by insertion of a cylindrical plug of fine-grain plastic foam that was 3 . 5 cm in diameter when uncompressed and slit to hold the leaf lamina or ear peduncle. A bag enclosed a whole ear or most of the lamina of a leaf after removal of the tapered leaf tip. The caged aphids were left undisturbed to mature and reproduce. When their eldest progeny had matured and started to reproduce, all cages were cut unopened from the plants and those that could not be examined the same day were stored in a freezer at -22°C.

Characteristics of grain aphid resistance in winter wheat 53 1

All cages from a replicate were examined on one day, either fresh or after storage for up to 10 days. Exceptionally, one to three replicates were stored up to 14 days, or cages from replicates of experiments with ears were examined on successive days. For each cage a record was made of the shoot G.S., the extent of any necrosis of leaf lamina, the number of aphids, the number of adult aphids and the combined weight, fresh or from storage, of all aphids from the cage. In two similar experiments with spring wheat and for some replicates of the two experiments with penultimate leaves, the aphids were weighed fresh, but then returned to a cellophane bag with a leaf and placed in the freezer for some days before being collected and weighed a second time.

Analyses of variance were made of the total numbers of aphids and their combined weights, i.e. biomass, taken as measures of growth, and the numbers of adults as measures of rate of development. Numbers of aphids were transformed as d(n+O. 5) for analysis, but analysis of untransformed data was also carried out for total numbers of aphids and for biomass.

Antixenosis These experiments were conducted in three seasonal groups. The first in summer, August

and September 1983, used leaves from plants grown without artificial lighting; a second, autumn group in November 1983 used plants grown with supplementary light, whilst the plants for mid-winter experiments conducted between 19 December 1983 and 26 January 1984 were almost wholly dependent on artificial lighting. The eight stocks tested for antibiosis were used for the first and second groups of experiments, but from December 1983, ten stocks were included without cv. Flanders.

Flag or penultimate leaves were chosen so that small differences in shoot G.S. were confounded with replications. Each replicate was set up in a transparent box 15 X 10.5 X 7 . 3 cm with about 1 cm depth of agar gel (5 g litre-') in the bottom and included one leaf of each wheat stock. Two portions about 7.5 cm long were cut from each leaf, discarding the extreme tip, base and a middle section of the lamina. The 16 or 20 portions were set on end in the agar to a randomised plan, each at a slight angle from the vertical so that their upper ends contacted the lid of the closed box. The surface of the agar between the rows of leaf portions was covered with filter paper strips that largely prevented walking aphids from being trapped in free water. Twenty-five or, for ten stocks, 30, young apterous adult S. avenae were released in each box at about 1600 h on the day of the experiment. In August and September the boxes, 10 per experiment, were placed on a laboratory bench near a north- facing window, but subsequently the 10 or 15 boxes in each experiment were placed in the glasshouse under fluorescent lights, photoperiod 0400-2200 h, in a screen that allowed lighting from above only. Numbers of adults on each leaf portion were counted without disturbance about 0900 h the following morning, and numbers of progeny were counted 24 h later, i.e. 40-42 h after release of the adults.

In each experiment, a x2 test was done for homogeneity of the summed numbers of adults amongst the types of leaf portion, eight or ten wheat stocks X proximal or distal portion, but analysis of variance was used for the numbers of progeny. The summed numbers of adults and progeny on seven stocks were used, after transformation as dn+O.5 and log, (n+l ) respectively, as basic data for overall analysis of variance combining three experiments from each of the three seasons, late summer, autumn and mid-winter.

Screening tests Resistance to S. avenae was also assessed simply in tests carried out as described by Lowe

(1 984a). Winter wheat was sown in trays of paper pots and after seedling emergence (1 0-1 4 days from sowing) was vernalised for 8-9 wk. Vernalised plants were potted into 8 .5 cm pots of John Innes No. 2 compost in batches and grown on in the glasshouse, under the conditions described above. Each batch contained 15 plants of each wheat stock, labelled with individual

532 H. J . B. L O W E

randomised numbers to conceal their identity. Compound fertilisers were applied in solution treating each batch as a unit, providing a regime comparable to that described for individually treated plants. Thus batches tested when young (2-9 days from potting) received no additional fertiliser, whereas the oldest batches received three treatments before testing. Ethirimol sprays were used as necessary to control mildew.

