Naturally Occurring Pest Bioregulators, ed. P Hedin, 126-135. ACS Symposium Series 449, 1991.
Chapter 9
Potato Glandular Trichomes Defensive Activity Against Insect Attack
Ward M. Tingey
Department of Entomology, Cornell University, Ithaca, NY 14853-0999

Insect-resistant cultivars offer a realistic and practical foundation upon which to build an economical and environmentally sound crop protection system. Insect resistance of a wild Bolivian potato species is conditioned by the presence of glandular trichomes on its foliage. Resistance is expressed both mechanically and behaviorally. Mechanical resistance involves release of a viscous exudate from the trichome gland upon contact by an insect. The trichome secretions accumulate initially on tarsi, first impeding movement and later entrapping the insect. In addition to immobilization, entrapment and resultant mortality, glandular trichomes dramatically alter normal insect behaviors, particularly those involving host acceptance and feeding. Pest species adversely affected by glandular trichomes include the Colorado potato beetle, green peach aphid, potato leafhopper, potato flea beetle, and spider mites. The mechanical and behavioral defensive properties of glandular trichomes are conditioned by the presence of specialized chemistry in trichome exudate. The Colorado University potato breeding program has introgressed the genetic information for expression of glandular trichomes into germplasm of the cultivated potato. Commercially acceptable levels of resistance have been maintained in 8 successive breeding cycles under development for horticultural adaptation. Elite hybrids can experience greater than 85% reduction in populations of insect pests, compared to those on susceptible commercial cultivars. Insecticide use on resistant hybrids can be reduced by at least 40% of that presently needed on susceptible cultivars.

To-date, over 180 wild, tuber-bearing species of Solanum are known (1) and while many of these have been screened for insect resistance over the pant ten years, the first large-scale systematic efforts to examine wild Solanum germplasm far insect resistance were initiated by E. B. Radcliffe at the University of Minnesota and reported in a series of papers first appearing in 1968 (2) These studies led to the identification of several excellent sources of resistance to aphids and leafhoppers. Prior to this time, very few species were known to resist attack by major potato insects and in most cases, the underlying plant phenomena conferring resistance were unknown although the presence of steroid glycosides in the foliage of some species had been associated with resistance as early as 1950 by European workers (3-7).

One of the wild potato species Radcliffe reported resistant to the potato leafhopper had been described in 1944 from a collecting expedition in Bolivia. J. G. Hawkes, the British potato systematist, described this species, Solanum berthaultii, as follows (8):

Very glandular species with pale violet-blue pentagonal sub-stellate corolla. Almost certainly formed as a natural hybrid between S. tarijense (Commersoniana) and a blue-flowered mountain species, possibly S. sparsipilum. Distribution: Bolivia, eastern slopes of Andes in rather dry valleys amongst bushes and in waste places from 2,100-2,700 m."

In 1971, R. W. Gibson at the Rothamsted Experimental Station reported that S. berthaultii was resistant to aphids and furthermore that resistance was associated with the presence of glandular trichomes (9). Subsequent work in Britain, the U.S., and Peru has expanded the list of pest species against which S. berthaultii is defended. Currently, this species is known to resist herbivory by at least 10 major groups or species of pests including leaf miner flies, the potato tubermoth complex, aphids, leafhoppers, fleabeetles, mites, and the Colorado potato beetle (10-13). In 1977, research was initiated as Cornell University to systematically explore the nature of insect resistance in this wild potato species and to examine its potential valor in management of insect pests.

Glandular Trichomes of Solanum berthaultii

Resistance of S. berthaultii to insects and mites is associated with the presence of two types of glandular trichomes on foliage, fruiting organs, and stolons. Type A trichomes are about 120 to 210 mµ in length and bear membrane-bound tetralobulate gland about 50 to 70 mµ in diameter at their apices (14). Release of secretory material occurs upon mechanical breakage of the gland at its junction with the stalk The tetralobulate gland is not renewed following rupture.

Type B trichomes, in contrast, are longer (ca. 600 to 950 mµ in length) and bear an ovoid droplet of exudate (20 to 60 mµ in diameter) at their tips (14). The naked droplets of exudate are extremely adhesive and transfer readily upon contact by an insect. Following mechanical or solvent removal, droplets of type B exudate are renewed so their original dimensions within 8 days (15).

