Pine root tip weevil, Hylobius rhizophagus, and pine root collar weevil, H. radicis, attack certain Scotch pine stands simultaneously causing more mortality than expected from either insect alone. Recommendations for curtailing this insect complex include favoring red pine, planting Scotch pine far from brood sources, and avoiding stump culture of Christmas trees

Mosher, Daniel G

Wilson, Louis F

English

Daniel G Mosher

Louis F Wilson

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Scotch Pine Deterioration in Michigan Caused by Pine Root Weevil Complex

ValpoScholar

The Great Lakes Entomologist 
Volume 10 
Number 4 - Winter 1977 Number 4 - Winter 
1977 
Article 1 
December 1977 
Scotch Pine Deterioration in Michigan Caused by Pine Root Weevil 
Complex 
Daniel G. Mosher 
Michigan Department of Natural Resources 
Louis F. Wilson 
USDA Forest Service 
Follow this and additional works at: https://scholar.valpo.edu/tgle 
 Part of the Entomology Commons 
Recommended Citation 
Mosher, Daniel G. and Wilson, Louis F. 1977. "Scotch Pine Deterioration in Michigan Caused by Pine Root 
Weevil Complex," The Great Lakes Entomologist, vol 10 (4) 
Available at: https://scholar.valpo.edu/tgle/vol10/iss4/1 
This Peer-Review Article is brought to you for free and open access by the Department of Biology at ValpoScholar. 
It has been accepted for inclusion in The Great Lakes Entomologist by an authorized administrator of ValpoScholar. 
For more information, please contact a ValpoScholar staff member at scholar@valpo.edu. 
THE GREAT LAKES ENTOMOLOGIST 
SCOTCH PlNE DETERIORATION I N  MICHIGAN 
CAUSED BY PlNE ROOT WEEVIL COMPLEX 
Daniel G. Mosher and Louis F. Wilson 1 
ABSTRACT 
Pine root tip weevil, Hylobius rhizophagus, and pine root collar weevil, H. radicis, attack 
certain Scotch pine stands simultaneously causing more mortality than expected from either 
insect alone. Recommendations for curtailing this insect complex include favoring red pine, 
planting Scotch pine far from brood sources, and avoiding stump culture of Christmas trees. 
Scotch pine, Pinus sylvestris L., which is attacked by many insect species is especially 
susceptible to injury of the pine root tip weevil, Hylobius rhizophagus Millers, Benjamin and 
Warner, and the pine root collar weevil, H. radicis Buchanan. The former insect has only 
recently become a noteworthy pest in Scotch pine plantations in Michigan; the latter insect 
has been devastatingscotch pine for many years. 
Currently, both species are attacking some Scotch pine stands simultaneously, causing 
pine deterioration. Both attack the underground portion of the tree. H. rhizophagus larvae 
feed on the root tips initially causing reduced height growth and flagged shoots, followed by 
mortality. H. radicis larvae feed o n  the root collar and root bases. Their feeding girdles the 
root collar area causing whole tree mortality or  blowdown from the weakened base. 
A survey was conducted in Michigan in 1976 to examine the extent and degree of injury 
caused by these weevils in order to determine if remedial measures will be needed. This 
paper is a study based on  a portion of that  survey. 
MATERIALS AND METHODS 
After examining numerous Scotch pine plantations in Michigan, 1 2  were selected for 
study from 1 0  counties over a wide area in the northern portion of the state's Lower 
Peninsula (Fig. 1). A systematic survey was conducted in each planting. Tree injury due to 
these two species of  Hylobius was separated using specific symptoms for each insect. Trees 
attacked by rhizophagus flag unevenly, while trees injured by radicis entirely turn color at 
once. Trees which had been dead more than one year were entirely brown or needleless, and 
cause of death could no t  be determined. 
Sample data were taken at  onechain intervals; the number of samples varied by the size 
of the planting. Degree of flagging was used to estimate injury from the pine root tip weevil. 
This was done by recording the number of flags per each of the nearest 1 0  trees at  each stop 
as: none (0 flags), light (1-3 flags), moderate (4-10 flaps), or heavy ( > I 1  flags). These data 
