Interactions between Plant Resistance and Natural Enemies in Datura wrightii

None of the herbivorous insects of D. wrightii have any specialized natural enemies, but all are attacked by several species of generalist natural enemies, including several insect predators (Figs. 1 – 4).  Because the glandular trichomes and their acyl sugars of the sticky phenotype were detrimental to some of the insect herbivores of D. wrightii, we asked if the same was true of those herbivores’ natural enemies.  In an extreme case, the natural enemies might be even more deterred by the glandular trichomes than the herbivores.  This would indicate that glandular trichomes may disrupt the suppression of herbivores [1], such that the herbivores acquire enemy free space on the sticky phenotype [2].  My former postdoctoral researcher, Aaron Gassmann, initiated our studies on the tritrophic interactions of D. wrightii.

Figure 1.  Nabis sp.

Figure 2.  Orius sp.

Figure 5.  Numerous G. pallens on D. wrightii late in the season.

Figure 3.  Habronattus sp.

Figure 4.  Geocoris pallens, the most abundant predator on D. wrightii.

The most abundant predator in the field was the omnivorous Western Big-Eyed Bug, Geocoris pallens, which feeds on the eggs and young larvae of L. daturaphila and the nymphs of T. notatus.  The densities of G. pallens on D. wrightii also sometimes exceed that of L. daturaphila [3] (Fig. 5).  For Lema daturaphila and Tupiocoris notatus, the predation rates of several generalist predators were lower on sticky plants compared to velvety plants (Figs. 6-9) [3].  The lower rates of predation were associated with reductions in the residence time and foraging efficiency on sticky plants compared to velvety plants, and not due to any direct toxic effects of the glandular exudate on the predators. 

Figure 6.  Mortality of larvae of L. daturaphila when placed on sticky and velvety plants in the presence of selected insect predators.  From Gassmann & Hare 2005.

Figure 7.  Survival of larvae of L.  daturaphila larvae on sticky or velvety plants in the presence or absence of G. pallens. Redrawn from Gassmann & Hare 2005

Figure 8.  Survival of T. notatus on sticky or velvety plants in the presence or absence of Orius sp.  Redrawn from Gassmann and Hare 2005

Figure 9.  Survival of T. notatus on sticky or velvety plants in the presence or absence of G. pallens.  Redrawn from Gassmann and Hare 2005.

From other research we knew that both predators and parasitoids can be attracted to plants damaged by herbivores by induced cues [4], so we developed some background information on the traits that are inducible in D. wrightii.  Initially, we asked 1) are the plants inducible, 2) are the different trichome phenotypes differentially inducible, and 3) are the glandular trichomes themselves inducible? 

In greenhouse studies, proteinase inhibitors were completely inducible by insect damage or the plant hormone, methyl jasmonate (Fig. 10), and induction increased the activity of polyphenol oxidase and the quantities of acyl sugars [5].  Both trichome phenotypes were similarly inducible, but the glandular trichomes themselves and alkaloid concentration were uninducible.  We concluded that D. wrightii exhibits an array of induced responses similar to those of other solanaceous species, especially when young, but the inducible traits of D. wrightii are independent of trichome phenotype.

Figure 10.  Concentration of induced serine proteinase inhibitors in sticky and velvety plans induced by exposure to methyl jasmonate.  From Hare and Walling 2006.

Using a laboratory collection system, in which the inlet and exhaust flow of purified air were regulated, we were able to collect volatile organic compounds (VOCs) from 16 plants at a time (Figs. 11-12). Young D. wrightii plants in the laboratory emit a suite of volatile compounds following damage by herbivores or exposure to methyl jasmonate (Figs. 13-14).  Both the quantity as well as the composition of the volatile blend vary among  D. wrightii genetic lines [6], and this variation was heritable.  Within genetic lines, however, neither the quantities nor the blends differed between trichome phenotypes [6].

Figure 11.  16-channel flow regulator to independently control the inflow and outflow of purified air to VOC collection chambers. From Hare 2007.  The instrument is portable and was also used in field studies.

Figure 12.  Laboratory collection of volatiles from 16 plants.  From Hare 2007.

