This presentation explains why the sap sucking insects excrete honeydew. Digestion of the sucrose in sap sucking insects. Chemical transformation of sucrose in the gut contribute to aphid osmoregulation other examples which excrete honeydew.

1. Why sap sucking insects excrete honeydew??
Presented by
M. Satwika - RAD-21-20
B. Madhuri - RAD-21-18
Department of Entomology

2.  In general, digestion and absorption of food takes place in the midgut.
 In sap sucking insects, the food directly passes from the anterior part of
the midgut to the hindgut i.e. morphological adaptation of sucking pests
(filter chamber). Hence, less absorption of the food occurs.
 Sap sucking insects takes food continuously to compensate the more loss
of water, sugars and amino-acids.
 Eg: On susceptible plants, aphids often feed continuously from single
sieve elements for extended periods. The evidence comes principally from
the “honeydew clock” experiments, where each honeydew droplet is of
uniform size at regular intervals of once per 15–40 min.
Why??

3.  When their mouthparts penetrate the phloem, the
sugary, high pressure liquid is forced out of the anus
of the aphid.
 Honey dew accumulate in the rectum prior to
elimination. The rectal epithelium of aphids
is very thin, offering minimal barrier to water
and ion movement.
 As a result, sap sucking insects excretes honeydew (rich in
sugars) after passing through filter chamber.

4.  The excreted liquid, on drying, leaves a white deposit on the plant
leaves is called manna.
 Manna is excreted by coccid Trabutina mnannipara feeding on
tamarisk in sinai which contains 99.3 % carbohydrates.
 The reason for the wasteful method of feeding due to deficiency of
organic nitrogenous compounds in plant juice.
 Therefore, insect takes large quantities of plant juice continuously in
order to get sufficient aminoacids and proteins.
 As a result, it excretes manna with excess carbohydrates.
 This was not found in the case of Pineapple mealybugs, since large
quantities of free amino acids and sugars were found to be present in
their honey dew.

5. Digestion of the sucrose in sap sucking insects
 α-glucosidases are the carbohydrases which hydrolyze specific
disaccharides and oligosaccharides.
 Sucrase is an example of α-glucosidases.
 Sucrase break sucrose in the midgut of aphids.
 Sucrase generally mediate hydrolysis because water is the typical acceptor
for the sugar residues, but sugar at high concentrations can act as a
acceptors, resulting in the formation of oligosaccharides.
 Thus, the aphid sucrase can mediate the formation of trisaccharides and
longer chain oligosaccharides which are commonly voided in the honey
dew.
 However, the ingested sucrose is undetectable or accounts for a very small
proportion of the honeydew sugars, suggesting that the voided sugars are
the product of extensive enzymatic modification in the insect gut.

6. Sucrase
 In general : Sucrose Glucose + Fructose
(Disaccharide) Midgut (Monosaccharide) + (Monosaccharide)
Sucrase
 In aphids : Sucrose Trisaccharides + Oligosaccharides
(Disaccharide) Midgut
(Honey dew)
 The glucose moiety of much of the ingested sucrose is channelled into
oligosaccharide synthesis and voided via the honeydew, while the sucrose
derived fructose is assimilated quantitatively from the gut in to the aphid
tissues.

7. Chemical transformations of sucrose in the gut contribute to aphid
osmoregulation
 The osmotic pressure of phloem sap is 2–4 times greater than that of aphid body
fluids. As a result, an aphid would be expected to lose water to the lumen of the
alimentary tract and shrivel as it fed.
 The transformation of ingested disaccharides (sucrose) to oligosaccharides by
sucrase or transglucosidase would tend to reduce the osmotic pressure of the gut.
This is called transglucosidase activity.
 By this activity, osmoregulatory collapse is circumvented by downregulation of
the osmotic pressure of the gut contents, such that the aphid honeydew and
haemolymph are isosmotic.

8. Other examples:
 In Madagascar, some gecko species in
the genera Phelusma and Lygodactylus
are known to approach flattid plant-
hoppers on tree-trunks from below and
induce them to excrete honeydew by
head nodding behaviour. The plant-
hoppers then raises its abdomen and
excretes a drop of honeydew almost right
onto the snout of the gecko. Trophobiosis

9. The genomic sequencing of Drosophila and mosquito for cellulase gene
 Genetic capacity to produce cellulase enzyme is not universal, among the
insects.
 The genomes of all Drosophila and mosquito species have been
sequenced to date lack an identifiable cellulase gene.
 GHF9 (Glucoside hydrolase family) cellulase gene is found in honey bee
genome. Many experiments were conducted by dropping the GHF9 gene
in Drosophila and Anopheles. But, honey bee cellulase gene is not
conserved in these species, as all of which lack cellulase.

10. References:
 Chapman R. F. 2014. The Insects: structure and function. Cambridge
university press. lec-16,463-499.
 Douglas A. 2003. The Nutritional Physiology of Aphids. Advances in
Insect Physiology. 31:73-140.
 Kunieda. T et al. 2006. Carbohydrate metabolism genes and
pathways in insects: Insights from the honey bee genome. Insect
molecular biology. 15 (5): 563-76.