were converted t o  a flagging index for each stand for relative comparisons. The index was 
calculated as follows: I = x + 2y + 42, where x ,  y, and z are the percentages of light, 
medium, and heavy flagging respectively. The index varies from zero (no flagged trees) to  
400 (100 percent of the trees heavily flagged). 
Percentage of pine root collar weevil attacks other than mortality was determined by 
digging around the root collar of 5 to  1 0  trees per stand and searching for weevil feeding 
damage according to  a technique developed by Kennedy and Wilson (1971). 
l ~ n s e c t .  and Disease Specialist, Forestry Division, Michigan Department of Natural 
Resources, Lansing, Michigan; and Principal Insect Ecologist, North Central Forest Experi- 
ment Station, USDA Forest Service, East Lansing, Michigan, 48824. 
1
Mosher and Wilson: Scotch Pine Deterioration in Michigan Caused by Pine Root Weevil
Published by ValpoScholar, 1977
THE GREAT LAKES ENTOMOLOGIST Vol. 10, No. 4 
Fig. 1. Locations of Scotch pine 
/ 1 I I I I I 1  
plantations used in this study. 
RESULTS AND DISCUSSION 
Both weevils are now widespread and damaging numerous stands throughout the north- 
ern portion of Michigan's Lower Peninsula. All the plantations sampled in this study showed 
flagging symptoms of rhizophagus injury (Table 1). The individual stand injury indices 
varied from 46 to 338 (maximum is 400). This index is more sensitive to injury than 
percentage of trees attacked as it considers the degree of flagging. For example one stand 
with the index of 338 had over 99 percent of the trees iniured, but 70 percent were heavily 
flagged. 
H. radicis occurred in 10 of the 12 dantations indicating that an Hvlobius comdex was 
developing in Michigan. The plantings &th this weevil had from 10 p&cent to logpercent 
of the trees attacked (Table 1). The two plantations with just rhizophagus attacks showed 
the most flagging injury and lowest mortality. Those with the highest radicis attacks showed 
the most mortality (Table 1). 
Each insect species is capable of killing the tree, but both attacking together appear to 
increase mortality. This situation seemed to occur in plantations where radicis attacks were 
2
The Great Lakes Entomologist, Vol. 10, No. 4 [1977], Art. 1
https://scholar.valpo.edu/tgle/vol10/iss4/1
1977 THE GREAT LAKES ENTOMOLOGIST 171 
Table 1. Slortality and flagging of Scotch pine from the pine root collar weevil and pine root tip 
~ e e v i l  in l ?  plantations in Michigan, 1976. 
Tree M& (%) Trees Artacked H. rhizophagus 
b)- H. radicir H. H. - Flagged Trees-- Nuntber of 
(7 rhizophagus radicis Undetermined Total Percent Mean Indexa Plantations 
alndes r a n s s  from 0 to  400. At 400 all trees would be heavily flagged 
100 percent (Table 1). But, because rhizophagus was not as abundant in these plantings 
much of the undetermined mortality could be blamed on radicis which readily kills Scotch 
pine at high population levels. The pattern of damage on several stands, however, suggests 
that both interact Mortality frequently occurred in pockets in stands infested with both 
weevils The oldest mortality started in the center of the pocket and spread outward. This 
type of mortality is indicative of rhizophagus (Kearby and Benjamin, 1963) but is not 
known for rad ic i~  which causes scattered mortality. H. rhizophagus appears to kill more 
sloaly amen alone or with low populations of radicis. Three of the stands had damage 
indices e s c e e h g  300 and more than 97 percent had flagged trees, yet less than 1 percent 
had died by the time of the survey (Table 1). 
There is probably Little direct competition between these two insects as rhizophagus 
feeds on the roots and root tips while radicis feeds on the root collar and root bases. They 
undoubtedly compete indirectly, however, as their combined effort severely restricts nutri- 
ent uptake and transport, a situation which brings about rapid decline in Scotch pine. 
The pine root tip weevil seldom injures saplings and prefers pole-sized trees (Kearby and 