Figure. 1. Nature of glandular trichome‑mediated aphid resistance in S. berthaultii. Key: (1) Carboxylic acid sucrose esters (CASE), (2) Viscous type B trichome exudate, (3) Increased aphid movement and attempts to escape, (4) Adhesive aphid tarsi, (5) Enhanced rupture of type A trichome membrane?, (6) Polyphenoloxidase + 02 + substrate, (7) Aphid alarm pheromone, E‑(β)=farnesene, (8) Encasement of tarsi by trichome exudate, (9) Greater effective tarsal size, (10) Decreased aphid mobility, (11) Occlusion of mouthparts by trichome exudate, (12) Starvation and death. Adapted from (38).

Defensive Biology of Solanum berthaultii

Many species of small-bodied pests experience classical responses to host resistance phenomena daring or encounter with foliage of S. berthaultii. These include host avoidance and restlessness, reduced feeding, delayed development, reduced adult weight and reproductive performance, increased mortality, and diminished longevity. In several cases, these responses are associated with entrapment or immobilization by trichome exudate.

Entrapment/Immobilization. Aphids and leafhoppers experience significant mortality by entrapment in trichome exudate. Initial contact with the foliage leads to an encounter with the tall Type B hairs which confer a high degree of adhesiveness to the tarsi (Fig. 1). Tarsi coated with this viscous material are very effective in breaking the junction between the head and stalk of a Type A trichome that resulting in release of Type A exudate onto the insect's body (16). Within several hours, the exudate darkens and hardens by virtue of its phenolic oxidation chemistry, immobilizing these inserts or severely restricting their movements. The dispersal rate of aphids with exudate-encased tarsi on S. berthaultii and of hybrids is only 1/3 that of aphids with unencased tarsi on nonglandular plants (17). Trichome exudate also accumulates on the mouthparts and may totally occlude the stylets of insects with sucking mouthparts, thus preventing feeding (18). The defensive activity of S. berthaultii foliage increases along with the densities of both types of trichomes and with increased volume of Type A trichome glands. Young aphids and leafhoppers experience greater mortality from an encounter with glandular trichomes than do adults. probably because of a more limited ability to escape the viscous and adhesive trichome banter (16).

Behavioral and Sensory Disturbance. In addition to entrapment and immobilization, the glandular trichomes of S. berthaultii interfere with host acceptance and condition an avoidance for oviposition and feeding. Agitation and avoidance by aphids is conferred by a complex of sesquiterpenes located primarily in Type A trichomes and released when the gland is broken (19-20). The aphid-immobilizing potential of trichome exudate is magnified by the presence of these compounds because the resulting behavioral excitation and increased locomotion promote a greater frequency of encounters with undisturbed Type A trichomes. Interestingly, Type A trichomes of some commercial potato cultivars contain high levels of the aphid alarm pheromone, E-β-farnesene (20). However, aphid acceptance and feeding behavior on these plants are not adversely affected because the tetralobulate glands on foliage of the cultivated potato do not readily rupture upon contact

Type B trichome exudate of S. berthaultii also conditions abnormal behaviors in aphids and leafhoppers, particularly delay in host acceptance as measured by an increase in time to the first probe and a decrease in feeding time (Table I) (21, 22). The avoidance/deterance responses of aphids are conditioned by the presence of sucrose esters of short-chain branched carboxylic acids in the type B exudate (Table II) (23, 24). Sensory receptors on the tarsi and/or antennae of aphids are the likely target sites of sucrose esters in type B exudate (24).

Table I. Effect of type B trichome removal on feeding behavior of fourth-instar
potato leafhopper nymphs on resistant and susceptible potato clones. Adapted from (22).

Clone Type B trichomes Preprobe
time (min)
Total feeding
time (min)
% of nymphs
S. tuberosum X S. berthaultii (F3)      
  Intact 14.2 a 5.7 b 25 b
  Removed 11.2 b 5.3 b 95 a
S. tuberosum      
  Intact 1.2 c 21.0 a 100 a
  Removed a 0.6 c 16.9 a 100 a

 a A glass microscope slide was gently pressed to the leaf surface to
simulate the conditions used to remove type B exudate from the F3 hybrid.

