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 Photograph:
George L. Teetes. Note: all pictures link to larger images.
Good day to you who are participating in this electronic lecture.
Recognizing there are entire courses and books on the subject of plant resistance to crop
pests, this lecture will cover only the essential aspects of the topic. The role of plant
resistance as a direct control tactic in insect integrated pest management (IPM) will be
emphasized. Two case studies of the use of insect-resistant varieties as a component of an
IPM strategy will be provided.
Ecological Results of Agriculture: In contrast to natural ecosystems, most
agricultural crop production systems are ecologically unstable, non-sustainable, and
energy dependent. Man has through plant domestication and cultivation practices interfered
in many ways with species diversity and natural defense mechanisms of plants. Cultivated
crops originated from genetically diverse plant types. However, crop plants are now grown
in large, genetically homogeneous stands, a practice that decreases genetic and species
diversity and increases the likelihood of economically significant insect pest
infestations. Defense mechanisms of plants are re-created in resistant plants. Plant
defense mechanisms include escape in space and time, incompatible biological associations,
physically and chemically derived barriers, and accommodation by replacement or repair of
damaged plant parts. By re-creating plant defenses, genetic resistance to insect pests
plays, in an environmentally compatible manner, a vital role in the attempt to enhance
ecological stability in agricultural crops.
An IPM Direct Control Tactic: Plant resistance to insects is one of several
cultural control methods. Cultural control methods involve use of agronomic practices to
reduce insect pest abundance and damage below that which would have occurred if the
practice had not been used. In IPM, plant resistance to insects refers to the use of
resistant crop varieties to suppress insect pest damage. Plant resistance is intended to
be used in conjunction with other direct control tactics.
Definition of an Insect-Resistant Plant: Definitions of an insect-resistant
plant are many and varied. In the broadest sense, plant resistance is defined as "the
consequence of heritable plant qualities that result in a plant being relatively less
damaged than a plant without the qualities." In practical agricultural terms, an
insect-resistant crop cultivar is one that yields more than a susceptible cultivar when
confronted with insect pest invasion. Resistance of plants is relative and is based on
comparison with plants lacking the resistance characters, i.e., susceptible plants.
Effect of Insect Pest-Plant Host Relationship: Insect-resistant crop
varieties suppress insect pest abundance or elevate the damage tolerance level of the
plants. In other words, insect-resistant plants alter the relationship an insect pest has
with its plant host. How the relationship between the insect and plant is affected depends
on the kind of resistance, e.g. antibiosis, antixenosis (non-preference), or tolerance.
Antibiosis resistance affects the biology of the insect so pest abundance
and subsequent damage is reduced compared to that which would have occurred if the insect
was on a susceptible crop variety. Antibiosis resistance often results in increased
mortality or reduced longevity and reproduction of the insect.
Antixenosis resistance affects the behavior of an insect pest and usually is
expressed as non-preference of the insect for a resistant plant compared with a
susceptible plant.
Tolerance is resistance in which a plant is able to withstand or recover
from damage caused by insect pest abundance equal to that damaging a plant without
resistance characters (susceptible). Tolerance is a plant response to an insect pest.
Thus, tolerance resistance differs from antibiosis and antixenosis resistance in how it
affects the insect-plant relationship. Antibiosis and antixenosis resistance cause an
insect response when the insect attempts to use the resistant plant for food, oviposition,
or shelter.
Advantages to the Use of Insect-Resistant Crop Varieties: Use of
insect-resistant crop varieties is economically, ecologically, and environmentally
advantageous. Economic benefits occur because crop yields are saved from loss to insect
pests and money is saved by not applying insecticides that would have been applied to
susceptible varieties. In most cases, seed of insect-resistant cultivars costs no more, or
little more, than for susceptible cultivars. Ecological and environmental benefits arise
from increases in species diversity in the agroecosystem, in part because of reduced use
of insecticides. Increases in species diversity increase ecosystem stability which
promotes a more sustainable system far less polluted and detrimental to natural resources.
