The appearance of the Mediterranean fruit fly (medfly) and other tephritid-type fruit flies in general have become so commonplace that the insects are essentially a fact of life for most Californians, particularly in the Bay Area, the Los Angeles Basin, and the greater San Diego region.

Originally from central Africa, the medfly is now distributed throughout that continent, the Mediterranean regions of Europe, South and Central America, Hawaii, and western Australia with intermittent appearances in Florida and near-continuous discoveries in California.

With the exception of the Florida finds and a small outbreak in Brownsville, Texas, in 1966, this pest has never been detected in other states with medfly-friendly climates.

The medfly is a serious threat to California agriculture since the insect lays eggs in mature, market-ready fruits and vegetables. The eggs hatch and the maggots destroy the host.

Medfly hosts exceed 250 crops including stone and pome fruit, citrus, avocado, plum, walnut, tomato, pepper, and persimmon. According to the California Department of Food and Agriculture, the combined value of medfly hosts in California is nearly $10 billion.

Since 1982, about 250 emergency projects on tephritids have been conducted including more than 60 for the medfly and 150 projects for the oriental fruit fly. In 2009 and 2010, a total of 123 tephritid adults were captured, representing nine species in 11 counties that triggered 59 delimitation (trapping) and 16 eradication programs.

Although medfly discoveries and the intervention programs that follow are a nuisance for urban dwellers in the quarantine zones, the insect can be devastating for the hundreds of commercial growers located within five miles of a tephritid outbreak.

For these growers, medfly/tephritid finds that typically first occur from July to October can have catastrophic economic consequences with quarantines typically lasting until early summer of the following year. Like an unexpected lightning bolt, these medfly finds can instantly place at risk the livelihoods of hundreds of growers, packers, and distributers.

In 2009, a Fallbrook, Calif., grower estimated the loss on two acres of avocados at about $14,000. Restrictions due to the 2007 medfly quarantine in Dixon, Calif., located in Solano County in the Sacramento Valley, reduced the numbers of tomatoes in tubs on harvest trucks from 20 percent to 33 percent. Quarantine restrictions prevented growers from traditionally mounding the tomatoes and the availability of tarp to cover the tomatoes was limited which caused crop loss.

A farmer in the Dixon area who sells produce by the box to 800 urban customers had to reduce his price in half since it was illegal to ship produce included on the list of 260 medfly hosts.

In 2009, some growers in Fallbrook, Calif., were faced with overlapping pest quarantines for the medfly and the Asian citrus psyllid which included different regulatory requirements on treatments, durations, and costs.

Established residency

The reason medfly outbreaks have become commonplace over the last two decades, despite having been non-existent before 1975, is that several populations of the pest have established residency in California. Not only are the pests adapted to local conditions, but the insects are also pre-positioned to immediately expand when local conditions become favorable for population growth and inoculating nearby regions.

Evidence of outbreak clustering and sub-detectable populations is revealed in the history of medfly captures. From the first appearance in 1975 until the present, the medfly has been found in 168 of the 480 California cities, or in other words one out of three cities in the state. It has been discovered in multiple years in 47 cities.

The medfly can persist at insidiously low levels and is revealed in capture-span statistics. For three cities - San Jose, Santa Monica, Los Angeles – the capture span has ranged from 27 to 34 years. For 13 cities the capture span ranged from 10 to 18 years.

The medfly was discovered in Dixon in 2007. In 2010, the insect was found in the southern most reaches of California in Calexico in Imperial County. Both findings raise the likelihood that the medfly has spread to these two key agricultural regions.

Historical overviews of medfly outbreaks in the Los Angeles Basin and California as a whole reveal two important patterns. The first is that, as noted above, the medfly has been repeatedly captured for the past 15 years in the same regions where it was captured the previous decade.

These overlapping regions in California include the San Jose metropolitan area to the north and the greater Los Angeles Basin in the south.

The second important pattern is the pest has been found in new regions; most of which are relatively close to previously infested areas yet some are more distant. These include the cities of Escondido, Oceanside, and San Diego in San Diego County, as well as in Calexico, Novato in Marin County, and Dixon.

These repeat finds in old regions combined with the new finds point to an ongoing slow-motion invasion of the medfly. The invasion characteristics of this pest are consistent with the literature on invasive weeds; the pattern is of a ‘sleeper’ pest that remains at low levels for long periods but may eventually ‘awaken’ to grow aggressively and spread widely.

The medfly is not the only tephritid that continues to reappear frequently. Seven additional pest tephritids exhibit similar patterns which also point to establishment in California. The pattern includes reappearances over many years in the same region or location, outbreaks in new locations adjacent to previous outbreak areas, and the absence or near-absence of captures in other climate-friendly areas of California and the U.S.

Fruit fly species found in California likely established as ‘sleeper’ residents include the West Indian and Mexican fruit flies captured in each of seven and 43 different years, respectively, and the striped, peach, melon, guava, and oriental fruit flies, captured in each of seven, 12, 13, 22 and 45 years, respectively.

Few messages concerned with medflies and other tephritids in California are more relevant than the one contained in the old proverb ‘to be forewarned is to be forearmed.’

Knowledge of the threats from medflies and other tephritids and the understanding that these threats will never disappear should encourage California growers and policymakers to better understand this reality.

Pragmatic contingency plans are needed to allow the production and sale of California agricultural commodities so farmers can economically survive.