Figure 13.  Total volatiles production in the laboratory from uninduced plants of D. wrightii or plants induced with methyl jasmonate from eight backcrossed lines.  From Hare 2007.

Figure 14.  Percent of beta caryophyllene in the VOC blends of plants in the laboratory from uninduced plants or plants induced with methyl ljasmonate from eight backcrossed lines of D. wrightii.   From Hare 2007.

We also measured the production of induced VOCs in the field (Fig. 15).  Under natural conditions, the production of induced VOCs is constrained to the early stages of plant growth, when plants are growing vegetatively (Fig. 16).  The ability to produce volatiles late in the season can be restored to some extent by “rejuvenating” plants by pruning them  back to the root crown and allowing them to resprout (Fig. 17) [7].  In no case did VOC production differ between plant trichome phenotypes.  Unlike many other plant species studied, the long growing season of D. wrightii (February – November) may have allowed easier detection of the ontogenetic constraint on induced production of volatiles than for better-studied plant species with shorter, more ephemeral life histories.  The fact that the production of induced VOCs was poorly matched with the patterns of seasonal abundance of both the herbivores of D. wrightii and their natural enemies led us to determine season-long patterns of predation on L daturaphila by G. pallens. 

Figure 15.  Collection of volatiles from D. wrightii in the field.  From Hare 2010.

Figure 16.  Total production of volatiles in the field from sticky and velvety D. wrightii plants at monthly intervals during the growing season.  From Hare 2010.

Figure 17.  Total production of volatiles in the field from sticky and velvety D. wrightii plants that were either cut back and allowed to resprout (“rejuvenated”) or left unmanipulated (“unrejuvenated”) at monthly intervals during the growing season.  From Hare 2010.

We measured predation by natural populations of G. pallens on eggs of L. daturaphila on induced and uninduced plants in the field.  We measured egg predation and viability by comparing the mortality of eggs in open and closed clip cages (Figs. 18, 19).  Open cages allowed G. pallens to attack the eggs but controlled for any effects of mechanical damage to the plants by the clip cages themselves.  We also collected volatiles from induced and uninduced plants as above, and the experiment was repeated five times over the growing season. 

Higher levels of VOC production were associated with higher levels of predation throughout the growing season, though the effect of VOC production was more pronounced in the spring than later in the summer (Fig. 20) [8].  By contrast, baseline levels of predation in the absence of volatiles were higher in the summer (60 – 70%) than earlier in the spring (14.5%), and we noted once again, that, late in the season, plants were already colonized by G. pallens prior to the initiation of our experiments (Fig. 5). The effect of trichome phenotype varied among dates but was not significant overall, suggesting that prior laboratory experiment may have overestimated the impact of the trichome dimorphism on predation by G. pallens in the field. 

Figure 18.  Open and closed clip leaf cages to measure predation on eggs of L. daturaphila.  From Hare and Sun 2011.

Figure 19.  Experimental egg mass of L. daturaphila in an open clip cage showing full access of the eggs to G. pallens.  From Hare and Sun 2011.

Figure 20.  Relationship between egg predation by G. pallens and total VOC production at monthly intervals in the field.  Results are from an analysis of covariance in which the covariate was total VOC production.  The statistically significant common slope is shown.  The Y-intercepts are the adjust treatment means showing the expected “baseline” level of predation in the absence of VOC production for each month of the season.  VOC production declined seasonally as above, but predation increased as G. pallens became more abundant on plants as the season progressed (see Fig. 5).  From Hare and Sun 2011.

Conclusions

Datura wrightii shares a suite of inducible traits with related solanaceous species.  The long growing season helped reveal the constraint of these inducible traits to early periods of growth and development.  The activity of natural enemies was reduced consistently on the sticky phenotype under laboratory conditions but less consistently so in the field.  High levels of VOC production by D. wrightii in response to herbivore damage may aid in the discovery of herbivore-damaged plants early in the season, but the seasonal decline in VOC production does not limit the activity of predators after the predator community become established on plants later in the season [8].   

