Case Study: Gardner’s grid system and plant selection efficiency in cotton (Verhalen et al., 1975)
5.5 Evaluating Resistance
5.5.2 Evaluating antixenosis and antibiosis
During a breeding programme plants may be screened for resistance to insect pests but most experimental work on resistance mechanisms takes place either with released cultivars or with resistant culti- vars prior to release. Experiments to evalu- ate antixenosis and antibiosis are usually labour intensive and can only be carried out on relatively few cultivars and hence are rarely included in routine screening programmes (although there are excep- tions, e.g. screening in alfalfa and rice).
Antixenotic resistance can be assessed in preference tests either in a choice or no- choice situation (Ng et al., 1990; Jackai, 1991) or by comparing the behaviour of the insect on plants having a range of sus- ceptibilities. For example, experiments can be designed which compare the num- ber of adults alighting on plants (e.g. Singh et al., 1994) or their oviposition response
Susceptible cultivar
(New generation)
Antixenotic cultivar
Time
ETL
Pest density
Susceptible cultivar
Antibiotic cultivar
ETL
Time
Time
Tolerant ETL
Non-tolerant ETL
Pest densityPest density
Fig. 5.16.Effects of three functional resistance mechanisms on pest population dynamics where the top graph shows the effects of antixenosis; in the middle, antibiosis; and at the bottom, tolerance (modified from Kennedy et al., 1987).
on a range of cultivars, either in a choice or no-choice situation (e.g. Jager et al., 1995). The disadvantage with no-choice situations is that spurious results may be obtained if the response of the insects to a particular cultivar is conditioned by the presence of other cultivars. Tests such as these have been carried out in both the laboratory and the field. The importance of choice in influencing insect response to resistant and susceptible plants in the field has been demonstrated in the work of Cantelo and Sanford (1984). Intermixed and isolated pure stands of resistant and susceptible varieties of potato, cabbage and lima beans were sown to test for resis- tance to the potato leafhopper, Empoasca fabae, the imported cabbage worm, Pieris rapae, and the Mexican bean beetle, Epilachna varievestis, respectively. The
response of the insects to the mixed and isolated pure stands was different in each case. The plant array did not have a signif- icant effect on the size of the Mexican bean beetle population in the resistant and susceptible cultivars of lima bean, but antixenosis was more apparent against the potato leafhopper in isolated pure stands of susceptible and resistant potato lines than in mixed resistant and susceptible lines. The opposite was true with the imported cabbage worm on cabbage. When planted in a mixed stand the preference for egg deposition on susceptible cultivars was greater than in pure isolated stands of either resistant or susceptible types.
This example illustrates the differences between insect responses to their host plants and the need to identify the effects of choice on the preference before Ð0.4 Ð0.2 0 0.2 0.4
Development time
Juvenile
mortality Fecundity
Adult mortality 3.5
3.0
2.5
2.0
1.5
1.0
0.5
Rate of increase
Proportion change in parameter
Fig. 5.17. The effect of changes in life history parameter values on the intrinsic growth rate of a pest population. The zero value for each parameter relates to that given in the text: development time (2 weeks), fecundity (140 eggs per week), juvenile mortality (0.5 per week) and adult mortality (0.5 per week). The population growth rate with these parameter values is r = 1.6 (after Thomas and Waage, 1996).
designing experiments to evaluate the potential of antixenosis as a resistance mechanism.
Antixenosis has also been measured in terms of the number of insects leaving cul- tivars, both in the laboratory and the field.
The rationale behind this approach is that insects that have located a susceptible plant will be less inclined to leave it than an insect on a resistant plant, hence the numbers leaving susceptible and resistant plants should differ. However, differences in both the field and the laboratory are not necessarily large. Müller (1958) observed the arrival and departure of the black bean aphid, Aphis fabae, on two bean cultivars in the field, a resistant Rastatter and a sus- ceptible Schlandstedter. He observed that while equal numbers of alatae land on both varieties only 1% remained to reproduce on the resistant variety while 10%
remained on the susceptible variety. The mean staying time for those adults that landed and then took off again was higher on the susceptible beans (6!s min) than on the resistant beans (3!s min) and these dif- ferences were significant. So even though large numbers landed on both cultivars the difference in the numbers taking off was quite small, although sufficient to account for differences in population sizes on the two cultivars. A laboratory study of the reproduction and flight of alatae of the grass aphid, Metopolophium festucae cere- alium, from a number of grasses and cere- als also indicated that adults even reselect after settling on susceptible host plants (Dent, 1986). Most alatae flew from the grasses Festuca rubra (94%) and Festuca arundinacea(84%) after producing only a few nymphs. Whereas between 38% and 64% of alatae flew from Lolium multiflo- rum, oats, Lolium perenne and wheat, these alatae deposited more nymphs on the hosts before flight. This study indicated the importance of measuring both the flight response and reproduction, since an evalu- ation of flight response alone might provide misleading information about antixenotic resistance. Also the ranking of host according to alatae nymph production
differed from that of apterous virginoparae on the same host plants (Dent and Wratten, 1986) emphasizing the dangers inherent in assessing host resistance only in terms of the antibiotic effects on the non-selective life stage of an insect, e.g. apterous aphids or lepidopterous larvae.
