8513

Managing Roof Rats and Deer Mice in Nut and Fruit Orchards

Abstract

Effective rodent management is critical for the control of

damage from these agricultural pests to nut and fruit trees

in California and the rest of the United States. As yet, no

one has developed an effective management plan for roof

rats ( Rattus rattus ) and deer mice ( Peromyscus spp.) in

California orchards. In this publication, we describe an

effective management plan using 0.005% diphacinone-

treated oats placed in elevated bait stations. In particular,

we look at the specific types of damage that rodents cause

in orchards—information you need to know in order to

implement an effective baiting program—and we give a

cost estimate for a baiting program. The elevated baiting pro -

gram that we propose should provide effective and cost-effective

control for roof rats and deer mice in nut and tree fruit orchards,

while posing little risk to the natural environment.

Invasive, non-native vertebrate species cause an estimated

$39 billion in damage in the United States each year ( Pimentel

2011). Rats ( Rattus spp.) are extremely common invasive pests

found throughout most of the United States in both urban and

agricultural areas and are thought to be among the main causes of

damage. In fact, one report puts the annual costs of damage caused

by rats at $19 billion ( Pimentel, Zuniga, and Morrison 2005 ).

More specifically, rats and mice are known to cause consid -

erable damage in nut and fruit trees in the United States ( Kern

2012 ; Pearson, Gorenzel, and Salmon 2000 ; Tobin, Koehler, and

Sugihara 1997 ). In nut crops, damage to developing macadamia

nuts from invasive rats has been estimated to be between 5 and

10% ( Tobin, Koehler, and Sugihara 1997 ). Native rodent species

can also cause a lot of damage, with estimates for damage from

deer mice ( P. spp.) of $20.64 per acre ($51 per hectare) in some

almond orchards in Fresno County, California ( Pearson, Goren -

zel, and Salmon 2000 ).

Clearly, effective rodent control is critical for the prevention

of damage to agricultural crops. Worldwide, rodenticide baits

are the mostly commonly preferred means for rat and mouse

control, given their low cost and high efficacy ( Stenseth et al.

2003 ). In California, past and present control strategies have

focused on flooding burrow systems (F. Rinder, Fresno County

Agricultural Commissioner’s office, pers. comm.), since few

baiting options are available for these species. Flooding bur -

rows, though, this is an inefficient use of time and resources.

The development, efficacy verification, and registration of a

baiting material and strategy for rat and deer mouse control in

California orchards would be of great use.

NIAMH QUINN, University

of California Cooperative

Extension advisor, South Coast

Research and Extension Center,

Irvine; and ROGER A. BALDWIN,

UCCE wildlife specialist,

UC Davis.

ANR Publication 8513 | December 2014
http://anrcatalog.ucanr.edu

ANR Publication 8513 | Managing Roof Rats and Deer Mice in Nut and Fruit Orchards | December 2014 | 2

The efficacy and palatability of commercially

available rodenticides can vary greatly, and bait effec -

tiveness is often specific to particular pest species ( Pitt,

Driscoll, and Sugihara 2011 ). Many rodenticides have

been developed to control rodent populations (e.g.,

brodifacoum, bromethalin, chlorophacinone, diphaci -

none, and zinc phosphide [Gill 1992; Pitt, Driscoll, and

Sugihara 2011 ]), and several studies have assessed the

materials’ ability to control rats and mice in natural

areas ( Radvanyi 1980 ; Spurr et al. 2013 ). However, until

now no peer-reviewed studies have tested the efficacy

of rodenticides for roof rat control in nut or tree fruit

crops, and few if any studies have been conducted on

deer mice. We recently initiated an investigation into

the efficacy of three rodenticide baits for control of

roof rat and deer mouse activity in almond orchards

and found that the 0.005% diphacinone oat bait, sold in

many county Agricultural Commissioner’s offices, was

highly effective ( Baldwin et al. 2014). This study made

use of elevated bait stations, which proved effective at

supplying bait to target species while substantially limit -

ing access to rodenticides for many nontarget species.

The rodenticide formulation described here can be used

against roof rats and deer mice in orchards.

In this publication we provide information on

how to identify damage from roof rats and deer mice

in nut and tree fruit orchards, and how to effectively

implement a baiting program to control these pests.