Coupled with the consequences of entrapment by Type A exudate, (i.e. high mortality, limited population development, and the disabling influence of trichome exudates on locomotion), the avoidance behaviors conditioned by sesquiterpenes in type A trichomes and by sucrose esters in type B trichomes contribute to the reduced vector efficiency of the green peach aphid for potato virus Y as measured by impaired acquisition and transmission ability (Table III) (25, 26). This interplay of several trichome defensive phenomena adds further value to the plant breeding attributes of S. berthaultii because it provides an additional harrier of disease protection for hybrid germplasm carrying genetic resistance to the potato virus Y.

Table II. Deterrence of green peach aphid feeding by sucrose esters from type B glandular
trichomes of S. berthaultii (Pl 473331) applied to diet membranes. Adapted from (24)

No feeding sheaths/mm2 Pa
Treated Untreated
100 0.21 2.24 0.0001
33 0.38 3.36 0.0001
10 1.31 1.77 0.06
3 1.39 1.17 0.33
1 1.66 1.30 0.15

a Probability that sucrose esters have no effect on the distribution of feeding sheaths by G-test, goodness of fit.

Table III. Final potato virus Y (PVY) incidence as a percent of exposed target plants for four source plant/target plant combinations
of S. tuberosum and S. berthaultii (PI 310927). Target plants exposed to green peach aphids for 4 weeks. Adapted from (26)

Source plant/target plant % PVY incidence S.E. N
Tuberosum/Tuberosum 39 3.67 3
Tuberosum/Berthaultii 30 9.15 2
Berthaultii/Tuberosum 17 4.16 2
Berthaultii/Berthaultii 6 2.78 3

Nature of Resistance to the Colorado Potato Beetle. Over the past two decades, the Colorado potato beetle, Leptinotarsa decemlineata (Say), has become the major limiting factor in potato production in the northeastern and mid-Atlantic regions of the U.S. (27) and is an increasingly serious constraint in Europe. The magnitude of the problem has stimulated considerable interest in S. berthaultii as a source of resistance to this pest. When confined on S. berthaultii, both adults and larvae accumulate type A and B exudate on their tarsal pads and claws but they do not become entrapped nor does their mobility appear affected. Rather, the expression of resistance against the Colorado potato beetle is more subtle and characterized initially by abnormal behaviors. Females display a marked reluctance to accept, feed, and oviposit on S. berthaultii. Numbers of eggs per egg mass abcd. Values followed by the same letter are not significantly different (p > 0.05) by R X C test of independence using the G statistic for feeding and mortality and analysis of variance. LSD for weight.

Table 1V. Effect of removing type A (wipe) and type B (methanol dip) glandular trichomes
of S. berthaultii (Pl 310927) on feeding, growth, and mortality of neonate Colorado potato beetle. Adapted from (29)

Treatment N % Feeding % Mortality Weight (mg)
None 65 40 a 63 a 1.6 a
Wipe 50 60 ab 48 ab 1.9 a
MeOH dip 57 72 ab 32 bc 1.2 a
MeOH dip and wipe 39 90 cd 15 cd 1.4 a

Such subtle effects of the trichome barrier might explain our recent finding that feeding is a much smaller portion of the activity budget of Colorado potato beetle larvae on S. berthaultii than on non-glandular susceptible cultivars (Table V) (34) and thus provide an explanation for several of the major impacts of resistance on this pest, i.e. decreased food consumption leads to the reduced growth rates, retarded development, and their cumulative suppressive effects on survival, fecundity, and population dynamics.

Generic Manipulation of S berthaultii and Variety Development

The breeding program at Cornell University to introgress genetic information for insect resistance of S. berthaultii was initiated in 1977 by crossing selected clones of S. berthaultii (as males) with several tetraploid (2n=48) clones of S. tuberosum (35-37). The successful production of tetraploid hybrids between these two species results from unreduced male gamete production in diploid (2n=24) S. berthaultii. All subsequent breeding has been with tetraploids. The F2 generation was produced by random intermating of the F1 generation. The F3 and F4 generations were produced by intercrossing progenies selected for presence of type B droplets and for horticultural adaptation. The F3 generation also included outcrossing to selected F2s of S. andigena x S. berthaultii to introduce genes for resistance to potato virus Y. In subsequent generations, the breeding method has involved a system in which back-crossing to S. tuberosum has been alternated with intercrossing within the hybrid population. Parents used in these latter generations had high densities of both types of trichomes, large Type B droplets and the presence of sucrose esters in type B exudate, high levels of trichome phenolic oxidation activity, insect, disease and nematode resistance, and horticultural adaptation. We are currently 8 generations advanced beyond the FI.