The IPM concept stresses the need to use multiple tactics to maintain insect pest
abundance and damage below levels of economic significance. Thus, a major advantage to the
use of insect-resistant crop varieties as a component of IPM arises from the ecological
compatibility and compatibility with other direct control tactics. Insect-resistant
cultivars synergize the effects of natural, biological, and cultural insect
pest-suppression tactics. The "built-in" protection of resistant plants from
insect pests functions at a very basic level, disrupting the normal association of the
insect pest with its host plant. The compatible, complementary role plant resistance to
insect pests plays with other direct control tactics is, in theory and practice, in
concert with the objectives of IPM. All crop cultivars should contain resistance to insect
pests.
Plant resistance to insect pests has advantages over other direct control tactics. For
example, plant resistance to insects is compatible with insecticide use, while biological
control is not. Plant resistance to insects is not density dependent, whereas biological
control is. Plant resistance is specific, only affecting the target pest. Often effects of
use of insect-resistant cultivars are cumulative over time. Usually the effectiveness of
resistant cultivars is long-lasting.
The role of plant resistance to insects in IPM has been well defined, at least in theory.
However, the specific role a resistant cultivar plays in a particular IPM situation is
crucial to successful deployment of the resistant cultivar. The impact of the resistant
cultivar on standard cultural, biological, and insecticidal control methods should be well
defined. Likewise, the impact of each of these control tactics on the resistant cultivar
also must be defined.
Several definitions have been used to convey the relative level of resistance in a plant.
However, the problem of quantifying resistance continues to be a problem influencing
farmer acceptance of insect-resistant cultivars. A better way to define resistance levels
in agronomically improved resistant cultivars is through quantified comparisons of insect
pest damage or plant yield loss of susceptible cultivars. Once insect pest abundance or
damage to yield-loss relationships have been determined, economic threshold levels can be
determined and combined with factors such as crop value and insect pest control costs to
develop dynamic thresholds for use by producers. Dynamic thresholds provide a description
of resistance and can reduce crop loss risk because limitations are known and remedial
action can be taken when necessary. By using this system to define relative differences in
insect pest resistance between cultivars, it may be possible to simply indicate that a
resistant cultivar has a higher economic threshold level than a traditional susceptible
cultivar.
Case Studies: Provided here are two examples of the role of insect-resistant
cultivars as a component of an IPM strategy. Many other examples could be used.
 Photograph:
Sorghum midge, Stenodiplosis sorghicola (Coquillett). (Not
available at this time)
Sorghum midge in sorghum: The sorghum midge, Stenodiplosis sorghicola
(Coquillett), is a ubiquitous and serious insect pest of sorghum, Sorghum bicolor (L.)
Moench. It is a key insect pest of sorghum throughout the southern production region of
the United States. Each ovipositing female sorghum midge lays about 50 eggs between the
glumes of flowering spikelets of cultivated sorghum or johnsongrass, Sorghum halepense
(L.) Pers. Larval feeding on the developing ovary inside the spikelet prevents normal
kernel formation and results in direct grain loss. A single larva is sufficient to destroy
a kernel; the entire crop can be destroyed.
A sorghum midge life cycle is completed in 16-18 days under favorable temperatures. This
relatively short generation time allows for multiple generations of the insect each
season. Sorghum midge abundance can increase each generation when flowering sorghum hosts
are available and weather conditions are favorable. Sorghum midges can become especially
abundant when successive plantings of the crop allow the flowering period to be extended
in a locale.
Johnsongrass is essential for survival of the sorghum midge. In the spring, sorghum midges
in an area emerge from overwintering before cultivated sorghum is flowering and lay eggs
in flowering spikelets of johnsongrass. Typically, a second generation is produced in
johnsongrass before cultivated sorghum in an area is flowering. Sorghum midges disperse to
the first flowering fields of cultivated sorghum, and within a single additional
generation usually have increased enough in abundance to cause economic damage to sorghum.
