Efficacy of plant protection substances against virus transmission by aphids infesting potato

Wirkung von Insektiziden auf die Übertragung von Viren durch Kartoffel besiedelnde Blattläuse

Material and methods

After starving for 2 ± 1 h a single aphid was carefully placed, with the help of a micro aspirator, in the middle of each leaf disc. Then a 15 minute video recording was made of the behaviour of each aphid.

Unlike the method described by Powell et al. (1995) the video cameras were not each placed at a right angle above a leaf disc, but at an angle of 45°.

Both adult apterous and alate viviparous aphids of both species were used. In order to standardize alate aphids in terms of their tendency to fly or settle they were taken from the culture and transferred to another cage containing potato plants. After they settled on the plants these aphids were removed and starved prior to use in the experiments. The experiments were done at two different temperatures: 16 ± 1°C and 25 ± 1°C.

The results for aphids that did not place their rostrum on the surface of leaf discs during an experiment were not included in the analysis.

The results indicated that the behaviour of different sized aphids was similar. Both, the number of probes as well as the mean duration of probes did not differ significantly. The adult aphids showed differences in reproduction and in the ability to resist starvation (or water loss). These characters are unlikely to affect the behaviour relevant to the question being addressed.

Large aphids can only be produced by either rearing aphids in isolation, in small colonies or at a low temperature. Because of this and the above results weight of the aphids was not standardized in the following experiments.

Comparison of EPG (electrical penetration graph) and video recordings indicate that the behaviour of aphids with a gold wire attached to their dorsum is affected. Aphis fabae that are not attached to a wire start probing sooner and penetrate their host plant (Vicia faba) much faster (Hardie et al., 1992). Although the mean duration of the probes of unwired aphids was shorter their total duration was longer. Experiments with Rhopalosiphum padi also indicate that wired aphids ingest less phloem sap and excrete less honeydew (Prado, 1997). Own experiments with A. frangulae and A. solani indicate similar effects of wiring on the behaviour of aphids (not presented here). They were restricted in their movement and tried to release themselves from the wire when observed over a long period. Therefore the use of the EPG-technique is inappropriate for studying behaviour, for which video recording is more suitable.

Aphids monitor a host plant before colonising it. According to Klingauf (1987) this selection procedure consists of several different phases:

Each aphid goes through this procedure before accepting a plant and reproducing. For the acquisition or transmission of non-persistent viruses aphids have to penetrate plant tissue with their stylets (probe). The different phases in the selection procedure are correlated with the position of the antennae (Fig. 1). When examining the leaf surface aphids point their antennae in the direction in which they are walking and keep the rostrum ventrally docked (Fig. 1A). When probing aphids keep their antennae upright or place them slowly along their back (Fig. 1B, 1C) (Klingauf, 1987; Hardie et al., 1992; Powell et al., 1993). Our analyses indicate that aphids not only probe with their antenna in this position, but also often keep their rostrum in a stationary position (ventrally) when their antennae are held along their backs (Fig. 1D). The experimental set-up described by Powell et al. (1995) results in an artificially 10% increase in the number of probes recorded (see Fig. 1D). This was avoided in this study by recording the behaviour using cameras placed at an angle of 45°.

Fig. 1. Classical features used to characterize feeding behaviour (short distance).

The death of an aphid was not recorded in any of the experiments. Not presented here, the first dead aphids (show no response if touched) were noticed after 60 minutes. Exposed to Ripcord 10 the aphids reacted to both dosages (900 and 180 ml/ha) after 10 minutes, as their movements became uncoordinated (staggering, rushing, alternating contraction of legs). Often they adopt a posture that reduces the contact of their body with the leaf surface (Fig. 2). This is described as a “pain position”, and is adopted by animals living in hot environments biotopes (e.g. lizards and darkling beetles in the Namib Desert of South-West Africa). Also the sycamore aphid, Drepanosiphum platanoides, behaves similar when exposed to strong sunlight and the ambient temperature is high (Dixon, pers. comm.).

Fig. 2. „Pain position“ of aphids on leaf discs treated with Ripcord.

After transfer to leaf discs all the aphids paused for a short period in a resting position (Fig. 1D) and then tested the different sources of stimuli. For this they palpated the leaf surface with the distal end of their rostrum (Fig. 3) and their fast moving antennae were pointed forward. Analyses of EPG-curves show that the stylets at this time are not penetrating the plant tissue. It is possible that, although taken carefully from the culture, their mouth parts were damaged. Such aphids never touched the leaf surface with their rostrum and were not included in the analysis.

Fig. 3. Apical segments of rostrum (left) and enlarged view of last rostral segment (right) with sensillae (S) at the distal end.