Benjamin, 1%9). However, k e e  of the plantations in this study had trees that were 
notably d (average 13 ft  tall) for the unusually heavy flagging from rhizophagus attacks. 
The remaining nine planting in this study were young pole stands and averaged 22 f t  tall, a 
size more amenable to attack Further examination revealed that the three younger stands, 
and one other one, had been stump cultured 10-15 years previously so the present crop of 
trees were turnups. That is, a crop of Christmas trees had been taken out and the lower 
limbs left on the stump to grow into a second tree crop. Thus, these trees, though only 
sapling in size, had pole-sized root systems uhich are the size preferred by rhizophagus. 
COXXUSIONS AND RECOMMENDATIONS 
An Hylobiu complex vas  discovered between radicis and rhizophagxs in the Lower 
Peninsula of Michigan where abundant Scotch pine is grown. Together, these weevils are 
becoming Michigan's most serious problem in young pole-sized and stump cultured Scotch 
pine plantations- 
There are a few ways of potentiaay eliminating or at  least curtailing this complex in 
future planting programs. For instance, planting red pine in preference for Scotch pine (or 
jack pine which is also a highly susceptible species), will greatly reduce the complex. Red 
pine, unless planted near or among Scotch or jack pine, is fairly resistant to both weevils. 
Resistant varieties of Scotch pine are not known as yet. If Scotch pine is planted for 
pulpwood or sawtimber, Michigan's Upper Peninsula would be a better location. Neither 
weevil is yet a pest there and Scotch pine can still be grown safely. 
We do not recommend growing Scotch pine in the northern protion of the Lower 
Peninsula, except for Christmas trees, unless exceptional precautions are taken. We know we 
can plant susceptible pines far from a pine root collar weevil infestation and have a low 
3
Mosher and Wilson: Scotch Pine Deterioration in Michigan Caused by Pine Root Weevil
Published by ValpoScholar, 1977
172 THE GREAT LAKES ENTOMOLOGIST Vol. 10, No. 4 
probability of heavy injury before maturity. This occurs because radicis is not a very mobile 
insect (Wilson, 1968). However, we do not yet know much about the mobility of rhizo- 
phagus and further research is needed. 
Stump culture used for growing two crops of Christmas trees should be avoided as the 
second-crop trees with their larger root systems favor the pine root tip weevil. Stump 
culture also favors other insects such as the pales weevil, H. pales (Herbst) and the Zimmer- 
man pine moth, Dioryctria zimmemani (Grote), further complicating the situation (Bell 
and White, 1966). 
The pine root collar weevil can be managed by early basal pruning, a technique involving 
the removal of a few of the lower whorls of branches and needle litter around the base of 
the tree (Wilson and Miller, 1968; Wilson and Rudolph, 1970). This treatment should be 
started before trees reach pole size and can be used as either a preventive or control 
measure. 
LITERATURE CITED 
Bell, L. E. and D. P. White. 1966. Technical manual for Christmas tree farmers. Nitrogen 
Division, Allied Chemical Corp. 
Kearby, W. H. and D. M. Benjamin. 1963. Preliminary investigation of Hylobiusrhizophagus 
Millers, and its effects on coniferous plantations. Proc. N. Cent. Br. Entomol. Soc. Amer. 
18:67-68. 
Kennedy, P. C. and L. F. Wilson. 1971. Pine root collar weevil damage to red pine planta- 
tions in Michigan related to host age, temperature, and stand location. Can. Entomol. 
103: 1685-1690. 
Wilson, L. F. 1968. Habits and movements of the adult pine root collar weevil in young red 
pine plantations. Ann. Entomol. Soc. Amer. 6 1: 1365-1369. 
Wilson, L. F. and W. E. Miller. 1968. How to control European pine shoot moth and pine 
root collar weevil. USDA Forest Serv., Leaflet LF-3. 
Wilson, L. F. and V. J. Rudolph. 1970. Early basal pruning to control forest pests. J. 
Forestry 68:632-634. 
4
The Great Lakes Entomologist, Vol. 10, No. 4 [1977], Art. 1
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Valparaiso University

