The invasive brown marmorated stink bug (BMSB) has made waves in recent years with its introduction to Ontario and remains on the radar for many growers. However, this new guy on the scene is not the only one increasing presence in orchards. Damage caused by native stink bugs, such as the green stink bug (Acrosternum hilare) (Figure 1) can often be more of an issue across the province in some years.
What’s a stink bug?
Stink bugs are relatively large, shield-shaped insects with small, narrow heads (Figure 2). Nymphs are smaller, rounded and wingless, often a different colour than the adult. The host range for stink bugs is quite extensive, including ornamental and fruit trees, grapes, berries, vegetables and field crops. Most species are well-adapted to a diversity of landscapes so can easily move between hosts to become well established in an area.
In an orchard, beneficial or predatory stink bug species are also present, feeding on aphids, mites and caterpillars. These can be distinguished from their plant-feeding (pest) relatives by the thickness of their proboscis, or beak (Figure 3). Predatory stink bugs have a very thick proboscis for attacking prey while plant-feeding stink bugs have a needle-like proboscis for piercing-sucking.
Stink bugs overwinter as adults in protected areas such as buildings or woodlots, becoming active when temperatures increase above 20⁰C in the spring. They are usually present in orchards following bloom or after nearby cereal or forage harvest. However, signs of injury do not often become obvious until later in the summer or after harvest. If you are beginning to see signs of damage now, feeding likely happened weeks earlier and the pest is now gone.
What’s the damage?
Stink bugs (both nymphs and adults) cause direct damage to the fruit using their piercing-sucking mouthparts. Digestive enzymes inserted into the plant result in the formation of small indented areas (Figure 4) at the feeding site with a light brown corky area just under the skin (Figure 5). Cutting the fruit open shows a feeding tube rarely more than halfway to the core.
Figure 4. Early stink bug damage appears as discoloured or bruised areas on the apple (left) or pear (right).
Figure 5. Older stink bug damage appears as dark, sunken areas with corky flesh beneath the skin.
Since stink bugs are highly mobile, perimeter rows tend to be at greatest risk of damage as these pests migrate in from bordering fields, buildings or woodlots. Damage is also more common in the upper canopy.
It is not uncommon to mistaken stink bug damage with bitter pit, especially during a year like this where the fruit is large. However, unlike bitter pit which tends to be most prevalent around the calyx end of the fruit, stink bug feeding can occur anywhere on the apple or may be most common around the shoulder. Use a hand lens to look for signs of a puncture mark in the centre of the indent. Also, the internal corking caused by stink bug injury will meet the skin, whereas bitter pit corking will not. Pay particular attention to where damage was observed in the orchard. Stink bug damage tends to be perimeter-driven whereas bitter pit can occur anywhere in the orchard, not just around the edges.
What’s the difference?
Most adult stink bugs are brown or green in colour and can be hard to distinguish between species (Figure 6). While management strategies for different pest species will not differ significantly, the period of activity and when damage may occur can vary so it is important to identify what is present in the orchard. Not only that but there can be non-stink bug look-alikes in an orchard, including numerous beneficial insects such as the spined soldier bug (Figure 7 & 8).
How to manage stink bugs
Unfortunately, this is not really a good news story. With limited effective monitoring tools and registered products, control of stink bug can be quite difficult in an orchard.
A few things to keep in mind if considering an insecticide spray:
- There are currently no thresholds. In tree fruit, the presence of adults or nymphs in the crop itself is enough to trigger an insecticide spray.
- Border sprays are sometimes sufficient for limiting damage, unless the pest has become established in the crop.
- Sprays will only control individuals present at the time of application, or shortly thereafter. New waves of adults can migrate into the orchard from adjacent areas.
- Nymphs are typically easier to kill than adults.
- Many insecticides that may be effective against stink bug such as pyrethroids are also harmful to natural enemies active during the summer and should be used only when necessary.
- For efficacy ratings of products available to apple growers, refer to the Insecticide Efficacy Table in the Ontario Crop Protection Hub.
Monitoring for stink bug activity is key. These pests are quite mobile and great at hiding within the canopy so can be difficult to spot in the field, especially when numbers are low. However, low numbers of certain species may still equate to economic injury, especially during dry spells in a season. Make sure to look for signs of stink bug injury when regularly scouting, especially along perimeter rows near woodlots, hedgerows and buildings.