Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK
Corresponding author: firstname.lastname@example.org
Characteristic volatile chemicals are credited with the major role of guiding herbivorous insects to their host plants. Studies comparing how insects find host plants growing in backgrounds of clover and bare soil indicated that it is not chemical, but visual stimuli that govern where the insects land. A mechanism of host-plant finding is proposed in which chemical cues start and end the process of host-plant selection and in which the missing (central) link, based on "appropriate/inappropriate" landings, is governed by visual stimuli. This missing link operates via the insects landing indiscriminately on green surfaces but avoiding brown surfaces, such as soil. Hence, host plants growing in bare soil are colonized by more insects than similar plants growing amongst green non-host plants. Five earlier mechanisms of host-plant finding, plus the "Resource concentration hypothesis" and the "Enemies hypothesis" are discounted. The new theory answers also why wild host-plants growing under natural conditions are not decimated by pest-insects, a question that has puzzled entomologists for decades.
Brassica pests, intercropping, deterrent chemicals, Resource Concentration Hypothesis, appropriate/inappropriate landing theory
Pliny the Younger (23-79AD) wrote in his Naturalis Historiae that when rape (Brassica napus L.) and vetch (Vicia sativa L.) were grown together, many insects that occurred normally on these plants were not found (Schoonhoven et al. 1998). This was probably one of the earliest clues that by increasing plant diversity within a cropping system it might be possible to lower the numbers of pest insects that find crop plants.
In recent years, many researchers have shown that the numbers of herbivorous insects found on crop plants are reduced considerably when the background of the crop is allowed to become weedy, when the crop is intercropped with another plant species, or when the crop is undersown with a living mulch (Finch & Collier 2000).
It has been suggested that when diverse backgrounds of the types mentioned above “disrupt” (Vandermeer 1989) the searching insects, the action is mediated through 1) the non-host plants physically impeding the searching insects (Perrin 1977); 2) visual camouflage (Smith 1976); 3) root exudates from the non-host plants altering the physiology of the host-plant (Theunissen 1994); 4) odours of the non-host plants directly deterring the searching insect (Uvah & Coaker 1984); or 5) the odours of the non-host plants "masking" those of the host plant (Tahvanainen & Root 1972). Two general hypotheses, have also been proposed to explain these reductions in insect numbers. The "Resource concentration hypothesis" (Root 1973), which indicates that more insects are found where the "resource" (host plants) is most concentrated and the "Enemies hypothesis" (Root 1973), which indicates that fewer herbivorous insects are found on host-plants growing in diverse backgrounds, because many of the herbivorous insects are eaten by the higher numbers of predators arrested also at such sites.
In this review, we will discuss briefly the seven hypotheses put forward to date. We will then go on to describe a theory based on “appropriate/inappropriate” landings which we believe is the key, or “missing link”, to host-plant selection by herbivorous insects. A more detailed description of the theory can be found in our original review (Finch & Collier 2000). This greatly shortened version has been included in the current proceedings for completeness, as it was the main subject the senior author was asked to address at the meeting.
Materials and methods
Laboratory and field experiments were done to determine how growing cabbage plants (Brassica oleracea L.) (Cruciferae) in backgrounds of bare soil and subterranean clover (Trifolium subterraneum L.) (Papilionaceae), affected host-plant finding by eight pest species belonging to four insect orders.
All insects were produced in the Insect Rearing Unit at HRI Wellesbourne. The insects tested were the small white butterfly (Pieris rapae L.), the large white butterfly (P. brassicae L.), the cabbage root fly (Delia radicum L.), the mustard beetle (Phaedon cochleariae Fab.), the diamondback moth (Plutella xylostella (L.)), the garden pebble moth (Evergestis forficalis L.) the cabbage moth (Mamestra brassicae L.) and the cabbage aphid (Brevicoryne brassicae L.). Depending upon species, between 30-200 insects were used per replicate in each experiment. The cabbage (Brassica oleracea var. capitata Alep.) plants were grown in 7.5cm pots, tested at the "five true-leaf" stage, and left in their pots throughout the experiments. The laboratory experiments were done in a large rotating cage or in smaller Perspex® cages. The rotating cage (160 cm x 160 cm x 63 cm high) contained a 145 cm diameter turntable, which rotated once every four minutes. The rotation ensured that all treatments placed on the turntables were exposed equally to the insects, which aggregated near the strip lights used to illuminate the test chamber. The Perspex® cages were sufficiently large (80 cm x 48 cm x 54 cm) to house one seed-tray of clover and one of soil. Field experiments were done in large (600 cm x 315 cm x 180 cm high) field cages and in the open field.