Batches to be tested were moved to another glasshouse and set on a subirrigated bench with pots packed together for maximum contact. Aphid-infested plants from mixed-age cultures of S. avenue clone CASS were shaken over each batch to give the initial infestation, and the number of aphids on each plant was scored on a 0-9 scale, 9 = very many aphids, after 8-15 days. Where different ages were compared, two or three batches placed side by side were infested together and subsequently scored on the same day. For each batch, the mean score was found for each stock and also the percentage of the 15 plants scored as resistant (G4). These quantities were used as data for further analysis, the latter after angular transformation.

RESULTS

Screening tests carried out between 18 May and 4 October 1983 demonstrated differences in resistance to S. avenue among the eight wheat stocks used for more detailed study (Table 1). Three tests completed in August had ears emerged on the majority of plants (G.S. 58) with a varying proportion past anthesis (G.S. 68); tests before and after this were scored with the plants at booting (G.S. 41-45). There were no variations in assessment attributable to the differing states of development of the plant, and results from all nine tests were combined for analysis. Maris Huntsman was the most susceptible and Rapier the most resistant cultivar. There were no significant differences (at P = 0.05) among the five resistant stocks, and whilst the ranking of all eight was essentially the same with either mean score or the proportion of ‘resistant’ plants (Lowe, 1984a), separation of the susceptible cvs was greater with the mean score.

Table 1 . Assessments of resistance lo S . avenae from screening tests, average values of mean score and percentage of apparently resistant plants in nine tests

Stock Mean score* % ‘resistant’ plants (angles)

Maris Huntsman 6 . 3 Armada 5 . 8 Flanders 5 . 3 Bounty 4 . 5 A4501-4E 4 . 4 Virtue 4 . 2 Kador 4 . 0 Rapier 3 . 9

19.9 25.1 32 .4 4 9 . 2 4 7 . 0 5 1 . 9

56 .9 56 .3

S.E. (56 D.F.) 0.15 3 . 3 0

* 0-9 scale, 9 = very many aphids

Antibiosis Aphids stored in the freezer lost only up to 10% of their fresh weight over 2 wk storage,

and aphids from the stored material had the appearance of freshly killed aphids when examined. In contrast, aphids reweighed after 18-20 days storage appeared flaccid. As the effects of storage were deliberately confounded with variation between replicates, it was not likely that weight loss in storage affected the conclusions drawn below.

Characteristics of grain aphid resistance in winter wheat 533

Growth and development of S. avenae were affected by antibiosis similarly among the eight wheat stocks (Table 2), although these aspects of aphid performance may respond independently to environmental variation (Dixon, Chambers & Dharma, 1982). The stocks differed consistently among experiments and in analysis of separate experiments, variation among stocks had significant effects (at P = 0.001) on aphid numbers in all but two experiments and biomass in all ten. Cvs Maris Huntsman, Armada and Flanders were susceptible in all respects, and Virtue and Rapier were resistant. Bounty was also resistant, but less so than Rapier and Virtue as, on both leaves and ears, the average biomass was greater on Bounty than on the other two cvs and on ears, more aphids were produced on Bounty. Contrary to expectation, Kador was as susceptible as Maris Huntsman and Armada, and the non-glaucous line A4501-4E was susceptible to aphids on leaves although in tests of ears it was as resistant as Bounty. The leaves were little affected by trimming and caging, and estimated loss of healthy leaf area from all causes was 1-5% except in the first experiment (February-March) where 5-1 0% of leaf area had deteriorated.

Table 2. Measurements of the performance of S . avenae on ears and leaves of winter wheats, as production from a single caged nymph

Number of aphids Biomass, mg Number of adultst --- Stock on flag leaf on ear on flag leaf on ear on flag leaf on ear

Maris Huntsman Armada Flanders Bounty

Virtue Kador Rapier

A4501-4E

53.2 93.9 31.5 43.7 47.7 85.9 26.4 42.6 47.5 81.7 28.6 41.5 41.7 69.1 23.8 34.8 47.4 67.0 30.4 36.9 39.6 55.5 20.2 29.8

