The black olive scale, known by its scientific name Saissetia oleae, is a typical parasite of olive trees. It has a distinctive appearance that sets it apart from other species of scale insects. If not properly controlled, it can cause severe damage to both olive production and the trees themselves.
In this article, we will describe the black olive scale. Additionally, we will cover its life cycle, the damage it inflicts on plants, agronomic prevention methods, and biological defense strategies.
Description of the Black Olive Scale
Saissetia oleae is an insect of the order Hemiptera, family Coccidae. It’s commonly known as the black olive scale due to its appearance, resembling the well-known spice. It has spread primarily in regions with milder climates, although it can be found wherever olive trees are cultivated. On plants, you can observe the adult female, about 5 mm long and oval-shaped. The body of this small parasite is highly convex, with distinct ridges on the back that form an “H” shape. The color is brown, but it appears almost black when in the egg-laying phase. The male has wings, but it’s rarely seen on plants. The nymphs are small and light-colored, turning darker as they age.
Damage Caused by the Black Olive Scale
The main host of the black olive scale is the olive tree. In addition to olives, this parasite also damages citrus trees, especially lemon trees, as well as oranges, mandarins, chinotto, kumquat, and more. The damage is caused by feeding punctures on branches and leaves. The black olive scale attaches to the leaf, usually on the lower side and along the midrib. While feeding, it extracts sap from the plant, leading to stunted shoot growth and subsequently reduced fruit production. Premature fruit drop is another clear sign of damage caused by this pest, affecting olive harvesting. Heavily infested plants may exhibit a decline, with dry branches and leaves. An indirect damage caused by Saissetia oleae is the excessive production of honeydew. This creates conditions of suffocation and poses a risk of burning to vegetation. Honeydew, moreover, attracts ants which, in turn, form a symbiotic relationship with the scale insect, inducing it to produce more honeydew. This leads to the formation of sooty mold, deteriorating the overall condition of the plant.
The black olive scale usually overwinters as nymphs. It rarely enters diapause as immature females or during egg-laying. The complete development of nymphs occurs in the spring and summer of the following year. During the summer, females mature, begin reproducing through parthenogenesis, and lay eggs which they protect beneath their bodies. Egg laying is typically completed by the end of July. Among the overwintering nymphs, only those emerging in spring complete their development and reproduce. These give rise to a second generation. As such, this type of scale insect completes one or two generations per year.
Certain climatic conditions favor the proliferation of the black olive scale. Mild winter climates, for example, allow the parasite to overwinter comfortably. Harsh winters, on the other hand, result in a high mortality rate for the insect. Additionally, the Saissetia oleae benefits from humid summers with moderate temperatures. Extremely hot and dry conditions above 38°C (100.4°F) cause high nymph mortality.
Preventing Black Olive Scale Infestations
When dealing with the presence and risk of black olive scale infestation, proactive measures through agronomic prevention are essential. Firstly, attention to planting density is crucial when establishing a new olive grove. Avoid planting trees too closely together, as reduced light and poor canopy ventilation encourage infestations. Olive tree pruning should be carried out with the right intensity, always aiming to aerate the canopy. In this sense, annual pruning is preferable. Biennial or longer intervals between pruning sessions result in vegetative accumulation, which increases the presence of the parasite. In case of infestation, pruning interventions can help eliminate the pest by removing and destroying the most heavily infested branches. Lastly, both excessive nitrogen fertilization and irrigation should be avoided. Excessive vegetative growth boosts the presence of this bothersome scale insect.
Natural Predators of the Black Olive Scale
Fortunately, nature offers several natural enemies of the black olive scale. However, their presence is only possible through natural farming methods, without the use of chemical pesticides. Here is a list of these natural enemies of scale insects:
- Chilocorus bipustulatus, Exochomus quadripustulatus (specific species of ladybugs);
- Scutellista cyanea, Moranila californica, Coccophagus spp., Diversinervus spp., Metaphycus spp. (parasitic wasps);
- Eublemma scitula (moth).
Biological defense against the black olive scale starts with careful monitoring of crops. Unlike the olive fly, where monitoring on olive trees is done using special traps, scale insect population control is done by visual inspection. Hence, it’s necessary to establish the presence of the insect on the canopy. If the pest is found, infestation trends are monitored using samples of 100 leaves. These samples should be taken every 7 to 10 days during the optimal period for bio treatments, which is summer. This process helps determine the degree of infestation and the biological stage of the insect.
Eliminating the Black Olive Scale
To eliminate the black olive scale from olive trees, the use of white mineral oil is recommended. This is a highly effective organic insecticide. After monitoring, treatment is advised only if the number of live nymphs found on leaves is greater than 4 to 5 per leaf (sample of 100 leaves). The mineral oil is most effective when applied to young nymphs, found on the plant between July and August. This is also the period when eggs hatch, so monitoring should be intensified. Once 70% to 80% of the eggs have hatched, treatment can be applied immediately. To target all eggs, treatment should be repeated at a 10-day interval. To ensure efficacy, the canopy should be evenly wetted. Additionally, consider that Saissetia oleae is mostly found beneath the leaves. The mineral oil acts upon contact, so treatment should be limited to infested plants. Applying it preventively to otherwise clean plants is unnecessary. For heavily honeydew-soiled vegetation with sooty mold presence, treatment with soft potassium soap can follow the white mineral oil treatment.