In each laboratory test, a pot containing a test plant was inserted in the centre of each seed-tray of clover or bare soil. In each field-cage, 32 host plants were arranged, at 50 cm spacing, in four rows of eight plants; alternate plants being surrounded by either clover or bare soil. Most experiments lasted five to ten days and involved at least ten replicates. The insect eggs were counted daily. To reduce bias, host-plants removed from one background were placed into the opposite background when re-introduced into a test cage. The field experiments were done using sixteen 25m2 plots, arranged as a 4 x 4 Latin square. The four treatments, each replicated four times, were plots of cabbage plants undersown with subterranean clover, plots undersown with white clover (Trifolium repens L.) and two treatments in which the plants were surrounded by bare soil. One of these last two treatments was subjected to the full insecticide and fungicide schedule (positive control) and the other was left unsprayed (negative control).
In all experiments and for all eight species, fewer (P=0.05) eggs (colonies for the cabbage aphid) were found on cabbage plants (15-20 cm tall) surrounded by green clover (10-12 cm tall) than on similar plants surrounded by bare soil (Finch & Kienegger 1997). The percentage reductions in egg numbers ranged from 39±5% for the diamondback moth to 94±3% for the cabbage moth (Finch & Kienegger 1997). When the small white butterfly was presented with cabbage plants of different sizes, the clover (10-12cm tall) reduced the numbers of eggs laid by only 48±4% on 25cm tall cabbage plants and had no effect on 35 cm tall plants. In addition, the numbers of eggs laid by the cabbage root fly, the diamondback moth and the large white butterfly on host plants presented in brown (dead) clover (230±33, 87±9 and 98±15, respectively) did not differ (P=0.05) from those laid (255±46, 81±9 and 94±14, respectively) on host-plants presented in bare soil (Finch & Kienegger 1997).
Results similar to those mentioned above were obtained in field experiments, in which undersowing with clover enabled commercially-acceptable cabbage plants to be harvested, without having to apply insecticide, fungicide or herbicide.
Discussion of the earlier hypotheses
Although authors indicated that diverse backgrounds affected host-plant selection in the ways described earlier, it is hard from both the current results and published data to refute the view that all species are affected similarly (Finch & Collier 2000). Of the earlier proposals, "visual camouflage" implied "concealing an object" (Smith 1976), whereas our mechanism (see later) is dependent on all surfaces being clearly visible to the searching insect. It is doubtful also whether physical interference per se (Perrin 1977) contributes greatly, as the brown clover had the same plant architecture as the green clover, but did not deter the searching insects. Similarly, by leaving the test plants in their pots throughout the experiments, root exudates from the non-host plants could not cause physiological changes in the host plants (Theunissen 1994). In addition, no evidence has been produced during the last 18 years to support the suggestion (Uvah & Coaker 1984) that the non-host plants produce their effects through chemical deterrence. Even backgrounds of plants such as sage (Dover 1986), thyme (Dover 1986) and onions (Uvah & Coaker 1984), selected specifically for their pungent odours, have failed to deter insects from landing on their host plants. The host-plant odour being "masked" by that of the non-host plant (Tahvanainen & Root 1972) also does not seem to be the mechanism, as similar effects were obtained when host plants of the cabbage root fly were surrounded by weeds, spurrey (Spergula arvensis L.), peas (Pisum sativum L.) or clover, all of which have different odour profiles (Finch & Collier 2000). More striking, however, is that the effect was produced also when host plants were surrounded by green, but not brown, plant models (Kostal & Finch 1994), or by sheets of green paper (Ryan et al. 1980; Kostal & Finch 1994 & 1996), neither of which release plant odours. The current differences also cannot be explained by the "Resource concentration hypothesis" (Root 1973), as the host plants are at the same density in both situations. Similarly, the "Enemies hypothesis" (Root 1973) is difficult to support, as it would be against established principles, to suggest that predators are found mainly on the host-plants in the clover when most prey colonize the host-plants in the bare soil. Contrary to the earlier claims (see Altieri 1994), differences in colonization alone appear sufficient to account for the lower numbers of pest-insects found when host plants are grown in diverse backgrounds.
