Leaf quality and behaviour

Field observations at Warwick HRI conducted between 2005 and 2008 (discussed in Chapter 6) indicated that there was some influence of plant quality on the inter-plant distribution of thrips with a clear preference for young and fresh plants over older plants or those in poorer condition. This experiment was

undertaken to further investigate this phenomenon, and the potential effect of plant quality, on thrips behaviour. The data obtained show a clear correlation between what has been observed in the field and this experiment. Thrips exposed to leek

81 foliage of varying quality behaved very differently. On rotten leaves the thrips‟ behavioural repertoire was seriously curtailed, they did not engage in any feeding- related behaviours at all. Although both types of feeding-related behaviour, were observed on dry leaves, the number of instances was so low that in fact they might be considered insignificant (Table 4.14). The fact they were observed at all lends weight to the argument that, as discussed in section 4.4.2, thrips are capable of engaging in all their behaviours at 20oC but are, in this case, restricted by a secondary factor, the quality of the host plant. Overall changes in observed

behaviour were also considerably reduced on the two low-quality samples of leek, again indicating that the host plant was unsuitable. It might be expected that an unsuitable host plant would encourage increased emigration, yet flight activity showed a significant increase only on dry leaves. Rotten leaves induced comparable, though slightly lower, levels of flight activity to healthy leaves. One explanation for this might be that dry leaves represent a more pressing threat to thrips survival than leaves which are merely of poor quality but still moist. Microclimates formed around the leaf surface may be more similar in humidity and temperature on rotten leaves than on dry ones and an increased danger of desiccation may be a more pressing incentive to emigrate.

Behaviours on the oak leaf samples were much as might be expected for a non-host plant. Behavioural repertoires were curtailed, with no feeding or feeding related behaviours at all, and the overall number of instances of behaviour was much lower than on leek leaves. The reason for this, and also the significantly increased incidence of flight activity, which was twice that on healthy leek leaves, is the

unsuitability of the host material and the capacity of the animal, at 20oC, to engage in flight behaviour as a means of locating a new host plant. It is highly likely that, if the observation period on oak leaves were to be extended a little longer, all the thrips would have been observed engaging in flight. Riefler and Koschier‟s (2009) newly published paper on behavioural patterns of T. tabaci on leek and cucumber also identifies differences in the amount of time the thrips spent in various behaviours on two different crops, indicating that the host plant environment has a direct influence on behaviours.

82 4.4.4 Behavioural sequences

There are sequences in the order in which thrips engage in their behaviours and aspects of these behaviours and sequences are influenced by environmental temperature. On a fundamental note, the majority of observed behavioural sequences are what one might expect to see, for example, the high percentage of thrips engaged in wing grooming directly before flight.

Limitations imposed by the size of the dataset required to confidently and robustly identify sequences involving multiple behaviours over extended periods means that only direct sequences of two behaviours were considered here. Despite this, it is clear that temperature has a direct effect on such sequences and how they are expressed. The sequence highlighted in Figure 4.16 illustrates this well. The relationship between two behaviours was not merely an expression of the varying number of behaviours exhibited at the different temperatures, since in many cases, sequences became more or less prominent as temperatures changed. An example is the relationship between grooming and walking, which accounted for only about 16% of transitions at 7.5oC and yet accounted for about 27% at 25oC, a clear growth in association between these behaviours as temperature increased. The reason for this discrepancy may be that certain groups of behaviours are naturally expressed together in order to achieve certain goals. For example, searching must be associated with feeding if nutrient and energy gathering are to be successful. As temperatures change, it may well be that relationships within these behavioural complexes change too. An example would be that as temperatures become more conducive to feeding, then locomotive behaviours will become increasingly associated with searching behaviours so that suitable feeding sites can be located. If such thinking is applied to the observed diel periodicity in intra-plant distribution, for example, as discussed further in Chapter 5 then daily changes in temperature can be seen as the direct instigator of necessary pre-flight locomotion by which the thrips reach suitable positions on the plant for take-off.