Instruction

How to Manage Olive Fruit Fly Infestations

Instruction

How to Manage Olive Fruit Fly Infestations

Love growing olives but hate the olive pests? (Join the club!)

The olive fruit fly (B. oleae) is olive growers’ #1 pest problem worldwide. In the Mediterranean, the olive fly is responsible for fruit losses of up to 80% of cultivars used as table olives. Mon dieu!

Why?

In large part, this pest is invisible, making it very difficult to combat or eradicate. It’s very small (under 5mm) and it reproduces by laying its eggs under the skin of the olive fruit. It simply can’t be seen due to its elusive maneuvers. In fact, many experienced olive producers don’t know they have an infestation until it’s too late.

Olive Fruit Fly

Above: Olive Fruit Fly (B. Oleae)

Puncture on olive

Above: Olive punctured by Olive Fruit Fly (B. Oleae)

Larvae in Olive

Above: Maggot (before it has evolved into an Olive Fruit Fly) living inside Olive

Infestation

Larvae hatch within a few days in warm weather and eat their way through the fruit as maggots before emerging in late summer or fall where they may find other foods besides olives to consume. Some overwinter (in soil or in fallen fruit – so it’s important to remove any remaining fallen fruit each year). Infestations are as high as 80% in the Mediterranean. California’s infestation is dramatically lower. In 1999, olive fruit fly was found in California though and is now present in most California olive-growing areas.

The most visible sign of infestation (and the time of greatest opportunity to damage fruit) is when the fruit begins to soften and turn color, from September to November. Of course, by late fall the damage is long completed. Hence, it’s imperative to take preventative measures earlier in the growing season.

Harvest & Food Safety

Many producers are unaware they have an infestation until they harvest and/or begin the curing process for olives meant for food production. It is then they discover the problem.

When harvesting young olives in the early ripening stage, the maggots may not yet have matured, but they have destabilized the fruit, ensuring they won’t withstand the curing process. Maggots feeding within the olive will destroy its value as a table olive because the fruit may break apart during the curing process.

In Europe there is some tolerance for infestation in the food safety chain. But in the U.S., tolerance of infestation is virtually zero.

Preventative Measures & Mitigation 

First, the key in the future is preventative measures. Second, how do you mitigate current infestation circumstances and reduces losses?

Although your fruit may no longer be usable for table olive production, it may be usable for olive oil. Olives that will be pressed for oil may have from 10-30% olive fruit fly infestation without a problem unless fruit pressing is delayed following harvest. However, olive oils extracted from olives with high infestation levels are very difficult to commercially classify as virgin olive oils, due to both sensorial and chemical quality parameters.

Damaged fruit that are stored for longer than a few days may have increased levels of acid within them that will alter the flavor of their oil. (There is a direct correlation between acid accumulation and the build-up of microorganisms [e.g., bacteria (Xanthomonas), yeasts (mostly Torulopsis and Candida), and molds (mainly Fusarium and Penicillium)] that develop in damaged fruit.)

The percent infestation by olive fruit fly maggots is not indicative of the amount of flavor change due to microorganisms. Total phenol content in olive oils is inversely related to the olive fly infestation level of the olive fruits.

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Identification

Olive fruit flies are very small: approximately 3/16” or 5 mm in size. They have a reddish brown color, with large, reddish eyes and small antennae. Along with clear wings, much of the rest of the body is brown with dark markings.

Insecticide

Table olive growers commonly apply weekly insecticide treatments for olive fruit fly control.

Spinosad is a protein-based insecticide derived by fermentation of the Actinomycete bacterium Saccharopolyspora. Spinosad kills susceptible species, including fruit flies, by causing rapid excitation of the insect’s nervous system. Here’s the catch: Insects must feed on the bait mixture and ingest the insecticide in order for it to be effective.

Two approaches to the problem are applied in the adult olive orchard. The first is to use traps containing spinosad bait percolating from a capsule. The trap is used either to monitor the presence of the olive fruit fly or to eradicate it completely. Monitoring of the presence of the fruit fly is achieved either by placing a trap every 3–5 ha or by examining 100 randomly picked fruits at each monitoring station.

Each fruit is searched for the number of bites, presence of any eggs, and the dead or alive larva content. Placing a trap every four trees can eradicate the fly efficiently and prevent future damage. In years when the fly causes no substantial damage, the economic cost of this method is not justified.

The second approach is to spray ground bait with the same active ingredient, spinosad. Also, Foliar spot spray is applied in the middle of each tree row using an all-terrain vehicle. Additional fruit examination after spraying can lead to ground foliar cover spray of a broad-spectrum organophosphate (Rogor® containing Dimethoate) if necessary.

There is a disadvantage of organophosphate spray: it’s non-specific targeting. As well as eradicating the olive fruit fly, it also eradicates its natural enemies, such as the parasite wasp and some acari, thus reducing biodiversity.

Another shortcoming of using this type of organophosphate spray is the high residue content in the fruit, which can be transferred to the oil produced. As a general rule, Rogor® should be applied at a rate of up to one spray per season, but hopefully it will not be required.

Since the Manzanillo variety is especially sensitive to olive fruit fly damage, this specific block or orchard (depending upon how you’ve laid out your trees) receives one or two sprays per season in addition to bait spraying.

Kaolin Clay

Kaolin clay is registered for olive fruit fly control in California as an organic treatment and has been used to protect plants from various insect pests. It works as a protective barrier film (brand name: Surround WP) and is produced by Engelhard Corporation. It requires regular application to be effective. Directions call for applications every seven to 14 days throughout the season.

Surround WP contains highly refined kaolin clay, with a small particle size, as well as a spreader sticker. Data from small-scale trials in California indicated very good success with the product. It is applied every five to six weeks starting at pit hardening, when the fruit becomes vulnerable. The efficacy of Surround WP is still being investigated for OLF control in California, and no recommendations for its use can be made currently.

We have heard from local growers of its pros and cons from their point of view: It does leave the tree looking greyish-white due to the clay application. If appearance it important, this may not be the appropriate mitigation tool for use. Additionally, some growers have reported that after a few years of application, the olive fruit fly failed to return to their orchards.

Find instructions and more details here and here.

Temp & Location

One important factor in insecticide treatment relates to the environmental temperature. In general, olive fruit fly activity decreases substantially if the air temperature is above 35-38 °C; therefore, there is no need to apply insecticide at such temperatures.

Olive flies survive best in cooler coastal climates, but they are also found in the hot and dry regions of Greece, Italy, Spain, Mexico, and California. The fly activity threshold is approximately 60°F (15°C). The optimum temperature for development is between 68° and 86°F (20° and 30°C). Temperatures above 100°F (38°C) are detrimental both to the adult fly and to the maggot in the fruit.

But they fly! They are very mobile and have the ability to seek cooler areas. Reports of fly movement range from 650ft (200m) in the presence of an olive host, to as much as 2.5 miles (4km) to find hosts. Do you know proximity to other hosts in your area?

Conclusions

Regardless of the preventive measures taken, you can see that this is an annual, ongoing process to mitigate. Develop a plan to inspect fruit, use preventive measures and have processes to reduce losses.

Resources

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