Instead of the seven hypotheses described previously, we believe that a mechanism that we have described as "appropriate/inappropriate landings" is the central link in host-plant selection by insects. Our theory of host-plant selection can be divided into a chain of actions involving just three inextricably-bound links. In the first link, volatile chemicals emanating from plants indicate to flying receptive insects that they are passing over suitable host-plants (Figure 1-1). Once the odour of the host-plant in the air becomes sufficiently concentrated, it induces the insect to land (Figure 1-2). In this way, the volatile chemicals bring the insects into the close vicinity of the host plants. However, during the last few milliseconds, when the insects are only a short (often < 1m) distance away from the plant, instead of maintaining their directed response to volatile stimuli, herbivorous insects switch to a directed response to green objects, which in most cases means to plant leaves. It is not surprising that vision takes over at this stage, as most flying animals use vision to "pin-point" a suitable object on which to land. Therefore, insects that fly over plants growing in bare soil will be stimulated to land on host plants, the only green objects available to them (Figure 1-3a), as most herbivorous insects avoid landing on brown surfaces, such as soil.
Figure 1. Schematic diagram to illustrate how diverse backgrounds, here represented by clover (Trifolium spp.), influence host plant finding by the cabbage root fly. Numbers represent insect actions 1-7 (see text). Figure copied from Finch & Collier 2000.
When host plants are growing in bare soil, most landings will be what we have classed as "appropriate" and so the host-plants will in effect "concentrate" the insects. In contrast, insects flying over host plants surrounded by clover, land in proportion to the relative areas occupied by leaves of the host (Figure 1-3a) and non-host (Figure 1-3b) plants, as specialist herbivorous insects do not discriminate between the two when both are green. Hence, any landings made on the non-host plant, here represented by clover (Figure 1-3b), are classed as "inappropriate". The amount of time the insects spend on the leaves of the non-host plants before taking off again is governed by whether the insects receive acceptable or antagonistic stimuli through their tarsal receptors. Once the insects are again airborne (Figure 1-4), if they are stimulated to land after flying only a relatively short distance (Figure 1-5 & 6), they could land on a host plant. In all situations, however, the plant on which the insect first lands, even if it is a "host plant", may not stimulate the insect sufficiently, via its contact chemoreceptors on the tarsae or head appendages, to arrest it, and the overall process will be repeated. If this represented the complete system, then under “no-choice” situations in the field, it could just be a matter of time before the numbers of eggs laid on host plants growing in diverse backgrounds were similar to those laid on host plants growing in bare soil. However, this does not occur, as there is a second phase to host plant finding.
Figure 2. Schematic diagram to illustrate how diverse backgrounds, here represented by clover (Trifolium spp.), influence host plant acceptance by the cabbage root fly. Numbers represent the four (mean no.) leaf-to-leaf flights made by the fly to ascertain whether the plant is a suitable substrate around which to lay its eggs. Figure copied from Finch & Collier 2000.