This appears to be an efficacious, cost-effective, and

safe baiting protocol for control of roof rats and deer

mice in orchard crops, something that has thus far been

unavailable to growers.

Identifying damage

Accurate identification of the species responsible for

damage is essential to the development of an effective

pest management program. If your management plan

focuses on the wrong species, it is likely to be ineffec -

tive and it may pose hazards to nontarget species and

even be an illegal misuse of the material, based on the

rodenticide label information. Fortunately, the presence

of roof rats and deer mice can often be detected through

indirect monitoring techniques. For example, roof rats

often burrow at the bottom of trees, and these burrows

are typically 2 to 3 inches in diameter (figure 1). Bur -

rows of the California ground squirrel ( Otospermophilus

beecheyi ) are sometimes this same size, but usually they

are a bit larger (average diameter = 4 inches). Also, if

ground squirrels are present, you will see them running

around above ground and hanging out in burrows.

Discarded almond shells at the entrance of a

burrow can help you determine the depredating species

(figure 2), but distinguishing between damage from

deer mice and roof rats can be difficult. Deer mouse

burrow openings typically average around 1.5 inches

in diameter. If burrow openings of this size are present,

the depredating species may be the deer mouse. Vole

(Microtus spp.) and deer mouse burrow openings are

similar in size, but voles are not typically found in

almond orchards, so long as ground cover is limited. If

burrow openings are larger (2–3 inches), the roof rat is

the likely culprit.

If you are still unsure about which species is

responsible for damage in your orchard, you can place

rat- and mouse-sized snap traps around areas where

you have detected damage; this may in fact be the only

way for you to discern between damage done by deer

mice and by house mice ( Mus musculus ). Rather than

snap traps, you can also use a remote-triggered game

camera to determine the damaging species. Based on

identification of the captured or photographed individ -

ual, you can then decide on an appropriate management

plan. Further information on identifying roof rat and

deer mouse species is available online at the UC IPM

Pest Note website ( www.ipm.ucdavis.edu/PMG/menu.

vertebrate.html ).

Bait station design

The bait stations used in our field trials were tubular

structures manufactured specifically for Orange County

Vector Control (Baldwin et al., 2014). The bait station

consisted of high-density polyethylene plastic tubes that

were 13 in (33 cm) long and 3.94 in (10.8 cm) inside

Figure 1. Example of a rat burrow at the bottom of an almond tree.

Figure 2. Example of almonds predated by rodents.

ANR Publication 8513 | Managing Roof Rats and Deer Mice in Nut and Fruit Orchards | December 2014 | 3

diameter (figure 3). A steel end cap was fixed onto each

end of the tube. Each end cap was penetrated with a

1.89-in (4.8-cm) opening, big enough to allow the roof

rats and deer mice to enter the station and small enough

to reduce or even eliminate any inadvertent loss of bait

from the bait station. On the inside of the metal cap,

under the opening, a 4.5-in long metal shelf is present.

This also helps reduce bait loss. As of this writing, these

bait stations are available for sale in a limited supply

from the Los Angeles County Agricultural Commission -

er’s office. We are exploring additional supply options.

Bait application

The current label for 0.005% diphacinone oat bait only

allows baiting during the non-bearing season. This

means that growers need to be proactive when deal -

ing with rodent infestations. It is the responsibility of

the grower to be aware of the presence of endangered

species in orchards where they intend to implement a

control program, since the bait may prove hazardous

to nontarget species. The killing of an endangered spe -

cies may result in a fine and imprisonment under the

Endangered Species Act 1973.

The use of elevated bait stations will eliminate

access to bait for many protected mammal species, such

as kangaroo rats ( Dipodomys spp.). Although other

protected species, such as the Tulare grasshopper mouse

(Onychomys torridus tularen -

sis), are not usually associated

with climbing trees, growers

must be vigilant in areas where

these and other protected

species are found. Growers can

consult the California Depart -

ment of Pesticide Regulations

PRESCRIBE website ( www.

cdpr.ca.gov/docs/endspec/pres -

cint.htm ) for any endangered

species restrictions associated

with bait application.

We recommend placing

bait stations either 98 feet (30

m) or 164 feet (50 m) from each

other, throughout the orchard.

