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The blueberry gall midge (BGM) Dasineura oxycoccana (Johnson) is an important pest of southern highbush blueberry in some regions of Florida. The injury from larval feeding on floral buds can reduce yields by up to 80% with significant infestations and can also lead to reduced plant vigor and increased susceptibility to secondary infections. In Florida, BGM populations have been recorded on blueberry farms throughout the north-central and central regions. Infestations are also commonly observed in nurseries.
Preventive control measures include maintaining blueberry beds by regularly adding fresh mulch or employing polyethylene plastic or fabric mulch; selecting cultivars that exhibit greater resistance to BGM; and ensuring proper timing of reduced-risk insecticide applications prior to the onset of floral bud break, followed by another application ten days later to safeguard natural enemies within the system. Chemical control measures can also be employed when two or more adult BGM are seen in a trap.
Adult BGM live for two to three days, and once mated, females lay eggs in the flower and leaf buds. While females typically choose buds that are in their second to third developmental stage, they have been observed laying eggs as early as the first developmental stage (Figure 1). One bud can contain eggs from multiple females, and up to 33 larvae have been observed emerging from a single bud. The larvae take about 10 days to develop, after which they emerge from the buds and undergo pupation in the soil. Under ideal temperatures, BGM can complete a generation within two to three weeks.
Blueberry gall midges break out of their summer and fall diapause state after a sudden “false winter” chilling event, when there is a cold period when temperatures fall between 68–77 °F for one to two days, then quickly rise again. Because males emerge before females, finding mostly males in a planting is a sign that the midges have just begun to exit diapause, and more females will likely be seen after two weeks. If there are subsequent periods of warm temperatures, the first generation of eggs will hatch, and larvae will begin feeding inside the buds, which damages flowers and leaves. If development is allowed to proceed, BGM that emerged due to false winters may also contribute to higher spring populations, causing more significant yield losses in the upcoming season.
The damage inflicted by BGM on blueberries has been mistakenly attributed to freeze damage or insufficient chilling (Lyrene and Payne 1995).
MONITORING AND MANAGEMENT PRACTICES
It is recommended to begin monitoring for BGM as early as mid to late November following a cold spell. Many growers in areas where BGM is an issue also begin making preventative insecticide applications at this time. Effective monitoring techniques include destructive sampling and the utilization of multiple traps.
With destructive sampling, flower buds are collected when they are in their second to third developmental stages. During this period, flower buds swell, and bud scales begin to separate—also known as “bud break” (Figure 1). Collecting buds in the late afternoon or early evening provides a better representation of what is in the system. During this time of the day, most females have laid their eggs, and larvae are well hidden inside the buds, protected from the hot daytime conditions. Most larvae exit the buds and pupate in the soil from early to mid-morning, when morning dew protects the larvae from drying out on their way down from flower and leaf buds (Figure 4). Buds should be placed in a ziplock bag and kept at room temperature. After two to four days, any larvae present will begin to emerge. While most larvae will emerge in the first few days, it can take up to two weeks for all the larvae to emerge from the buds.
Figure 1. Blueberry buds in various stages of development.
Credit: M. Lopez, UF/IFAS
Traps can also be used to monitor for the presence of adult BGM before they lay eggs in the buds. Clear, sticky, adult emergence traps are a cost-effective and informative monitoring strategy. Alternatively, bucket emergence traps are another method to monitor for the presence of BGM. These traps can effectively detect low populations of BGM and can accurately tell the grower if a new generation of larvae or adults have emerged on the same day they are deployed.
When placing traps in the field, growers should consider “hot spots” where midges can develop, indicated by depressions in the ground. Growers observed the highest infestations in these lower-lying areas, possibly due to trapped air pockets warmer than the surrounding environment. Growers can have between two to four clear sticky traps and one to three bucket emergence traps per acre. It is ideal to check the traps once every few days or once a week. Chemical control is recommended when two or more BGM are caught on both traps. For more information on making and setting traps, see Management of the Blueberry Gall Midge on Southern Highbush Blueberries in Florida (https://edis.ifas.ufl.edu/publication/IN1414).
DIAGNOSTIC CHARACTERISTICS
To confirm the presence of blueberry gall midge, a microscope is required as their differentiating physical characteristics are hard to see under a hand lens (Figure 2A–E). Both sexes are about 2–3 mm long, have orange abdomens, and bear black stripes across the top side of the abdomen (Figure 2A). Both sexes also have a distinctive R5 vein curved upward on their forewings (Figure 2B), which is different from the R5 vein on forewings of another similar pest of blueberries. Males are easily distinguishable from females by a forceps-like structure on their genitalia at the end of their abdomen and bead-like antennae with sparse clusters of sensory hairs, called sensilla, on each antennal segment of the flagellum (Figure 2D). The longest part of the antennae, the flagellum, is more visibly subdivided in its parts in males than in females. Females are slightly larger, have less visible sensilla, have more compact antennae, and have a noticeably swollen orange abdomen (Figure 2A, E).
