Options for organic management of blueberry maggot
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
The blueberry maggot is a key pest of blueberries because infestation of fruit by the larvae of blueberry maggot makes it unmarketable. Management of this insect, whether organically or conventionally, depends on monitoring and appropriate reaction to trapping of flies. Organic growers may be more likely to engage in cultural controls to disrupt the life cycle, but the goal is the same – to prevent berries from becoming infested. This article reviews the currently available organic management options against blueberry maggot.
The first blueberry maggot flies begin to emerge from the soil in late June or early July, and are often stimulated to emerge by rain. After emergence, flies spend six to ten days feeding on insect honeydew, secretions and other deposits on foliage and bird droppings. During this period, female flies become sexually mature and mate. Once mated, the females seek ripening blueberries in which to lay eggs. At this time, flies often immigrate into fields from surrounding wild habitats, but if infestations are established in fields the flies can emerge here and quickly move to the bushes. The female fly punctures the skin of the blueberry in order to deposit a single white elongate egg. In three to five days, the eggs hatch and the larva (maggot) begins feeding. As the larva feeds and grows, the berry begins to shrivel. Breaking open one of these berries will reveal a small white maggot set against the bluish-colored flesh of the fruit. After two or three weeks of feeding, the larva becomes full-grown and the berry is unmarketable. Infestations tend to increase rapidly to a peak by mid-August when larvae begin exiting the berries and dropping onto the soil. Once there, they burrow into the soil and pupate in the top few inches of soil.
Weekly checking of yellow sticky traps can be used to identify when adult blueberry maggot flies are active, and action should be taken soon thereafter to prevent infestation of the fruit. Yellow traps baited with ammonium odor are the most attractive. The traps can be purchased with a glue impregnated with the odor, and this lasts for about two weeks. After that, yellow plastic “chargers” can be used to provide the odor, so that the traps can be changed when they become filled with insects. To increase the chance of catching flies, hang traps in a V-orientation with the yellow side facing down. Good maintenance of traps will ensure that flies will be attracted to the traps, so it is important to clean traps at each visit and to maintain the odor bait.
Accurate fly identification is critical for blueberry maggot management because there are a few other species of fruit flies that look similar to the blueberry maggot. Being sure that a fly on the trap is not a blueberry maggot can help growers save money by avoiding unnecessary sprays, or make the appropriate decision to protect the fruit if the fly is a blueberry maggot. In Michigan, the cherry fruit fly is active in the field slightly before the blueberry maggot and may be captured on the same traps. These flies usually come from wild cherry and are therefore most likely to be trapped next to deciduous woods. Care should be taken not to mistake these flies, which are distinguishable from blueberry maggot by their wing patterns (see Figure 1). The apple maggot has wing patterns similar to the blueberry maggot but it is bigger, appears later in the season (timed to apple ripening) and is not attracted to blueberry. It is unlikely that any flies trapped in blueberry fields would be apple maggot.
Cultural controls can be powerful approaches to reducing infestation by blueberry maggot. Selection of very early- or very late-ripening varieties may minimize the likelihood that flies will be active when the fruit is ripening. It will be interesting to see how the adoption of late-season varieties affects this pest. Research in New Jersey, has identified populations of blueberry maggot that emerge much later than the typical populations, presumed to have been pushed later because of insecticide spraying.
Mulches may have direct and indirect effects on blueberry maggot. If an infestation is established in a field, application of a dense wood mulch or plastic over the top of the pupae in the fall or in the spring could trap the flies and reduce their ability to emerge in the following summer. However, there may also be indirect effects on the natural enemies that walk across the ground surface searching for larvae or pupae. In experiments conducted in Michigan blueberry fields, we found lower densities of ground beetles (predators) in plots with bare ground compared to plots with clover or ryegrass.
Options for chemical control of blueberry maggot under organic standards in Michigan include kaolin clay mineral particle film, neem, pyrethrum and spinosad products. Please consult www.omri.org for changes and updates in the brand name product listings.
Kaolin is a naturally occurring clay, resulting from weathering of aluminous minerals such as feldspar. Kaolin likely acts as an irritant to the blueberry maggot. After landing on a treated surface, particles of kaolin break off and attach to the insect’s body triggering intensive grooming that distracts the flies from laying eggs. Surround® WP Crop Protectant (Engelhard Corp.) is a kaolin product registered for use in Michigan that is available as a wettable powder. Application can be made with most commercially available spray equipment, but large amounts of water are required because thorough coverage is essential. Periodic shaking is recommended for a backpack sprayer or use of an automatic agitation mechanism for larger equipment in order to keep the material suspended in water. Hydraulic sprayers at full dilution apply a better covering than mist blowers using concentrated sprays. Surround WP is considered to provide excellent protection against blueberry maggot infestation. Growers selling their fruit as U-Pick or into the fresh market should be aware of the white residue left on the fruit by this product. In MSU trials, the residue was very difficult to remove from the calyx cup of blueberries.
Neem products are derived from the neem tree, which is native to southern Asia. Neem has been used for centuries for medical, cosmetic and pesticidal purposes. Neem pesticide products are made by crushing neem seeds, then using water or a solvent to extract the pesticidal constituents. Neem products obtained with different extraction techniques may result in different biologically active chemicals (or amounts of chemicals) being present in a product. Thus, the efficacy of different products may vary. Neem products registered in Michigan include Agroneem® (Agro Logistic Systems, Inc.), AZA-Direct™ (Gowan Co.), Neemix® 4.5 (Certis USA). Note that Ecozin 3 percent EC (AMVAC) is not OMRI-listed. Neem products are considered to provide fair protection against blueberry maggot infestation.
Pyrethrum is an insecticide derived from the powdered, dried flower heads of the pyrethrum daisy, which is native to southwest Asia. Pyrethrum is a fast acting contact poison that affects the normal function of the nervous system leading to paralysis. However, some insects are able to recover after the initial knockdown if the dose is too low. No pyrethrum products are labeled specifically for use against blueberry maggot in Michigan. However, PyGanic® Crop Protection EC 1.4 II (MGK Co.) is registered for use against Japanese beetle which often occurs at the same time. Pyrethrum products are considered to have fair to good activity against Japanese beetle and will also have some activity against blueberry maggot. Note that Evergreen® Crop Protection EC 60-6 (also by MGK Co.) is not OMRI-listed.
Spinosad is composed of spinosyns A and D, substances produced by aerobic fermentation of a filamentous bacterium found in the soil. Spinosad acts primarily through ingestion. It activates the nervous system of the insect, causing loss of muscle control. Spinosad products registered for control of blueberry maggot in Michigan include Entrust™ (Dow Agrosciences, LLC) and GF-120 NF Naturalyte™ Fruit Fly Bait (Dow Agrosciences, LLC).
Growers aiming to protect berries from blueberry maggot infestation should be aware that the residual control provided by these insecticides is generally less than that provided by conventional products. Thus, if blueberry maggot flies continue to be caught on monitoring traps, a weekly spray interval may be more appropriate to maintain protection of berries.