Deer mice typically have a

small home range (0.25 ac [0.1

ha] to 4 ac [1.6 ha]) ( Timm and

Howard 1994 ), so if the target

species for your control program is the deer mouse only,

you should use a 98-foot spacing. This ensures that any

deer mouse will have access to at least one bait station

within its home range. Roof rats have a larger home

range (e.g., 1.1–1.9 ac [0.45–0.78 ha]) (Whisson, Quinn,

and Collins 2007), so the 164-foot spacing will put at

least one bait station in each rat’s home range while

reducing the total number of bait stations required to

effectively treat the orchard. If both roof rats and deer

mice are present, use the 98-foot spacing.

The number of bait stations needed across the

length ( NLength.aa, where “ .aa ” indicates the two deci -

mal places allowed in the calculation result) and width

(NWidth.bb, where “ .bb ” likewise indicates two decimal

places) of each orchard can be determined using the

following formulas:

Length of orchard = NLength.aa Spacing

Width of orchard = NWidth.bb Spacing

Since NLength.aa and NWidth.bb are not likely

to be whole numbers, growers should round them

down to whole numbers before multiplying to deter -

mine the total number of bait stations (NBS) needed

for the entire orchard (note that in a square orchard,

NLength = NWidth ).

NBS = NLength × NWidth

Bait stations should be evenly spaced in the orchard

(figure 4). To determine the optimal placement of bait

stations, you need to calculate how far from the edge

of the orchard to place the initial bait stations (we will

call this the distance from edge, or DE ). For square

orchards, this is calculated by multiplying .aa by the

spacing used in that orchard (either 164 or 98 ft). The

Figure 3. Design of bait stations used to control roof rats and deer mice.

ANR Publication 8513 | Managing Roof Rats and Deer Mice in Nut and Fruit Orchards | December 2014 | 4

Width

Spacing DEL

DEW

Length

resulting number should be divided by 2 to allow for

spacing on both edges of the orchard (whether the two

edges of length or the two edges of width).

DE = .aa × spacing

2

If the orchard is not square, you will need to calcu -

late the distance from the edge for the orchard’s length

separately from the distance from the edge for its width.

To do this, multiply .aa by the spacing used and then

divide the result by 2 to give the distance from edge

lengthwise ( DEL ), and multiply .bb by the spacing used

and divide that result by 2 to give the distance from

edge, widthwise ( DEW ):

DEL = .aa × spacing

2

DEW = .bb × spacing

2

Here is an example of the calculations for a square,

80-acre orchard (1,867 ft × 1,867 ft):

Length of orchard = NLength.aa 1,867 ft = 11.38 ft Spacing 164

Width of orchard = NWidth.bb 1,867 ft = 11.38 ft Spacing 164

NLength × NWidth = NBS 11 × 11 = 121 bait stations

.aa × spacing = DE 0.38 × 164 ft = 31 ft 2 2

These formulas will calculate the number of bait

stations required for the orchard and also provide

an approximate location for each bait station. Note,

however, that the actual spacing between individual

trees and rows of trees will dictate the ultimate place -

ment of each bait station, which will in some cases be

in the tree that is closest to the calculated location. Use

bungee cords (figure 5) or wire to attach to bait stations

to tree branches. Bungee cord takes less time than wire;

wire is cheaper, but it is also harder to use and must be

frequently replaced. Nylon cable ties were not effective

for keeping bait stations securely attached to branches.

To prevent spillage, bait stations should only be attached

to branches that are at an angle of 45° or less from the

main trunk. Bait stations can be attached to the top or

the underside of the branch, but must be rotated so the

hole in each end cap is at the top (12 o’clock position).

An initial amount of one cup (approximately

0.25 lb [113 g]) of bait should be added to each bait

station. Check the bait stations on a regular basis (for

instance, every three days) to ensure a constant supply

of bait; you can adjust the amount of bait you put in

each station according to uptake. If necessary, you can

put as much as 1 pound (454 g) of bait per station at

one time. It is important to note that diphacinone is a

first-generation anticoagulant that requires multiple

feedings to give the target species a toxic dose. As such,

it is essential that you maintain a constant supply of bait

in the stations throughout the duration of the baiting

process.

You can deploy bait stations without bait for a few

days at first to allow rodents to become acclimatized.