The Great Lakes Entomologist Volume 10 Number 4 - Winter 1977 Number 4 - Winter 1977 Article 1 December 1977 Scotch Pine Deterioration in Michigan Caused by Pine Root Weevil Complex Daniel G. Mosher Michigan Department of Natural Resources Louis F. Wilson USDA Forest Service Follow this and additional works at: https://scholar.valpo.edu/tgle  Part of the Entomology Commons Recommended Citation Mosher, Daniel G. and Wilson, Louis F. 1977. "Scotch Pine Deterioration in Michigan Caused by Pine Root Weevil Complex," The Great Lakes Entomologist, vol 10 (4) Available at: https://scholar.valpo.edu/tgle/vol10/iss4/1 This Peer-Review Article is brought to you for free and open access by the Department of Biology at ValpoScholar. It has been accepted for inclusion in The Great Lakes Entomologist by an authorized administrator of ValpoScholar. For more information, please contact a ValpoScholar staff member at scholar@valpo.edu. THE GREAT LAKES ENTOMOLOGIST SCOTCH PlNE DETERIORATION I N  MICHIGAN CAUSED BY PlNE ROOT WEEVIL COMPLEX Daniel G. Mosher and Louis F. Wilson 1 ABSTRACT Pine root tip weevil, Hylobius rhizophagus, and pine root collar weevil, H. radicis, attack certain Scotch pine stands simultaneously causing more mortality than expected from either insect alone. Recommendations for curtailing this insect complex include favoring red pine, planting Scotch pine far from brood sources, and avoiding stump culture of Christmas trees. Scotch pine, Pinus sylvestris L., which is attacked by many insect species is especially susceptible to injury of the pine root tip weevil, Hylobius rhizophagus Millers, Benjamin and Warner, and the pine root collar weevil, H. radicis Buchanan. The former insect has only recently become a noteworthy pest in Scotch pine plantations in Michigan; the latter insect has been devastatingscotch pine for many years. Currently, both species are attacking some Scotch pine stands simultaneously, causing pine deterioration. Both attack the underground portion of the tree. H. rhizophagus larvae feed on the root tips initially causing reduced height growth and flagged shoots, followed by mortality. H. radicis larvae feed o n  the root collar and root bases. Their feeding girdles the root collar area causing whole tree mortality or  blowdown from the weakened base. A survey was conducted in Michigan in 1976 to examine the extent and degree of injury caused by these weevils in order to determine if remedial measures will be needed. This paper is a study based on  a portion of that  survey. MATERIALS AND METHODS After examining numerous Scotch pine plantations in Michigan, 1 2  were selected for study from 1 0  counties over a wide area in the northern portion of the state's Lower Peninsula (Fig. 1). A systematic survey was conducted in each planting. Tree injury due to these two species of  Hylobius was separated using specific symptoms for each insect. Trees attacked by rhizophagus flag unevenly, while trees injured by radicis entirely turn color at once. Trees which had been dead more than one year were entirely brown or needleless, and cause of death could no t  be determined. Sample data were taken at  onechain intervals; the number of samples varied by the size of the planting. Degree of flagging was used to estimate injury from the pine root tip weevil. This was done by recording the number of flags per each of the nearest 1 0  trees at  each stop as: none (0 flags), light (1-3 flags), moderate (4-10 flaps), or heavy ( > I 1  flags). These data were converted t o  a flagging index for each stand for relative comparisons. The index was calculated as follows: I = x + 2y + 42, where x ,  y, and z are the percentages of light, medium, and heavy flagging respectively. The index varies from zero (no flagged trees) to  400 (100 percent of the trees heavily flagged). Percentage of pine root collar weevil attacks other than mortality was determined by digging around the root collar of 5 to  1 0  trees per stand and searching for weevil feeding damage according to  a technique developed by Kennedy and Wilson (1971). l ~ n s e c t .  