When Hoyas are in flower, you will often find sticky substances on any plants near the Hoyas in bloom. But sometimes, you can see sticky substance on the leaves or stem, petioles and other parts of your Hoyas even though they aren’t in bloom.
If you ever wonder what this sticky substance is then read on as I asked the opinion of some of my Hoya mentors/advisers about it. I also consulted Mr. Google and here is what they reckon.
The sticky substance other than nectar from Hoya blooms can be Extra floral nectaries or could be resin or sap.
1. Extra Floral Nectaries or EFN
Extrafloral nectaries are sugar-rich liquid which is produced by the plant in order to develop a symbiotic relationship with ants in the wild. EFN are often mistaken for bug damage or even a disease in the plant.
EFN are secreted by nectar-producing glands physically apart from the flower, that have been identified in more than 2000 plants species in more than 64 families. EFN glands may be located on a leaf laminae, petioles, rachis, bracts, stipules, pedicels, fruits and in the leaves. Their size, shape and secretions vary with plant taxa. EFN content differs from floral nectar, varies by taxa and may or may not flow in daily pattern.
Two functions for the EFN have been hypothesized as an excretory organ for the plant to rid itself of metabolic wastes or to attract beneficial insects for plant defense. It is well documented that many insects use EFN and it is easy to observe beneficial insects such as ladybird bettles feeding on EFN.
Many species of vine have EFN and the evolution and selection for EFN is hypothesized to occur as a direct result from the ants using the vines frequently as natural pathway into the forest canopy. In the wild, ants build nests amongst the plants roots, which help hold the nest together. The plant, in turn, obtain nutrients from the nest, and the aggressive nature of the ants serve to protect the plant from other insects, such as caterpillars, that would otherwise eat the plant.
The occurrence of EFN appears to be controlled by a single gene in most plant species. EFN offer an important supplemental food source for beneficial insects and too, some pest species (including aphids and mealy bugs on Hoyas) particularly during extreme weather conditions such as drought or other times of the year when prey are scarce.
Extrafloral nectaries are most often found on the leaves and petioles of the plant, and although they can be quite small and discrete, you can spot them by their different coloured border, usually yellow, black, purple or green, or wine-colored, which add an extra pop of color to the plant!
In some cases you can observe an excessive production of nectar which can leave the surface of the plant covered with it. Nectar droplets can leave a yellow spot on the leaf, which does not look attractive, nevertheless it’s harmless and should not be confused with pests. Extrafloral nectaries are not harmful to the plant and represent a natural part of the plants life.
Location of EFN in Hoyas : upper leaf surface, peduncles, pedicels, stems or petioles
Secretion of extrafloral nectar observed on several peduncles of Hoya latifolia. Extrafloral nectaries could be the reason why ants always find and particularly visit the tip of peduncles whenever it's going to form new flower buds. (Credits for Photo and write up) – Ryu Cuerdo, Hoya Philippines.)
Other Sources of information on EFN:
2. RESIN or SAP
Another form of sticky substance that can exist in your Hoya leaves is resin or sap.
Usually, the colleter in the base of the leaf produces resin or sap when there is weather disturbances or change in temperature.
In some cases, secreting sap is one of the plant’s défense mechanism when they are infected by virus or fungus or insect bites.
(NOTE: you cant prevent the plant to secrete EFN or sap but you can prevent physiological damage to your plants, specially the leaves).
Avoid build up. Excessive EFN and sap can damage the leaves. Wash the leaves with dishwashing liquid or wipe with 70% isopropyl alcohol wipes and gently scrape the affected part.
Credits: Derek Cabactulan – Hoya Lovers of the Philippines and Beyond