The idea that the mediation of host-seeking behavior of natural enemies by induced plant volatiles is a coevolved response between plants and those natural enemies is controversial.  Applying the logic behind the evolution of pheromone signals between males and females of the same species, Jeremy Allison and I reviewed the literature to determine if induced VOCs might be coevolved signals between plants and natural enemies.  An alternative is that they are merely cues, such that one of many potential effects of induced VOCs is to recruit natural enemies to herbivore-damaged plants, though not the only effect.  Most examples in the literature supported the hypothesis that induced volatiles serve only as cues, learned by natural enemies, and not coevolved signals between plants and  natural enemies acting for their mutual benefit [9].  Additionally, if such cues are learned, then the variation in blends among plant genotypes may be largely irrelevant, unless some blends are more easily learned than others.

The key to understanding the evolutionary trajectory of induced volatile production for suppression of herbivores in D. wrightii or any plant species is to understand how plant fitness is affected by the activity of natural enemies of the plant’s herbivores.  It is particularly important to understand if the activities of natural enemies can consistently impose natural selection on the variation in the quantities and blends of induced volatiles that might better attract natural enemies.  There is a reasonable amount of data showing genetically-determined variation in VOC blends in several plant species but comparatively little evidence showing that such variation differentially affects the recruitment of natural enemies to herbivore-damaged plants.  There is even less evidence that plants with particular blends accrue a fitness advantage over plants emitting other blends, and D. wrightii is no exception. The gap in our knowledge about the consequences to plant fitness of the production of plant volatiles under natural, field conditions continues to limit our understanding of the evolution of this induced trait [4].  For D. wrightii, it appears that there is only a relatively short temporal window in which natural selection might favor the production of induced VOCs, and it is far from clear if differences in the abilities of D. wrightii plants to attract natural enemies in the spring would result in measurable differences in fitness over the full growing season.

1. Hare, J.D. (1992) Effects of plant variation on herbivore-natural enemy interactions. In Plant Resistance to Herbivores and Pathogens: Ecology, Evolution and Genetics (Fritz, R.S. and Simms, E.L. eds), pp. 278-298, University of Chicago Press.  DOI: 10.7208/chicago/9780226924854.001.0001.

2. Berdegue, M. et al. (1996) Is it enemy-free space? The evidence for terrestrial insects and freshwater arthropods. Ecological Entomology 21 (3), 203-217.  DOI: 10.1111/j.1365-2311.1996.tb01237.x

3. Gassmann, A.J. and Hare, J.D. (2005) Indirect Cost of a Defensive Trait: Variation in Trichome Type Affects the Natural Enemies of Herbivorous Insects on Datura wrightii. Oecologia 144 (1), 62-71.  DOI: 10.1007/s00442-005-0038-z

4. Hare, J.D. (2011) Ecological Role of Volatiles Produced by Plants in Response to Damage by Herbivorous Insects. Annual Review of Entomology 56 (1), 161-180.  DOI: 10.1146/annurev-ento-120709-144753.

5. Hare, J.D. and Walling, L.L. (2006) Constitutive and Jasmonate-Inducible Traits of Datura wrightii. Journal of Chemical Ecology 32 (1), 29-47.  DOI: 10.1007/s10886-006-9349-8.

6. Hare, J.D. (2007) Variation in Herbivore and Methyl Jasmonate-Induced Volatiles Among Genetic Lines of Datura wrightii. Journal of Chemical Ecology 33 (11), 2028-2043.  DOI: 10.1007/s10886-007-9375-1.

7. Hare, J.D. (2010) Ontogeny and Season Constrain the Production of Herbivore-inducible Plant Volatiles in the Field. Journal of Chemical Ecology 36 (12), 1363-1374.  DOI: 10.1007/s10886-010-9878-z

8. Hare, J.D. and Sun, J. (2011) Production of Herbivore-Induced Plant Volatiles is Constrained Seasonally in The Field but Predation on Herbivores is not. Journal of Chemical Ecology 37 (5), 430-442. DOI: 10.1007/s10886-011-9944-1.

9. Allison, J.D. and Hare, J.D. (2009) Learned and naïve natural enemy responses and the interpretation of volatile organic compounds as cues or signals. New Phytologist 184 (4), 768-782.  DOI: 10.1111/j.1469-8137.2009.03046.x.