Management tactics used to suppress abundance of sorghum midge include pest avoidance by
planting sorghum early and uniformly in an area, destruction of johnsongrass, abiotic and
biotic natural control involving indigenous parasites and predators, insecticide use, and
use of resistant sorghum varieties. The major management tactic is early and uniform
planting of sorghum in an area so that sorghum flowers before sorghum midge abundance is
high enough to cause economic damage. The nature of sorghum midge biology and
characteristics of sorghum panicle flowering make insecticide use less effective and more
expensive than desirable.
 Figure 1:
Sorghum midge density/damage relationship
Significant progress has been made in developing sorghum inbred lines that, under high
levels of sorghum midge infestation, produce sorghum midge-resistant hybrids with
excellent grain yield. This management tactic has received much attention during the past
decade. The major resistance mechanism of resistant varieties is non-preference for or
interference with adult oviposition caused by differences in spikelet flowering time.
Spikelets of resistant sorghum flower during early morning hours before adult female
sorghum midges occur in the field. Spikelets of susceptible sorghums flower later in the
morning when sorghum midges are present in the field. The consequence of the resistance
mechanism is that sorghum midge-resistant hybrids are at least five times less damaged
than susceptible hybrids at the same level of sorghum midge abundance.
Consequently, the economic threshold level for sorghum midge-resistant hybrids is five
times higher than for susceptible hybrids (Table 1). The elevated economic threshold
significantly enhances the effectiveness of other management tactics. In many cases, the
resistance level is high enough to prevent economic damage and eliminate the need for
insecticides. Also, when sorghum midges become very abundant, insecticide use requirements
are much less, and the application(s) is more effective than for sorghum midges infesting
susceptible sorghum hybrids.
In this case study, sorghum resistance to sorghum midge plays a fundamental role in the
IPM strategy. Resistance is not intended to always provide the sole means of control but
is used as a direct control tactic that, in combination with others, provides desired
protection of the crop from the insect pest.
Table 1. Economic injury levels for sorghum hybrids infested by sorghum midges.
Economic injury level for
sorghum midge-susceptible hybrids |
| Per acre control cost ($) |
Crop market value ($) per
acre |
| |
100 |
120 |
140 |
160 |
180 |
200 |
220 |
240 |
260 |
280 |
300 |
3 |
1.2* |
1.0 |
0.9 |
0.8 |
0.7 |
0.6 |
0.6 |
0.5 |
0.5 |
0.4 |
0.4 |
4 |
1.6 |
1.3 |
1.1 |
1.0 |
0.9 |
0.8 |
0.7 |
0.7 |
0.6 |
0.6 |
0.5 |
5 |
2.0 |
1.7 |
1.4 |
1.3 |
1.1 |
1.0 |
0.9 |
0.8 |
0.8 |
0.7 |
0.7 |
6 |
2.4 |
2.0 |
1.8 |
1.5 |
1.3 |
1.2 |
1.1 |
1.0 |
0.9 |
0.9 |
0.8 |
7 |
2.7 |
2.3 |
2.0 |
1.8 |
1.6 |
1.4 |
1.3 |
1.2 |
1.1 |
1.0 |
0.9 |
8 |
3.0 |
2.7 |
2.3 |
2.0 |
1.8 |
1.6 |
1.5 |
1.3 |
1.2 |
1.1 |
1.1 |
Economic injury levels
for sorghum midge-resistant hybrids |
3 |
6 |
5 |
5 |
4 |
4 |
3 |
3 |
3 |
3 |
2 |
2 |
4 |
8 |
7 |
6 |
5 |
5 |
4 |
4 |
4 |
3 |
3 |
3 |
5 |
10 |
9 |
7 |
7 |
6 |
5 |
5 |
4 |
4 |
4 |
4 |
6 |
12 |
10 |
9 |
8 |
7 |
6 |
6 |
5 |
5 |
5 |
4 |
7 |
14 |
12 |
10 |
9 |
8 |
7 |
7 |
6 |
6 |
5 |
5 |
8 |
15 |
14 |
12 |
10 |
9 |
8 |
8 |
7 |
6 |
6 |
6 |
*Number of
sorghum midges per panicle |
Greenbug, Schizaphis graminum (Rondani), in sorghum: This aphid has been
an insect pest of small grains in the United States for almost 100 years and a key insect
pest of sorghum since the appearance of biotype C greenbugs in 1968. Management approaches
to dealing with biotype C after 1968 included using extremely low dosage rates of
registered insecticides. Treatment need was based on knowledge of the insect's seasonal
abundance trends and economic threshold levels. This IPM approach reduced greenbug
abundance below numbers capable of causing economic yield loss and preserved natural
biological control agents that prevented greenbug resurgence and secondary pest outbreaks.