Table V. Activity of neonate Colorado potato beetle larvae on excised leaflets of either S. tuberosum, S. berthaultii Pl 310927, or Pl 310927 from which most of the glandular trichome exudate had been removed by wiping between tissue papers. Adapted from (34)

      No. of larvae feeding as % of
Host plant n % of larvae in
contact with leaflet
no. larvae
No. larvae
on leaflet
S. tuberosum 16 96 a 12.5 a 13.1 a
S. berthaultii        
Pl 310927(wiped) 16 79 b 8.5 b 11.1 a
Pl 310927 16 74 b 1.0 c 1.7 b

Means within the same column followed by the same letter not significantly different in multiple paired t tests at α = 0.017 (experimentwise α = 0.05).

The inheritance of glandular trichomes in S. berthaultii and in crosses with S. tuberosum has been studied by several groups (11, 35-37) Type B trichome density, droplet size and presumably the presence of sucrose esters appear to be controlled by relatively few recessive genes. Heritability estimates ranged from 20-30% for density of Type A and B trichomes and 60% for Type B droplet size. The presence of polyphenoloxidase in type A trichomes is controlled by a single dominant gene (38). The inheritance of sesquiterpenes in type A trichomes has not been studied.

Recent studies at Cornell indicate that with the exception of Type B trichomes, none of the trichome insect-resistance traits being selected in our breeding program has any deleterious associations with horticultural adaptation, In the case of type B trichomes, their presence in hybrid clones is associated with reduced yielding ability, late maturity, and other characteristics unique to the wild parent (39). Alternate breeding schemes and somaclonal variation are being explored to determine whether gene linkage is responsible for this undesirable association (39, 40).

Potential Pest Management Applications

Clones selected from these hybrid populations have demonstrated excellent levels of resistance to aphids, leafhoppers, and the Colorado potato beetle in field and laboratory studies. We have demonstrated season-long reduction in aphid populations on hybrid clones of up to 60%, compared with populations on commercial cultivars (17, 41). We have also demonstrated population reductions of leafhopper adults and nymphs of over 80% on hybrid clones, compared to those on commercial susceptible potato cultivars (22). This level of resistance eliminates the need for insecticides in management of leafhoppers. As for the Colorado potato beetle, we have documented the following negative impacts by resistant clones compared to commercial cultivars: 30% delay in time to 1st egg laying, 80% reduction in total egg production, 20% increase in larval development time, and 25% reduction in adult weight (Table VI). Under field conditions, these impacts translate into a nearly 90% reduction in densities of second generation larvae with accompanying protection from defoliation. Insecticide use on resistant hybrids can be reduced by at least 40% of that presently needed on susceptible cultivars ($80-160 per acre per growing season) (32).

Although glandular trichomes can interfere with predators and parasitoids of aphids and the Colorado potato beetle, the inhibitory effect is largely associated with entrapment by type B exudate and is minimized on plants bearing moderate densities of these trichomes (42, 43).

Table VI. Vital Statistics of Colorado Potato Beetle on S. berthaultii and S. tuberosum. Adapted from (31)

  S. berthaultii S. tuberosum
Age at 1st Egg Laying (days) 46 31
Total Eggs per Female 351 2063
Larval Development Time (days) 14.3 11.2
Female Pupal Weight (mg) 104 138
Female Adult Weight (mg) 88 105

Future Outlook

Our experience with S. berthaultii indicates that it is a useful source of insect resistance for genetic improvement of the potato. Major attributes of this resistance include its broad-spectrum nature and the diversity of its mechano-chemical impact across a range of pest species. However, insects as a group have demonstrated a remarkable ability to adapt to stress. And, in fact, the durability of some types of host resistance is not appreciably greater than that of insecticides. We believe, however, that adaptation by pests to the multiplicity of negative impacts conditioned by glandular trichomes is likely to be a lengthy process requiring substantial genetic changes involving behavior and morphology. For this reason, glandular trichome-mediated host resistance to such pest species is likely to be more stable than that conditioned by the presence of a single toxic or deterrent allelochemical.

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RECEIVED May 16, 199

Trichomes bibliography