Antixenosis, and to a lesser extent antibiosis, is often evaluated by studying the behaviour of insects on potentially resistant and susceptible cultivars. An introductory guide to measuring behaviour has been written by Martin and Bateson (1993) and further more detailed studies relevant to resistance studies are given in Wyatt (1997) and Eigenbrode and Bernays (1997). A few elementary points are men- tioned here in relation to evaluating resis- tance mechanisms.
The initial temptation to devise behav- ioural experiments testing for resistance must be avoided until the observer has had time to observe the insect and its interac- tions with the plants in question. This period of preliminary observation enables the observer to become acquainted with the insect’s behaviour and enable better formu- lation of appropriate questions and ensure the correct choice of measures and record- ing methods.
The first step after this observation involves describing the insect’s behaviour.
Martin and Bateson (1993) describe behav- iour in terms of structure, consequences and relations. The ‘structure’ describes the behaviour in terms of the subject’s posture and movements, e.g. flying, walking, feed- ing. The consequences are the effects of the insect’s behaviour on the plant (and vice versa), e.g. insect impaled, insect takes off, whereas the relations describe where, when and with whom an event is occur- ring, e.g. the insect on the adaxial leaf sur- face. Behaviour is, of course, continuous but it must be broken down and divided into discrete units to allow it to be mea- sured. There must be enough measurement categories included to describe the behav- iour adequately, and each should be pre- cisely defined to allow any other experimenter to make the same observa-
tion. There are four types of behavioural measure: latency, duration, frequency and intensity. Latency is the time from some specified event to the onset of the first occurrence of a behaviour. Givovich et al.
(1988) measure the behaviour of 50 alatae of Aphis craccivora, on each of three cow- pea lines and found that once transferred to the plant the aphids took longer to decide whether to feed on the two resistant than on the susceptible line. The measure of latency was also combined with a mea- sure of duration to assess the response of the aphid to the cowpea lines. Duration is the length of time for which a single occur- rence of the behaviour pattern lasts, and in the above experiment duration, total prob- ing time was longer on the two resistant than the susceptible lines. The frequency is
the number of occurrences of the behaviour per unit of time. The most common fre- quency measure used in behavioural stud- ies of homopterans is the number of probes made per unit time. Intensity, in contrast to the other behavioural measures, has no universal definition and is a subjective assessment, e.g. the extent of restlessness.
This type of measure should be avoided if possible unless it can be scored in terms of the number of movements of a particular kind per unit time. For example, Bernays et al. (1983) observed the climbing speed of Chilo partelluslarvae at different tempera- tures on two different sorghum cultivars (Fig. 5.18). Behavioural studies are often aided through the use of physical models (e.g. Harris et al., 1993; Vaughn and Hoy, 1993), electroantennagrams (EAG) or single
Fig. 5.18. The climbing speed of newly hatched larvae of Chilo partellus on two sorghum cultivars (CV.IS 1151, s; CV.IS 2205, d) over a range of temperatures (after Bernays et al., 1983).
cell recording (SCR) (Blight et al., 1995;
Pickett and Woodcock, 1993) and video recording (see Wyatt, 1997 and Eigenbrode and Bernays, 1997).
Tests for antibiosis mechanisms of resis- tance are usually carried out under no- choice conditions, with the insects confined on plants or plant materials inside a cage. Most cages consist of a fine mesh material that can be used to cover the plant or plant part as a sleeve, or a cage either to cover the whole plant or to isolate specific areas such as a section of leaf.
Tests for antibiosis among plant culti- vars usually assess the performance of pest individuals to obtain a mean estimate of development, reproduction and survival.
Insect development can either be measured as a rate or expressed in terms of insect size or weight. The development rate is usually considered in terms of the length of time taken between one stage and another on resistant and susceptible cultivars. For instance, the larval and pupal periods were shorter and adult longevity longer for indi- viduals of Chilo partellusreared on suscep- tible maize than on resistant maize (Sekhon and Sajjan, 1987). The more resis- tant varieties also reduced larval weight by 51 to 60 mg per larva and pupal weight by 49 to 52 mg per pupa. The lengths of insects have also been used as a measure of size in resistance studies, both in the bean flower thrips, Megalurothrips sjostedi (Salifu et al., 1988) and in the leafhopper, Amrasca devastans, a pest of okra (Uthamasamy, 1986).
Differences in antibiotic resistance between cultivars can also be assessed by measuring the fecundity and fertility of insects. This can be extended to popula- tion effects by taking mortality into account and obtaining rmvalues (birth rate – mortality rate; Birch and Wratten, 1984;
Holt and Wratten, 1986). Life table analy- ses have also been used to evaluate culti- vars for antibiotic resistance (Easwaramoorthy and Nandagopal, 1986).
Tests to determine antibiotic resistance may be undertaken utilizing artificial diets, leaf discs, excised leaflets,
lyopholized resistant plant materials or membrane filters with incorporated leaf extracts (e.g. Wiseman, 1989; Allsopp et al., 1991; Allsopp, 1994; Hammond et al., 1995). In addition a variety of innovative techniques have been developed measur- ing electrical signals to study insect prob- ing behaviour (e.g. Kimmins, 1989), the activity of enzymes (e.g. Wu et al., 1993) and honeydew excretion (e.g. Pathak et al., 1982; Cohen et al., 1997).
5.5.3 Morphological and biochemical