39.9 53.3 20.4 26.8 50.7 94 .0 29.3 46.5

2.22 3.10 1.82 3.04 1.93 2.94 1.66 2.47 1.87 2.51 1.47 2.03 2.02 3.17 1.62 2.13

S.E. (388 D.F.) 1.12 1.78 0.92 1.240 0.062 0.069

t Transformed as d m , measures relative rate of development

In multi-factorial analysis, the four experiments made on flag leaves were combined with the four on ears in pairs for successive times from February-March to May to allow for any seasonal effects. The differences among stocks were very highly significant with each of the three measures of aphid performance (Table 2). In weighted analysis, used to correct for the different effective durations of the separate experiments, the interactions of wheat stocks with plant organs were significant at P = 0.001; values of F, with 7 and 388 D.F. were 5.92, 7.37 and 9.13 respectively in analysis of biomass, numbers of adult aphids and total numbers of aphids (both the latter as dn+0.5) . These significant interactions were traced to disproportionately high numbers of aphids on ears of susceptible cvs, for adult aphids on Armada only and for total numbers on Armada, Maris Huntsman and Kador, whilst A4.501-4E had fewer aphids on the ears in proportion to the other stocks. Biomass was disproportionately low on ears of A4501-4E and high on ears of Armada and Kador. Although total numbers of aphids showed significant interaction of plant stocks with times, this was due to variation of resistance in Rapier and Virtue relative to each other. There was no seasonal trend.

Antixenosis The x2 tests of the summed distribution of the adults in the separate experiments usually

indicated significant departures from homogeneity (at P = 0-05), although this was not

534 H. J . B . LOWE

Table 3. Settling of S. avenae on detachedportions of wheat leaves, mean total numbers of aphids from repeated experiments

Adults* Deposited progeny* Distal Proximal Distal Proximal

Wheat stock portion portion Mean portion portion

Armada Maris Huntsman Bounty A4501-4E Virtue Kador Rapier

S E

6.24 4 .90 5.36 4.38 4 .04 3 .75 3.71 2.41 4.16 3.76 4 .10 3.42 4.64 3.04

7

0.254

5.57 4.87 3.40 3.06 3.96 3.76 3.84

0.180

5.12 4.51 4.76 4.41 4.22 4.00 4.12 2.97 4.27 4.09 4.23 3.65 4.40 3.67 C.

T

0.131

Mean

4.81 4.59 4.11 3.55 4.18 3.94 4.04

0.093

* Adults as \/(n+0.5), progeny as log,(n+l).

invariably so. Results from individual replicate boxes were very variable. However, analysis of the numbers of progeny in separate experiments showed a significant effect of varieties more often than was evident from the distribution of adults. Cv. Flanders was moderately susceptible but was dropped from the winter experiments.

Combining results from all three seasons, the distributions of adults and of their progeny on the second day were similar (Table 3). Contrary to the results of the antibiosis tests, Kador and A4501-4E were the most resistant stocks, and average numbers of both adults and progeny on A4501-4E, 9 . 7 adults and 43 progeny per experiment, were significantly less than on Kador, with 14.9 and 70 respectively. Armada was the most susceptible stock and, on average, had significantly more adults than Maris Huntsman, 32.7 and 24.5 aphids per experiment respectively. The leaf portions of both these susceptible cultivars had significantly more adults and progeny than the five resistant stocks.

Differences in the numbers of aphids between proximal and distal leaf portions were significant in the earlier experiments but not in the later, wintertime experiments (Table 4). In the late summer and autumn there were more aphids on distal portions, on average 1.87 adults and 7 .9 progeny per portion, than on proximal portions, 0 .98 adults and 4 .1 progeny. Overall, there were significant interactions between leaf portions and seasons, F = 11 . O for

Table 4. Total numbers of progeny of S. avenae deposited on detached portions of wheat leaf at diferent seasons, transformed as log, ( n + l )

AugustSeptember November Decembcr-January Wheat Distal Proximal Distal Proximal Distal Proximal stock portion portion portion portion portion portion

Armada 5.05 3.98 5.12 4.35 5 .18 5.19 MarisHuntsman 4.63 4.01 4.79 4.20 4.87 5.02 Bounty 3.96 3.80 4.35 3.68 4.33 4.54 A450 1-4E 3.83 2.81 4.27 2.61 4.27 3.50 Virtue 4.01 3.74 4.41 3.98 4.38 4 .56 Kador 3.59 2.81 4 .50 3.63 4 .58 4.52 Rapier 4 .16 3.62 4.39 3.23 4 .65 4.17