and Disease Specialist, Forestry Division, Michigan Department of Natural Resources, Lansing, Michigan; and Principal Insect Ecologist, North Central Forest Experi- ment Station, USDA Forest Service, East Lansing, Michigan, 48824. 1Mosher and Wilson: Scotch Pine Deterioration in Michigan Caused by Pine Root WeevilPublished by ValpoScholar, 1977THE GREAT LAKES ENTOMOLOGIST Vol. 10, No. 4 Fig. 1. Locations of Scotch pine / 1 I I I I I 1  plantations used in this study. RESULTS AND DISCUSSION Both weevils are now widespread and damaging numerous stands throughout the north- ern portion of Michigan's Lower Peninsula. All the plantations sampled in this study showed flagging symptoms of rhizophagus injury (Table 1). The individual stand injury indices varied from 46 to 338 (maximum is 400). This index is more sensitive to injury than percentage of trees attacked as it considers the degree of flagging. For example one stand with the index of 338 had over 99 percent of the trees iniured, but 70 percent were heavily flagged. H. radicis occurred in 10 of the 12 dantations indicating that an Hvlobius comdex was developing in Michigan. The plantings &th this weevil had from 10 p&cent to logpercent of the trees attacked (Table 1). The two plantations with just rhizophagus attacks showed the most flagging injury and lowest mortality. Those with the highest radicis attacks showed the most mortality (Table 1). Each insect species is capable of killing the tree, but both attacking together appear to increase mortality. This situation seemed to occur in plantations where radicis attacks were 2The Great Lakes Entomologist, Vol. 10, No. 4 [1977], Art. 1https://scholar.valpo.edu/tgle/vol10/iss4/11977 THE GREAT LAKES ENTOMOLOGIST 171 Table 1. Slortality and flagging of Scotch pine from the pine root collar weevil and pine root tip ~ e e v i l  in l ?  plantations in Michigan, 1976. Tree M& (%) Trees Artacked H. rhizophagus b)- H. radicir H. H. - Flagged Trees-- Nuntber of (7 rhizophagus radicis Undetermined Total Percent Mean Indexa Plantations alndes r a n s s  from 0 to  400. At 400 all trees would be heavily flagged 100 percent (Table 1). But, because rhizophagus was not as abundant in these plantings much of the undetermined mortality could be blamed on radicis which readily kills Scotch pine at high population levels. The pattern of damage on several stands, however, suggests that both interact Mortality frequently occurred in pockets in stands infested with both weevils The oldest mortality started in the center of the pocket and spread outward. This type of mortality is indicative of rhizophagus (Kearby and Benjamin, 1963) but is not known for rad ic i~  which causes scattered mortality. H. rhizophagus appears to kill more sloaly amen alone or with low populations of radicis. Three of the stands had damage indices e s c e e h g  300 and more than 97 percent had flagged trees, yet less than 1 percent had died by the time of the survey (Table 1). There is probably Little direct competition between these two insects as rhizophagus feeds on the roots and root tips while radicis feeds on the root collar and root bases. They undoubtedly compete indirectly, however, as their combined effort severely restricts nutri- ent uptake and transport, a situation which brings about rapid decline in Scotch pine. The pine root tip weevil seldom injures saplings and prefers pole-sized trees (Kearby and Benjamin, 1%9). However, k e e  of the plantations in this study had trees that were notably d (average 13 ft  tall) for the unusually heavy flagging from rhizophagus attacks. The remaining nine planting in this study were young pole stands and averaged 22 f t  tall, a size more