Insect Nutrition

 

Green plants are autotrophs. They use the energy of sunlight through the process of photosynthesis to manufacture organic molecules from carbon dioxide and water. Insects, however, are heterotrophs. They must consume other organisms (either living or dead) in order to acquire energy-rich molecules (nutrients) needed for survival, growth, and reproduction. A balanced diet must include an assortment of essential components in sufficient quantity to meet the metabolic needs of the animal. Major dietary components are listed below:

Carbohydrates

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Complex carbohydrates (polysaccharides) like starch and glycogen are broken down by digestive enzymes into simple sugars such as glucose, galactose, or fructose. These simple sugars can be processed through the Kreb’s Cycle and oxidative phosphorylation to yield energy in the form of ATP. Alternatively, they can be used as building blocks for construction of chitin, a major component of the insect’s exoskeleton. In some overwintering insects, a high concentration of sugar (e.g. trehalose) is sequestered in the blood and body fluids where it works like antifreeze to resist the lethal effects of cold temperatures.

Insects do not have digestive enzymes that can break down structural polysaccharides like chitin and cellulose (found in woody tissues and plant cell walls). Insects that feed on wood (termites, for example) rely on digestive enzymes secreted by symbiotic bacteria and/or protozoa living inside their digestive tract. Insects that feed on plant sap (leafhoppers and aphids, for example) must process large volumes of liquid in order to extract enough protein to meet their metabolic needs. A filter chamber mechanism allows excess water and sugar to bypass most of the insect’s digestive system for excretion as honeydew.

Types of Carbohydrates

Polysaccharides

Starch, Glycogen (energy storage)

Chitin, Cellulose (structural)

Oligosaccharides

Sucrose, Trehalose, et al.

Simple Sugars

Glucose, Galactose, Fructose, et al.

Proteins

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Protease enzymes in the insect’s digestive system break down proteins into their constituent amino acids. Cells use these amino acids to build or replace enzymes and hormones as well as proteins needed for muscle, egg yolk, ribosomes, cuticle, and many other purposes. Amino acids can be converted to carbohydrates by removal of the amino group and processed (like sugar) to yield energy in the form of ATP. Ammonium ions (NH4+), produced by this deamination process, are toxic waste products that are converted to urea, collected in the malpighian tubules, and excreted from the body as uric acid.

Of the twenty naturally occurring amino acids, at least ten must be present in an insect’s diet. These ten, called essential amino acids, include lysine, tryptophan, histidine, phenylalanine, leucine, isoleucine, threonine, methionine, valine, and arginine. The other ten amino acids are considered “non-essential” because they can be synthesized from other amino acids or similar chemical building blocks. Insect diets and human diets require the same ten essential amino acids!

Proteins

Actin, Myosin, Resilin, Arthropodin

Enzymes, Hormones, Ribosomes

Peptides

Brain hormone, Bursicon, et al.

Amino Acids

Alanine, Lysine, Histidine, Glycine, et al.

Lipids

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Fats (triglycerides) serve primarily as energy storage molecules.   They can be broken down into glycerol and fatty acids by lipase enzymes in the midgut.   The fatty acids may be further digested to acetyl by thiokinase enzymes in the mitochondria, and with the addition of co-enzyme A, processed through the Kreb’s cycle to yield energy in the form of ATP.   Fatty acids also serve as building blocks for cuticular waxes and the glandular synthesis of certain pheromones and defensive compounds.

Most vertebrates (including humans) are able to synthesize steroids from acetyl-coA.   Insects, however, appear to lack enzymes for this metabolic pathway and they must obtain steroid compounds (especially cholesterol) directly from their diet.   Steroid building blocks are used to make hormones (e.g. ecdysteroids) and growth factors.   Insects feeding on steroid-deficient diets generally survive as immatures but fail to molt properly into the adult stage.

Fats (triglycerides)

Oils, Waxes, Resins, et al.

Fatty Acids

Cell membranes, Pheromones

Steroids

Hormones, Cholesterol

Nucleic Acids

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Sugars (ribose and deoxyribose), nucleotides (adenine, guanine, cytosine, uracil, and thymine), and phosphates are the major products of nucleic acid digestion.   Individual cells reuse these components for synthesis of new DNA and RNA — information-storage molecules that contain the cell’s genetic code for growth and reproduction.   Dinucleotides such as nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide 3′-phosphate (NADP), and flavin adenine dinucleotide (FAD) are important oxidation-reduction co-enzymes that participate in the Kreb’s cycle and oxidative phosphorylation reactions.

Partial hydrolysis of the nucleotide adenine yields adenosine, a nucleoside that plays an important role in energy exchange within cells and tissues.   Adenosine triphosphate (ATP) is the “energy currency” of the cell, and cyclic AMP (adenosine monophosphate) is a “second messenger” that transports information from the cell membrane to the cell nucleus.

Nucleic Acids

DNA, RNA

Purines and Pyrimidines

Adenine, Guanine, Cytocine, Uracil, et al.

Mono- and Di-nucleotides

ATP, cyclic AMP

NAD, NADP, and FAD

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In addition to the four major categories listed above, insects also acquire water, vitamins, and minerals from their food.  Most terrestrial insects are highly-adapted for water conservation and get most of the water they need directly from their food.  A drop of morning dew is usually sufficient for the few insects who actively drink water.  Insects like flour moths and grain beetles survive on surprisingly small amounts of water.  Some species can live for months on only the “metabolic” water they generate as a byproduct of protein synthesis or chitin synthesis.  These chemical reactions free a single molecule of water each time a simple sugar is added to a polysaccharide (or an amino acid is added to a protein).

Vitamins include a number of complex organic molecules that animals need in very small amounts for specialized metabolic processes.  (Coenzyme A, for example, is made from pantothenic acid).  Humans (and most other mammals) must have a dietary source of both fat-soluble and water-soluble vitamins to ensure good health.  Insects, however, have the ability to synthesize their own fat-soluble vitamins (A, D, E, and K) from other compounds.  They still need a dietary source of most water-soluble vitamins such as thiamine (B1), riboflavin (B2), pyridoxine (B6), nicotinic acid, pantothenic acid, ascorbic acid (C), and biotin (H).

Minerals are inorganic substances that animals also need in relatively small amounts.  Iron and copper atoms are needed for cytochromes; nerves and muscles need calcium, sodium, and potassium ions; phosphorus is a component of cell membranes; and sulfur atoms play a significant role in both three-dimensional structure of proteins and sclerotization of the exoskeleton.