4.18 3 .54 4.55 3.67 4.61 4.50 Mean

S E 78 D F for any comparison within Seasons = 0 . 2 3 (L.S.D. = 0.65)

Characteristics of grain aphid resistance in winter wheat 535

Table 5. Assessments of resistance to S. avenae in winter wheats by glasshouse screening at diflerent plant ages

/ Plant ages \

G.S. <30 G.S. 30-37 G.S. 39-45 Mean % ‘resistant’ Mean % ‘resistant’ Mean % ‘resistant’

Cultivars Score plants (angles) Score plants (angles) Score plants (angles)

Maris Huntsman 5 . 8 34 5 .7 30 6 . 6 19 Joss Cambier 5 . 6 32 6 . 2 14 6 . 3 15 Armada 5 . 3 43 6 . 0 23 6 . 0 20

Bounty 4 . 2 54 4 . 6 46 4 . 5 52 Kador 4 . 1 55 4 . 1 56 4 . 0 57 Rapier 4 . 3 55 4 . 0 63 3 . 6 74

S.E. (60 D.F.) 0.21 5 . 0 0 . 2 4 5 . 6 0 . 2 0 4 . 5

adults, 10.4 for progeny, 2 and 78 D.F., and there was an interaction of wheat stocks with leaf portions for progeny only, F = 3.34, 6 and 78 D.F.

The interaction between wheat stocks and leaf portions was significant, F = 4.35, 9 and 114 D.F., in analysis of totals of adults (as dn+O. 5 ) from seven winter experiments, but was not significant in the summer and autumn. In the winter, there were significantly more aphids on proximal than distal portions of leaves from the susceptible cv. Joss Cambier and also a greater number on proximal portions of Armada, whilst there were more aphids on distal portions of leaves from only Rapier, A4501-4E and another non-glaucous line. Both Armada and Kador had significantly more aphids on distal than proximal portions in the summer and autumn experiments, but in mid-winter only A4501-4E had significantly fewer aphids on proximal portions.

Plant age Between 27 April and 30 July 1982 six screening tests included three resistant and three

susceptible winter wheat cvs at three plant ages, namely tillering, and early and late during stem extension (Table 5 ) . The ranking of the six cvs was unaffected by the age at which the plants were tested. However, the resistant cvs differed most from the susceptible stocks in the oldest batches that reached G.S. 39-45. With these well advanced batches there was complete and significant separation (at P = 0.05) of resistant from susceptible cvs and some differentiation within the groups; the mean score for Bounty was significantly higher than that for Rapier, and Rapier had significantly more plants scored as resistant than any other cultivar. In contrast, the youngest plants showed significant separation between the groups of resistant and susceptible cvs only with the mean scores. Batches of intermediate age that were included only in four experiments gave an intermediate separation of cultivars.

DISCUSSION

Cvs Bounty, Virtue and Rapier possessed antibiosis to S. avenae and their resistance was also evident as moderate antixenosis. Bounty differed consistently from Rapier and Virtue as it was less resistant in screening tests and in the antibiosis experiments, but performed similarly in the tests of antixenosis. In the field Bounty has tended to be more resistant than Kador (Lowe, 19826; 19846 and in a field experiment in 1982, Rapier and Virtue were susceptible relative to Bounty (Lowe, 19846). Taken together these observations give good reason to propose a difference between the resistance of Bounty and that of Rapier and Virtue.

536 H . J. B . L O W E

In contrast, cv. Kador and the breeding line A45014E were resistant primarily because of antixenosis. Resistance in these two stocks is most likely, therefore, to have a different basis from that of Bounty, Virtue and Rapier. Although antibiotic effects have been detected on Kador in growth-room experiments with young plants (Kay, Wratten & Stokes, 1981; Lee, 1983) and on maturing plants in the field (Stokes, Lee & Wratten, 1980; Lee, 1984), they were absent in the present experiments. A4501-4E differed from Kador in two respects. Antixenosis in A4501-4E remained effective even in the mid-winter tests, when the resistance of proximal relative to distal leaf portions was evident only with A4501-4E. The resistance of Kador varied markedly between the differing environmental conditions, and was most effective in summer. Line A4501-4E also showed antibiosis, similar to that of Bounty, in its ears but not in flag leaves. This difference between parts of the plant did not occur in any of the other cultivars tested, including Kador. These observations therefore indicate a difference also between the resistance of A45014E and that of Kador. Not enough is known of leaf physiology to relate either the differences in antixenosis between proximal and distal portions or the changes observed between seasons to any cause of resistance.