amenable to attack Further examination revealed that the three younger stands, and one other one, had been stump cultured 10-15 years previously so the present crop of trees were turnups. That is, a crop of Christmas trees had been taken out and the lower limbs left on the stump to grow into a second tree crop. Thus, these trees, though only sapling in size, had pole-sized root systems uhich are the size preferred by rhizophagus. COXXUSIONS AND RECOMMENDATIONS An Hylobiu complex vas  discovered between radicis and rhizophagxs in the Lower Peninsula of Michigan where abundant Scotch pine is grown. Together, these weevils are becoming Michigan's most serious problem in young pole-sized and stump cultured Scotch pine plantations- There are a few ways of potentiaay eliminating or at  least curtailing this complex in future planting programs. For instance, planting red pine in preference for Scotch pine (or jack pine which is also a highly susceptible species), will greatly reduce the complex. Red pine, unless planted near or among Scotch or jack pine, is fairly resistant to both weevils. Resistant varieties of Scotch pine are not known as yet. If Scotch pine is planted for pulpwood or sawtimber, Michigan's Upper Peninsula would be a better location. Neither weevil is yet a pest there and Scotch pine can still be grown safely. We do not recommend growing Scotch pine in the northern protion of the Lower Peninsula, except for Christmas trees, unless exceptional precautions are taken. We know we can plant susceptible pines far from a pine root collar weevil infestation and have a low 3Mosher and Wilson: Scotch Pine Deterioration in Michigan Caused by Pine Root WeevilPublished by ValpoScholar, 1977172 THE GREAT LAKES ENTOMOLOGIST Vol. 10, No. 4 probability of heavy injury before maturity. This occurs because radicis is not a very mobile insect (Wilson, 1968). However, we do not yet know much about the mobility of rhizo- phagus and further research is needed. Stump culture used for growing two crops of Christmas trees should be avoided as the second-crop trees with their larger root systems favor the pine root tip weevil. Stump culture also favors other insects such as the pales weevil, H. pales (Herbst) and the Zimmer- man pine moth, Dioryctria zimmemani (Grote), further complicating the situation (Bell and White, 1966). The pine root collar weevil can be managed by early basal pruning, a technique involving the removal of a few of the lower whorls of branches and needle litter around the base of the tree (Wilson and Miller, 1968; Wilson and Rudolph, 1970). This treatment should be started before trees reach pole size and can be used as either a preventive or control measure. LITERATURE CITED Bell, L. E. and D. P. White. 1966. Technical manual for Christmas tree farmers. Nitrogen Division, Allied Chemical Corp. Kearby, W. H. and D. M. Benjamin. 1963. Preliminary investigation of Hylobiusrhizophagus Millers, and its effects on coniferous plantations. Proc. N. Cent. Br. Entomol. Soc. Amer. 18:67-68. Kennedy, P. C. and L. F. Wilson. 1971. Pine root collar weevil damage to red pine planta- tions in Michigan related to host age, temperature, and stand location. Can. Entomol. 103: 1685-1690. Wilson, L. F. 1968. Habits and movements of the adult pine root collar weevil in young red pine plantations. Ann. Entomol. Soc. Amer. 6 1: 1365-1369. Wilson, L. F. and W. E. Miller. 1968. How to control European pine shoot moth and pine root collar weevil. USDA Forest Serv., Leaflet LF-3. Wilson, L. F. and V. J. Rudolph. 1970. Early basal pruning to control forest pests. J. Forestry 68:632-634. 4The Great Lakes Entomologist, Vol. 10, No. 4 [1977], Art. 1https://scholar.valpo.edu/tgle/vol10/iss4/1

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Scotch Pine Deterioration in Michigan Caused by Pine Root Weevil Complex

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