Blueberry maggots
Learn how to identify blueberry maggot and requirements to prevent its spread to non-infested areas.
Blueberry maggot (BM) is a direct pest of both highbush and lowbush blueberries. Infested berries are considered unmarketable, as there is no tolerance for larvae in either fresh market or processed fruit. While BM is native the eastern North America, including New Brunswick, Nova Scotia, Prince Edward Island, and the eastern United States, it has spread westward to parts of southwestern Quebec (1993) and parts of southwestern Ontario (1996).
The Canadian Food Inspection Agency (CFIA) has developed requirements to prevent the spread of BM to non-infested areas. D-02-04: Phytosanitary Requirements for the Importation from the Continental United States and for Domestic Movement of Commodities Regulated for Blueberry Maggot outlines what growers must do when they are producing blueberries in infested areas or on infested farms.
Despite these restrictions, BM has continued to spread. Blueberry maggot has now been detected at 23 sites, and approximately 30% of sites in southern Ontario. Eight new sites were identified in 2013 and three in 2012. All infestations of BM must be reported to the CFIA.
Identification
The BM has 4 life stages: egg, larva ("maggot"), pupa and adult fly. Monitoring involves checking for larvae in fruit and adults in traps, as other life stages are difficult to detect.
Mature larvae are light-coloured, 8mm in length and with no obvious head capsule (Figure 1). Their bodies are tapered or pointed at one end and blunt at the other. Two black mouth-hooks are visible on the pointed end, while six distinct brown breathing holes (spiracles) are found on the blunt end; these features are difficult to see without magnification. Small BM larvae are easily confused with the invasive spotted wing drosophila larvae (SWD), except these appear more tapered at both ends. There are several other larvae that can be found in blueberries (Table 1). There is usually only one BM larva per berry versus several larvae per berry with SWD.
Figure 1: Blueberry maggot larva in fruit. (Photo credit: Rufus Isaacs, Michigan State University)
| Insect pest | Head (other) | Legs (other) | Colour | Size of mature larva |
|---|---|---|---|---|
| Blueberry maggot | No head capsule (black mouth hooks) | No legs | Cream | 8 mm, pointed at one end, blunt at the other |
| Cranberry fruitworm | Brown head capsule | 3 pairs legs (plus abdominal prolegs) | Light green | 9-10 mm |
| Plum curculio | Brown head capsule | No legs | Cream | 7-9 mm, C-shaped |
| Spotted wing drosophila | No head capsule (black mouth hooks) | No legs | Cream | 5-6 mm, tapered at both ends |
The adult BM is a fly that is slightly smaller than a housefly, about 4-5 mm in length. The body is shiny black with a white dot on the thorax. Females are slightly larger than males, with four pale lines on the abdomen (versus 3 in males) and a needle-like ovipositor (Figure 2). The most characteristic feature is the dark brown F-shaped pattern on their wings, which can be used to distinguish it from several other fruit flies (Figure 3 - inset photos). The wing pattern of BM flies is very similar to that of apple maggot (AM). Other features used to distinguish between BM and AM adults include the length of the female's ovipositor and colour patterns on the legs. These characteristics may be difficult to see, and so confirmation by a taxonomic specialist is required.
Figure 2: Female blueberry maggot female. (Photo credit: Rufus Isaacs, Michigan State University)
Period of activity
There is one generation per year (Figure 3). Adults emerge from overwintering pupae in the soil from late June through early July, depending on local weather conditions and soil moisture. Newly emerged adults are sexually immature. Prior to mating and egg-laying, adults must feed on nectar, honeydew and organic matter for 7-10 days. Females lay up to 100 eggs over a three week period, sometimes into early September. Eggs are deposited under the skin of ripening blueberries beginning to colour. The developing maggot consumes the pulp of the berry, causing it to collapse. Larvae remain in the ripening fruit for several weeks, and may be present at the time of harvest. Infested fruit may ripen and soften prematurely. Mature larvae exit the fruit and drop to the soil, where they pupate at depths up to 5 cm. A small percentage of pupae will remain in the soil for more than one year.
Figure 3: Blueberry maggot life cycle. Wing patterns of similar species are shown. (Photo credit: Adapted from MAPAQ)
Blueberry maggot is a regulated pest
The CFIA conducts detection surveys for BM every year to identify new positive sites or townships outside of regulated areas. Areas are regulated by township or at the level of individual growers. A township is regulated if there is at least one infested natural (wild) site within the township OR there are host plants growing within 500 metres of an infested grower site. Natural sites are not managed, and therefore they represent a continuous source for re-infestation of nearby grower sites. If there are no host plants located within 500 metres of the infested grower site then only that site will be regulated, not the entire township.
At locations where BM has not been detected previously, the detection of a single fly on a trap can result in a stop shipment order from the CFIA, regardless of a grower's spray program: Growers are ordered to stop all movement of blueberry fruit off the farm. This can happen right in the middle of harvest.
Blueberry Certification Program
Where BM has been detected, either at the township or the individual site level, growers can apply to be on a Blueberry Certification Program (BCP) so that they can move regulated commodities outside of the area. Before the beginning of the growing season and prior to approval into the BCP, first time participants need to be trained by the CFIA on the components of the program including: biology and identification, field monitoring, cultural and chemical control measures, fruit sampling and testing and product grading to identify suspect fruit. Applications must be submitted at least two weeks prior to the expected emergence of the adults, and the approval must be renewed each year.
Monitoring for adult flies is the foundation for managing BM and is a key component in the BCP. Traps are monitored at least twice a week through harvest. Growers can choose to control BM using an IPM program or an approved calendar spray program. They can choose to have CFIA do the monitoring or do their own. In either situation, they are required to apply insecticides if and when BM is caught on traps and can continue to ship fruit unless it is found to be infested with larvae. Fruit sampling is generally conducted by the CFIA.
There are several insecticides registered for control of BM. Most commercial growers are now spraying to control SWD through the berry ripening period, and some (but not all) of the products used for control should also keep BM in check.
Cultural practices such as sanitation, pruning, and weed management can help reduce infestations on the farm. All unharvested berries should be removed from the field. Clean-picking and destruction of fallen berries or cull piles helps to destroy any larvae present in fruit and thus reduce re-infestation. Weeds provide sheltered areas for the adult flies. Pruning helps improve spray coverage.
Growers on the BCP must keep extensive records of all pest management activities and audits are conducted to ensure compliance. For details, visit the CFIA website.