This second phase can be illustrated (Figure 2) most clearly by data collected from a detailed study of the cabbage root fly. The figure shows that before accepting a host plant as a suitable site for oviposition, receptive female cabbage root flies make, on average, four spiral flights before laying eggs alongside the plant (see Figure 1-3c). Hence, the insects stand a much greater chance of “losing” the host plant in a diverse background as, on average, they repeat the initial appropriate/inappropriate landing procedure a further three times. Observations under laboratory conditions showed that for every 100 females that landed on a Brassica plant surrounded by bare soil, thirty-six (Figure 2 - left) received sufficient stimulation from the plant to be induced to lay eggs. In contrast, only seven (Figure 2 - right) out of 100 females that landed on host plants surrounded by clover managed to lay eggs. Fewer flies managed to lay in this situation, because following each short spiral flight, a proportion of the flies landed on the leaves of the surrounding clover plants. This failure to re-contact a leaf of a host plant after any spiral flight prevented the females from accumulating, within the allotted time, sufficient stimulation from the host-plant to be induced to lay eggs. Hence, the barrier that this fly faces when its host plants are grown in diverse backgrounds is not chemical nor mechanical, but behavioural, simply because during the innate series of spiral flights the fly must continue to accumulate more positive host-plant stimuli each time it lands.
The maximum distance recorded for insect orientation to host-plant volatiles in the field is only a few metres (Finch 1980). The amounts of volatile chemical needed to induce directed responses are invariably several orders of magnitude greater than those released naturally (Finch 1980). For example, in the relative laminar airflow of a wind tunnel, cabbage root fly could be induced to fly upwind when host-plant odour was released at 2.5g/day (Hawkes & Coaker 1979). Such an amount (2.5g/day) is equivalent to the volatile chemicals released daily from 100,000 plants (Finch 1980), or 2 ha of a commercial crop, concentrated into a point source. Furthermore, although the receptive flies moved towards the odour source, the results were unexpected, as more than 90% of the "flights" were shorter than 50 cm (Hawkes & Coaker 1979). Such behaviour supports the suggestion that the cue from volatile plant chemicals is to stimulate the insects to land. Finally, even when large amounts of chemical are released from insect traps in the field, many of the insects responding miss the trap on landing (Finch 1980, Prokopy et al. 1983) and do not enter subsequently (Finch 1995, Kostal & Finch 1996) a further indication that the insect uses visual rather than chemical stimuli when selecting its landing site.
The current "appropriate/inappropriate landings" hypothesis, which involves visual stimuli as the pivotal link, seems more convincing than mechanisms based solely on chemical cues. Its great advantage is that once an insect has landed, it has by-passed the major difficulty of obtaining directional cues from odours while still in flight (Murlis et al. 1992). In addition, disruptive air movements around host plants (Murlis et al. 1992), the small amounts of volatile chemical released (Finch 1980), the short distance over which the insect responds (Finch 1995), the closing speed of the flying insect (Finch 1980) and the fact that many receptive insects miss the "target" (Finch 1995; Kostal & Finch 1996), all suggest that the central link in host-plant finding is not governed by volatile chemicals but by visual stimuli. Hence, as only visual stimuli are important in the critical central link, there should be an infinite number of plant combinations that could be used to "deter" pest insects. In addition, it is still debateable whether it is easier for insects to locate a source of odour once they have landed. For example, of the Colorado potato beetles (Leptinotarsa decemlineata Say) that came within the 60 cm radius of detection of an individual potato plant, only half were “attracted” to the plant (Jermy et al. 1988).
To ensure that a high proportion of the searching insects land on non-host plants, the foliage of both plant types must be in the insect's field of vision at the time it lands. Hence, to obtain the maximum impact from undersowing in crop protection, the relative height of the two plant types is crucial to the success of the overall system. When the outline of the host plant is made obvious, either by mowing the intercrop to reduce plant competition (Theunissen et al. 1995, Finch & Kienegger 1997) or by allowing the host plants to protrude well above the background crop (Finch & Kienegger 1997), then the effect is lost.
Finally, before intercropping can be considered a viable alternative to applying insecticides for pest control in large-scale commercial production, further work is required to study in detail the effects of the selected plant combinations on crop pathogens and weeds, and to generate the agronomy required to ensure that the main crop receives adequate nutrients and water.
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