In relation to A. solani, A. frangulae is smaller and less mobile. At 16°C apterous females of A. frangulae took longer to start probing (p=0.014) and made fewer probes (p=0.006). The mean duration (p=0.001) and mean total duration of their probes (p=0.001) were longer. During the experiments apterous A. frangulae spent 74.8% of the time probing and apterous A. solani 44.2% (Fig. 4A and 5A).

Fig. 4. Effect of insecticides on the number and duration of the probes made by apterous (A) and alate (B) viviparous Aphis frangulae (total duration: 15 min.).

Fig. 5. Effect of insecticides on the number and duration of the probes made by apterous (A) and alate (B) viviparous Aulacorthum solani (total duration: 15 min.).

At 25°C the differences in behaviour between the species was smaller. Significant differences were found in the mean duration of probes (A. frangulae 421 sec. and A. solani 589 sec.; p=0.016). During these experiments A. frangulae spent 46.6% of the time probing and A. solani 63.6%.

At 16°C alate females of A. frangulae started probing later (p<0.001) and made fewer probes than those of A. solani (p<0.001). Whereas the mean duration of probes of alate A. frangulae lasted longer than those of A. solani (p=0.011), the differences in the mean total duration are not significant (p=0.277). Alate A. frangulae and A. solani spent 42.7% and 39.4% of the time probing, respectively, at this temperature (Fig. 4B, 5B).

At 25°C alate females of A. solani started probing sooner (p=0.049) and probed more frequently (p=0.001) than A. frangulae. Statistically the differences in mean duration and mean total duration of probes are insignificant. The alate females of A. frangulae and A. solani spent 43.6% and 40.0% of the time probing, respectively, at this temperature.

After the start of the first probe alate aphids did not differ in the duration of probes at both temperatures. This is most likely associated with the dispersal function of this morph. Whereas apterous aphids invest the majority of the energy they consume in reproduction, alatae invest more of this energy in their fat reserves, which fuel their search for a new host plant. By metabolising fat, alate aphids can also obtain water and are thus less affected than apterae by water loss at higher temperatures.

Comparisons of behaviour reveal that alate aphids start probing later than apterous aphids (Table 2). There was a tendency for alate to probe more frequently than apterae. In addition, the duration of successive probes increases on suitable host plants.

The differences in total duration of probes presented in Table 2 indicate that alatae spent more time testing the leaf surface than apterous aphids. In nature the alatae find new food resources. Therefore after their dispersal flight suitable host plants must be identified and colonized. Apterous aphids are born on a suitable host plant and will only occasionally need to search for a new host. These differences in the function of the two morphs are also reflected in the different numbers of rhinaria on their antennae, which are used to process olfactory stimuli.

At 16°C there were no effects of Ripcord 10 to the onset of probing by any morph (Table 3). The number of probes as well as the mean duration and the mean total duration of probes were significantly reduced for all morphs tested with exception of apterous A. frangulae.

At 16°C the inhibition was stronger for alate than apterous A. solani (Table 4). The alate aphids started probing later and made fewer and shorter duration. Confidor delayed the onset of probing by apterous A. frangulae. Alate aphids of this species showed a significant reduction in the number of probes.

At 16°C apterous A. frangulae exposed to Confidor showed a delay in the onset of probing. In both morphs of this species the duration of the probes was shorter after exposure to Ripcord 10 than to Confidor. In contrast to Confidor, Ripcord 10 caused a stronger reduction in the mean duration of probes made by A. solani, but only so for apterous aphids.

The effect of Ripcord 10 and Confidor on the number and duration of probes is presented in Fig. 4 (A. frangulae) and Fig. 5 (A. solani). When exposed to Confidor at 25°C apterous and alate A. frangulae made more probes than on the control leaves and those treated with Ripcord 10. A. solani did differ in its behaviour. Whereas both insecticides caused a strong reduction of the number of probes made by apterous aphids at both insecticides caused a strong reduction of number of probes at 25°C, but not at 16°C. Alate A. solani also made fewer probes when exposed to both insecticides, but Ripcord 10 had a much stronger effect than Confidor.

Confidor is taken up by the plants and translocated there. Aphids also take it up when feeding. This exposure route would need another test design than used here.

In comparison to the control the reduced dosage reduced the time to the start of probing by alate aphids. For both dosages tested the number and mean total duration of probes was significantly reduced compared with the control.

These results indicate that application of reduced dosages will not result in higher transmission rates of non-persistent viruses. But this is only true for this ex­peri­mental set up. There is no evidence that other formulations would differ in their effect on the behaviour of these aphids. Furthermore, it is clear that Ripcord 10 is a very effective contact insecticide. Further experiments should be done using lower dosages or insecticides with other modes of action and other aphid species.