Figure 2A–E. Diagnostic characteristics of BGM. (A) Abdominal view of female showing orange abdomen with stripes on tergites (upper side); (B) Wing margin of BGM with R5 vein curved upward. (C) Wing margin of Prodiplosis vaccinii (Felt) with R5 vein curved downward; (D) Forceps-like structure part of the genitalia (end of abdomen) in males, and pubescent antennae of male BGM; (E) Ovipositor and antennae of females.
Credit: (2A, D–E) M. Lopez; (2B–C) C. Roubos, UF/IFAS
MANAGEMENT PRACTICES
Management practices for BGM include the use of mulches, potentially resistant cultivars, and natural enemies. Use reduced-risk and conventional insecticides when all other strategies have proven ineffective.
MULCHES
BGM thrive in older, more decomposed mulch. This is because when the larvae exit the bud, they need to reach the soil level as quickly as possible to prevent their bodies from drying out. It is advisable to keep mulches below the 75% decomposed stage.
Growers have the option to use black or white polyethylene plastic mulch. Because BGM need a place to burrow in the soil, it is almost impossible for the larvae to reach the soil when plastic mulches or weed mat are used over the raised beds. Only when the bushes grow past the plastic mulched area, or if larvae exit the buds during rainfall or irrigation, can larvae reach the soil in the alleyways and continue their life cycle through pupation and emergence.
POTENTIALLY RESISTANT CULTIVARS
Six southern highbush cultivars were screened for resistance against this midge in north-central Florida, including “Farthing,” “Patrecia,” “Magnus,” “Sentinel,” “Optimus,” and “Colossus.” Research indicates that “Sentinel” and “Optimus” may have high levels of resistance, while “Magnus” and “Colossus” have moderate levels. “Farthing” and “Patrecia” appear to have the lowest levels of resistance. Temperature and relative humidity, the length of planting establishment time, the cultivar planted, and cultural control strategies all play a role in how severe a BGM infestation will be. We predict that plantings established for more than five years with little mulch turnover will host greater numbers of BGM. With warmer winters, more generations of this pest can exist in one season and lead to reproductive isolation, where the pest can potentially develop a close relationship with a specific cultivar.
NATURAL ENEMIES
In organic plantings, spiders have been observed constructing webs around the vulnerable blueberry buds and early floral development stages. Spiders may play a significant role in minimizing BGM numbers, but further research is warranted.
Parasitoids are the most well-known natural enemies of BGM. These tiny micro-wasps are active during both bud development and flowering on southern highbush blueberry plantings and are also excellent at parasitizing the larvae concealed within the blueberry buds where chemical control cannot reach They essentially kill their hosts but do not damage the blueberry buds in the process.
REDUCED-RISK INSECTICIDES
The use of targeted insecticides in field IPM programs enhances the protection of biological control agents. This approach can help decrease reliance on chemical control methods, as well as delay the development of pest resistance and resurgence in subsequent generations. Reduced-risk insecticides should be considered due to their ability to synchronize with naturally occurring micro-wasps (parasitoids).
In a field study conducted in 2020, several reduced-risk insecticides were evaluated for their efficacy on BGM and their key parasitoid. The study included a conventional standard Malathion and a no-application control.
The results of the study indicated that Movento® (spirotetramat) with Induce® (nonionic low foam wetter/spreader adjuvant) was able to control 59% of BGM adults (Figure 3) and 56% of BGM larvae hidden within the blueberry buds (Figure 4). Movento®’s mode of action is especially effective to target midges hidden deep within floral and leaf buds, which are impossible to reach using insecticides with different modes of action. The results also showed that Movento® with Induce® did not significantly reduce parasitoid numbers. Apta™ (tolphenpyrad) was the second best against BGM, reducing adults by 53% and larvae by 51%. Both Apta™ and Movento® with Induce® also performed significantly better than Delegate®. Note that Apta™ was one of the three most lethal to the micro-wasp parasitoids— the first being Exirel® (cyantraniliprole) followed by Admire® Pro (imidacloprid).
Figure 3 . Overall mean number of adult BGM recovered from clear sticky traps in 2020.
Credit: M. Lopez, UF/IFAS
Figure 4 . Overall mean number of larval BGM per replicate (25 flower buds/replicate) in 2020.
Credit: M. Lopez, UF/IFAS
Most of the information in this article is taken from Management of the Blueberry Gall Midge on Southern Highbush Blueberries in Florida (https://edis.ifas.ufl.edu/publication/IN1414). Please see that publication for additional details.
CREDITS:
OSCAR LIBURD
& DOUG PHILLIPS