The identification of sources of heritable resistance is the primary step in breeding pest- resistant cultivars, and in the case of resistance to S. avenue in T. aestivum, this proved easy once test methods had been developed (Lowe, 1981u, 1982a, 1984u, c). However, efficient exploitation of these potential sources requires early determination of homologies among them. Whilst the present experiments could indicate simply two primary types of resistance, antibiosis and antixenosis, with their expression varied by modifying genes in the different stocks, the other differences between stocks in expression of resistance were considerable. As these sources of resistance to S . avenue are directly available to wheat breeders in highly adapted germplasm, it would be better in the absence of further information to treat them as providing at least four different sources, namely from A4501-4E, Kador, Bounty and Rapier or Virtue. Clarification of the identity and inheritance of resistance in these and other source stocks will require closer definition of the effects of environmental and developmental variables.

Screening tests, whether in the glasshouse or in the field (Lowe, 1984b, c), do not permit separation of the different types of resistance. In the field, large scale screening for resistance to S. avenue would most probably depend on observation after ear emergence, when the aphids become easily visible as they colonise the ear and rapid scoring of whole plots is possible (Lowe, 1984~). The advantage in the glasshouse of screening plants well advanced towards this stage was confirmed here with winter wheat, since the differences of infestation especially between resistant and susceptible cultivars were greater on older plants, as they were with spring wheat (Lowe, 1984~) .

I f a number of different types of resistance exist, variation in the results of experiments that do not separate resistances is likely to occur, especially in the field where the environment is more complex and less stable than in the glasshouse. Such variations have occurred, for example, in the relative resistance of the two spring wheat stocks 320/30 and 708/41 (Lowe, 1984~ ; Lowe & Acreman, 1984) and in differences between field tests of screening methods using winter wheats. Rapier and Virtue appeared susceptible to introduced aphids in the field in 1982 when there were a number of severe thunderstorms during the development of S. avenue populations (Lowe, 19846). This was not confirmed in 1983 which was remarkable for warm, calm weather. At G.S. 66-68, Virtue had on average 6 .1 S. avenue per shoot, similar to Rapier and significantly less than the most susceptible stocks that had about 15 aphids per shoot (Lowe, 1984~) . Similarly, A4501-4E was strikingly resistant in the 1982 field tests and in 1981 (J. Bingham, personal communication), but was no more resistant to introduced aphids in the field in 1983 than in glasshouse tests (Lowe, 1984~). Numbers of S. avenue that developed naturally on plots of A4501-4E and Rapier in yield trials at three sites near Cambridge in 1984 were about half those on Maris Huntsman, Armada and Avalon (H. J. B. Lowe, unpublished). Variations of this kind may best be interpreted as due to

Characteristics of grain aphid resistance in winter wheat 537

interactions of environmental variation with different types of resistance. They present a powerful argument for a n attempt to combine these resistances through plant breeding to give stocks with greater resistance than those identified hitherto.

The technique of screening for resistance to S. avenue in the glasshouse has been used routinely (Lowe, 1984e) and methods for field screening are available although not tested on a large scale (Lowe, 1984b, c). The identification of four probably different germplasm sources among a sample of five resistant winter wheat stocks, all suited to modern British agriculture, is indicative of a rich resource of heritable resistance to S. avenue. This resource could be exploited in wheat breeding by use of the recently developed techniques of screening to produce resistant varieties. Improved resistance in the wheat crop would offer substantial advantages in developing effective control of S. avenue whilst avoiding undue use of insecticides (van Emden, 1983) and in this, would support forecasting (Carter, Dixon & Rabbinge, 1982) and the beneficial effects of natural predators (Carter, Gardner, Fraser & Adams, 1982; Chambers, Sunderland, Wyatt & Vickerman, 1983).

It is a pleasure to acknowledge the substantial technical contribution made by Mrs C. D. Scarisbrick and Mrs L. M. Patten in this work.

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(Received 23 May 1984)