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Insect-Plant Interactions
Plants as insect food
4th Lecture
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Plants as insect food
Two basic question
– First, What do plants offer to insects by way of
nutrition?
– Second, What do insects need for optimal
growth and reproduction?
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Plants as insect food
1- The chemical composition of plants
2- The nutritional requirements of insects
vary between species and developmental
stage and environmental conditions.
What plants provide and what insects
require
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Plants as insect food
It is appropriate to compare the chemical
composition of insects with that of plants
?
Fig. 5.1
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Plants as insect food
Plants
Insects
Protein
2-4%
8-14%
Caloric value
18.9 j/mg
22.8 j/mg
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Table 5.1
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Plants are suboptimal food
The food of insect herbivores consists of
dilute nutrients in a matrix of indigestible
structural compounds, such as cellulose
and lignin and variety of allelochemicals
Exert toxic effects
Interfere negatively with digestion
The quantitative ratios of nutrients in plant
differ than insects need
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Plants are suboptimal food
The nutritional requirements of insect are
generally the same as animals, insect are
not able to synthesize sterols. So, insects
extract sterols from their food beside to
amino acid, carbohydrates, lipids, fatty
acids, vitamins, trace elements.
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Plants are suboptimal food
Polyphagous larvae of corn earworm
(Helicoverpa zea) grow best on an artifcial
diet with a protein:carbohydrate ratio of
79:21. Conversely, Locusta migratoria
nymphs require 50:50.
Why?
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Plants are suboptimal food
Helicoverpa zea
Locusta migratoria
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Plants are suboptimal food
The corn earworm grow
Growth activity
fast and therefore
need protein-rich
food, the locust is
Earworm Faster less
characterized by
slower growth and
higher activity level
Locust
Slower more
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Plants are suboptimal food
Growth activity need
Earworm
Faster
less
Locust
Slower more
More protein
More
carbohyrate
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Plants are suboptimal food
The artificial diets that support growth
better than natural food plants young
cutworm larvae (Agrotis ipsilon) gain 12
times as much weight as those raised on
susceptible corn plant.
Fig. 5.3
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Plants are suboptimal food
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Plants are suboptimal food
Conclusion
The most susceptible plants are in fact are
well defended against insect attacks.
The most susceptible plants are poor food
sources from a nutritional point view.
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Protein
Most plant tissue consists of
Carbohydrate-Cellulose
Most insect tissue consists of protein
Protein in plants
Protein in insects
Aromatic amino acids are amino acids
which include an aromatic ring.
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Protein
The balance of amino acids that constitute
plant proteins differs from the dietary
requirements of insects
Insects need higher levels of aromitic
amino acid, such as phenylalanine and
tryptophan, than present in plant protein.
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Amino acids
phenylalanine
Lysine
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Protein
High nitrogen levels may coincide with
metabolically useless nitrogen compounds
such as alkaloids, or with tannins, in which
in some cases reduce digestive efficiency.
In this respect phloem –feeding species are
in more privileged position than chewing
insects, because nearly all nitrogencontaining compounds in phloem sap be
utilized.
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Protein
Xylem forms a less suitable food source than
phloem ( 0.1% and 1% of nitrogen in the phloem
and xylem respectively.
Leaf-hopper suck enormous of sap to meet their
nitrogen and carbohydrate demands. Feeding
rates can be as high as 300 to 1000 times the
body weight per day.
Individual of leaf-hopper consumes 3.9 ml water,
57 µmol organic carbon, and 21 µmol nitrogen
during a day of feeding.
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Protein
Leaf-hopper suck enormous of sap to meet their
nitrogen and carbohydrate demands. Feeding
rates can be as high as 300 to 1000 times the
body weight per day.
Individual of leaf-hopper consumes 3.9 ml water,
57 µmol organic carbon, and 21 µmol nitrogen
during a day of feeding.
The daily intake of carbon about 14% of the
amount of carbon present in the body of insect.
The daily intake of nitrogen about 29% of the
amount of carbon present in the body of insect.
Fig 5.4
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Protein
The amount of nitrogen extracted by 16 leafhopper individuals during three weeks
experiment amount of 48% of total plant
nitrogen in case of soybean plant.
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Protein
Caution
Faster larval (lepidoptera) growth, due to
high nitrogen contents, caused pupal
mortality and reduction in adult size.
Insect performance as a whole is in this
case not correlated with a higher growth
rate during the larval stage on nitrogenenriched host plants.
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Nitrogen is an indicator of food
quality
The application of nitrogen fertilizer to plants
can be expected positively to affect herbivore
performance. But negative effects have been
also reported.
115 studies in which insects grew better with
increased plant nitrogen.
44 studies indicated a decrease of in insect
performance with high nitrogen concentration.
Why?
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Why?
1- The flush feeder insects species that are
adapted to high nitrogen levels in their
food, would respond positively to an
increased amount of nitrogen being
transported to the growing tissues,
whereas, senescence feeders would
respond negatively to decreased export
of nitrogen from senescing tissues
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Why?
2- Nitrogen fertilization may cause kind of
physiological and morphological alternation
to plant and affect secondary metabolism,
resulting in increased production of defense
substances. Morphological changes may
include an increase of leaf surface and leaf
thickness.
Thus, nitrogen fertilization-induced changes
naturally alter the value of the plant as a
home for the herbivore and its natural
enemies
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Consumption and utilization:
Consumption
Fast-growing insects consume large
amounts of food. Their gut occupies most
of the body cavity.
Food passage through the gut is fast and
often takes only a few hours in leaffeeding insects.
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Consumption and utilization:
Consumption
Schistocerace
gregaria
Food passage
through the gut in
hours
1.5
An Aphid
1
Cicadellid
More than one
hour
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Consumption and utilization:
Consumption
Young Caterpillar consume at a rate of six time
their body weight per day.
Adult locusts eat daily about their own weight of
food.
Sap feeding spittlebugs may ingest xylem in
amount of 100-1000 times their body weight per
day.
As result, mature larvae often weight several
times their weight at hatching (Silkworm,
Bombyx mori).
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Consumption and utilization:
Utilization
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