Ambrosia beetles are a group of wood-boring insects that can attack and kill young fruit trees and broadleaved nursery stock. Although these beetles typically attack trees under physiological stress as a result of flooding, drought, winter injury or those weakened by pathogenic fungi, they can attack apparently healthy trees. Three species — Xylosandrus germanus, Xylosandrus crassiusculus, and Xyleborinus saxesenii, have recently been attacking high density apple orchards in New York, New Jersey and Michigan, causing substantial damage and cause for concern among growers. The species of greatest concern in the United States are the black stem borer — X. germanus (Figure 1) and the granulate ambrosia beetle — X. crassiusculus, both of which have previously been identified in Ontario. Granulate ambrosia beetle is a new introduction and its distribution has not been documented.

Black stem borer.
Figure 1. Black stem borer, Xylosandrus germanu. Photo credit: Pennsylvania Department of Conservation and Natural Resources.

Biology and damage

The adult beetles are very small, about 2 to 3 mm in length. In the springtime, mated females leave their overwintering sites in search of new trees to colonize. Females bore into the trunk or branches to create tunnels and branched chambers called galleries where her developing brood will grow.

This group of beetles do not digest the woody tissue of host plants. Instead, the beetles have an obligate mutualistic relationship with a group of ectosymbiotic fungi called ambrosia (hence their name). Ambrosia fungi are carried into the gallery on the beetle's body when she colonizes the tree and become the food source for the adult female and the developing brood. The presence of the fungi disrupts the nutrient and water flow in the tree, causing sap to collect at the gallery surface. The ambrosia fungi alone do not kill a tree. However, the combination of high beetle density and fungal activity can cause wilting and eventual tree death. The secondary introduction of other plant pathogens into these beetle galleries can also cause tree decline.

Ambrosia beetles have a wide host range that includes many woody plants. Trees near the perimeter of orchards, especially near woodlots, are at greatest risk of attack. Signs of infestation include 1 mm diameter entry holes, often with oozing sap, discoloured or blistering bark, and sawdust "toothpicks" from tunnel excavations that protrude from the holes (Figure 2). Some growers have reported that heavily infested trees appear to spontaneously collapse.

Signs of ambrosia beetle infestation.
Figure 2. Signs of ambrosia beetle infestation can include 1 mm diameter entry holes (left) that often ooze with sap, and sawdust "toothpicks" protruding from holes (right, Photo: K. Douce, University of Georgia)

2016 Ontario survey

This year, we monitored high-density apple orchards to gain a better understanding of the range, abundance and diversity of ambrosia beetles in southwestern Ontario. Ethanol-baited traps were installed in 13 commercial orchards across the province to track female flight and to identify the ambrosia beetle species present. Signs of new injury were assessed periodically in trees surrounding the traps. All five apple districts were represented (Figure 3).

Ontario apple growing districts.
Figure 3. Ontario apple growing districts (Ontario Apple Growers).

Results of this survey concluded:

  • Six species of ambrosia beetle were identified in the 5 growing regions (Figures 4-8), including:
    • Anisandrus dispar (European shot-hole borer or pear blight beetle)
    • A. sayi (no common name)
    • Xylosandrus germanus (black stem borer)
    • Xyleborus obesus (no common name)
    • Monarthrum mali (apple wood stainer)
    • Xyleborinus saxesenii (fruit tree pin-hole borer)
  • Most activity declined by late June with the exception of M. mali and X. obesus which flew later in the season (mid-June to early August).
  • The most abundant species were X. germanus, X. saxesenii and X. sayi (Table 1).
  • Although X. crassiusculus is one of the main pests found in the northeastern US, very few were found in the Ontario survey and only in District 4. These beetles were collected in a peach orchard, not apple.
  • It is possible that not all of these beetles are attacking apples, as the ethanol baits may attract beetles from nearby woodlots or adjacent hedgerows. However, X. germanus is a confirmed pest in commercial apple orchards in other areas.
  • The locations where specimens were collected had obvious damage from previous years with holes that were dried and inactive. However, no new damage was observed this year. This could be due to the mild winter causing less stress on the trees which therefore had better defences against beetle attacks.
  • The numbers of beetles caught were relatively low (not surpassing 50 from a single trap), while a similar trapping survey conducted in New York detected over 100 beetles during the same time interval. This indicates that perhaps 2016 was a year of low abundance.
  • More research is needed in Ontario on the distribution of this beetle complex and the extent of damage that can occur in high-density orchards, including what fungal pathogens are found in beetle galleries.
Figure 4. District 1 — Ambrosia beetle samples collected from three orchards with two traps at each orchard (n=6) for 10 weeks.
District 2 — Ambrosia beetle samples collected from two orchards with two traps at each orchard (n=4) for 8 weeks.
Figure 5. District 2 — Ambrosia beetle samples collected from two orchards with two traps at each orchard (n=4) for 8 weeks.
District 3 — Ambrosia beetle samples collected from 3 orchards with two traps at each orchard (n=6) for 12 weeks.
Figure 6. District 3 — Ambrosia beetle samples collected from 3 orchards with two traps at each orchard (n=6) for 12 weeks.
Figure 7. District 4 — Ambrosia beetle samples collected from 2 orchards with two traps at each orchard (n= 4) for 13 weeks.
Figure 8. District 5 — Ambrosia beetle samples collected from two orchards with two traps at each orchard (n=4) over 13 weeks.
Table 1. Abundance of ambrosia beetles by apple growing district, May 3–August 24, 2016.
 # weeks# trapsA. disparX. germanusX. saxeseniiM. maliX. obesusX. sayiTotal
District 1
10
6
10
63
97
26
22
26
244
District 2
10
4
5
43
27
6
27
62
170
District 3
16
6
26
42
42
6
29
41
189
District 4
13
4
14
20
15
0
0
62
111
District 5
14
4
11
1
79
1
1
34
127
Total
16
24
66
169
260
39
79
225
838

Management

Insecticides, in the form of trunk sprays, have limited effectiveness since beetles spend the majority of their lifecycle inside the host tree, where they are protected from topical sprays. Preventative application of repellents (for example, pyrethroids) may deter or kill females as they attempt to find a host and excavate new galleries. Regular monitoring of beetles is required to determine early and peak activity periods. Timing is challenging, particularly if several species are present.

In the absence of registered insecticides to target colonizing females, the only management strategy we have available in Ontario is the removal of dead branches and dying trees. Since beetles will complete their life cycle inside the wood of the tree, all infested tissue should be burned. Maintaining tree health will help prevent beetle colonization and will help trees with low levels of infestation to recover.

We gratefully acknowledge all growers who participated in this study. This project was funded through the OMAFRA-University of Guelph Undergraduate Student Experience Learning program. We would like to thank project mentor Dr. Cynthia Scott-Dupree, School of Environmental Sciences — University of Guelph and Michael Celetti, OMAFRA for technical assistance.

References

  • Agnello, A., Breth, D., Tee, E., Cox, K., & Warren, H. R. (2015). Ambrosia beetle - an emergent apple pest. N Y Fruit Qtly. 23: 25-28.
  • Agnello, A. (2016). Trunk call - Gallery Openings. Scaffolds Fruit Journal. 25(3): 2-5.
  • Ranger, M. C. (2010). Experience with Black Stem Borer in Ornamental Nurseries. Presentation from USDA-Agricultural Research Service.