However, failure of sorghum growers to adopt this IPM strategy and the continued use of
high rates of organophosphorus insecticides resulted in high levels of insecticide
resistance. Another management tactic was needed.
Development of greenbug-resistant sorghums was initiated in 1969. As a result,
greenbug-resistant hybrids were commercially available in 1976. Tolerance is the primary
resistance mechanism of greenbug-resistant sorghums. Tolerance is ecologically
advantageous because natural enemies are maintained that complement the resistance and
suppress greenbug abundance. Also, the incidence of secondary pest outbreaks is reduced.
 Figure
2:Greenbug density/damage relationship
Unfortunately, numerous greenbug biotypes have occurred. As used here, biotype refers
to individuals or populations that differ from others based on criteria other than
morphology, such as parasitic (virulence) ability. Greenbug biotypes, as I am using the
term, are characterized on the basis of differential host plant response within the
greenbug species. Currently, 11 biotypes of greenbug have been identified although only
biotypes C, E, I, and K are virulent on sorghum in nature. The importance of biotype K is
yet to be determined. Despite the consistent occurrence of greenbug biotypes,
greenbug-resistant sorghums have been a major component of sorghum IPM. The effect of
using greenbug-resistant sorghums on reducing insecticide use is shown in the following
figure.
 Figure 3:
Sorghum acreage treated in Texas
Future Possibilities: Much interest in biotechnology relative to
developing insect-resistant plants is in methods known collectively as genetic
transformation, rDNA methods, or genetic engineering. These methods enable transfer of a
resistance gene that could not be transferred by traditional sexual hybridization. As
important, however, are evolving molecular genetic techniques that are providing exciting
opportunities to better understand the genetic relationship between an insect and a
resistant plant. DNA marker technology is currently used to map and identify specific gene
structures conferring resistance traits in plants. Understanding the genetics of
resistance in plants will provide the knowledge to improve resistance deployment
strategies. However, it is becoming evident with regard to plant resistance to insects
that it is equally, if not more, important to understand the genetics of the insect
attacking the plant. However, regardless of the technology used to develop
insect-resistant plants, the technology will have to be used as a component of IPM in the
same way traditionally developed insect-resistant plants are used today.
As new technology has begun to be used, there is increasing concern over the likelihood of
formation of insect pest biotypes that can overcome resistance. Many factors are
associated with the ability of an insect to overcome plant resistance. A popular theory is
that resistant plants exert selective pressure for virulent individuals that become
predominant types (biotypes). This theory of biotype selection often is related to
theories of evolutionary biology. For example, one evolutionary biology theory is that
selection by insects causes plant populations to acquire resistance traits, and selection
imposed by these resistance traits gives the insect the ability to exploit these plants.
The implication is that insects with a long-term association with a host-plant species
have the genetic plasticity to overcome defense strategies of the host plant. That is,
resistance in plants that evolved with an insect could be susceptible to being overcome by
genetic changes in the insect pest.
Theories on gene conservation and management are major subjects of scientific discussion
and experimentation. Strategies for preservation of insect resistance in crop varieties
include use of multiple resistance genes conferring different causes of resistance,
sequential (time and space) deployment of resistance, and maintenance of refugia to
support avirulent individuals to mate with virulent individuals in the insect population.
Concern over insect biotypes probably will result in much wiser use of genetic
opportunities that now are available or will be available in the future. A major mistake
would be to attempt to use gene-altered plants as the sole control tactic for an insect
pest rather than a component of IPM. To do so would be to repeat the mistakes of the
insecticide era.
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Last modified: Monday, 16 December, 1996
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