After this initial period, keep replenishing the bait for

around 4 weeks or until consumption stops. After cessa -

tion of a baiting program, you can keep the bait stations

in place, but filled with non-toxic oats, in order to assess

re-invasion by nearby roof rats or deer mice. If you do

see evidence of re-invasion, you can continue baiting

until the start of the growing season.

Figure 4. Example of the layout of bait stations (black-filled circles) for an 80-acre, square-shaped orchard. The length and width of the orchard are both 1,867 ft. In this example, only roof rats are present, so we use 164-ft spacing between bait stations. Initial bait stations are 31 ft from the edge of all sides of the orchard (DEL and DEW).

Figure 5. Bait station attached to the branch of an almond tree using a bungee cord.

ANR Publication 8513 | Managing Roof Rats and Deer Mice in Nut and Fruit Orchards | December 2014 | 5

Bait stations may also be deployed at ground level,

but previous studies have found that elevated baits

are more effective against roof rats than bait placed

at ground level ( Campbell, Koehler, and Sugihara

1998 ). Additionally, bait stations placed at ground level

increase the likelihood that non-target animals (e.g.,

kangaroo rats) will fall prey to the bait.

Re-invasion rates can be high for many rodent

species, with immigration occurring as soon as two

months after control if adjacent areas are home to large

populations (Spurr et al. 2013). As such, it is important

to consider rat and mouse outbreaks and invasions at a

broader, landscape level. Coordinated control programs

implemented by neighboring landowners should

provide the best results against damaging rat and mouse

populations.

Cautions when using bait stations

In wet and humid conditions, the bait can absorb

moisture. If this occurs, replace it with fresh, dry bait to

maintain efficacy. Bait stations were designed to mini -

mize spillage of bait during rodent feeding, but you still

need to monitor for bait spillage and immediately clean

up any spilled bait.

Deer mice have been known to nest in bait stations.

Deer mice are known reservoirs of the Sin Nombre

virus, which causes the potentially fatal Hantavirus

Pulmonary Syndrome ( Childs et al. 1994 ; Nichol et al.

1993 ). This virus is transmitted through the inhalation

or ingestion of aerosolized saliva, feces, or urine of deer

mice. Because of this, it is important to make sure that

any wind is behind the bait application technician when

he or she opens the bait stations, so that any aerosolized

particles will be carried away, downwind.

Although concerns of secondary toxicity are typi -

cally quite low with first-generation anticoagulants

(Baldwin and Salmon 2011 ; Lima and Salmon 2010 ;

McMillin et al. 2008 ; Silberhorn, Schnabel, and Salmon

2006 ), they still hold some inherent risk. As such, the

survey of pest carcasses is always an important part of

implementing a baiting program using anticoagulants.

During a baiting program, most rodents die below

ground but a few will be found above ground. Check for

aboveground pest carcasses once or twice a day. Remove

the dead rodents and dispose of them by burying or

burning (if permitted). Alternatively, you can wrap

carcasses in newspaper or double-bag them and dispose

of them in the trash. Latex or nitrile gloves should be

worn when handling dead rodents to prevent potential

infection from the various diseases or parasites they

m a y c a r r y.

Cost analysis

Initial start-up costs for an effective baiting program

will vary depending on which pest species are present,

since deer mice require more bait stations per acre

than roof rats alone. At the time of this writing, the

individual bait stations cost $4.72 each, and 1 pound of

the 0.005% diphacinone oat bait cost $1.75. Estimated

start-up costs, then, using the 164-foot spacing, would

be approximately $624.07 per 80-acre orchard (table

1). Initial costs for deer mouse management would cost

considerably more: $1,794.17 per 80-acre orchard (table

1). These estimates are based on initial placement of

one cup of bait (approximately 0.25 lb) per bait station.

If rates of infestation are high, you may need to place

additional bait. Also, these estimates do not include

tie-up or labor costs, which will vary considerably

depending on the tying method you use (bungee cords

or wire) to fasten bait stations to tree branches.

It is important to note that subsequent costs for bait

programs go down dramatically, since the initial cost

of bait stations accounts for the bulk of the expense of

a start-up baiting program. The bait stations described

are very durable and can be used across multiple fields

and multiple years, further reducing the long-term cost

of baiting programs. Ultimately, we believe this baiting

strategy will allow for effective, economical manage -

ment of roof rats and deer mice in nut and tree fruit

orchards in most areas where these species are found.

This approach also substantially reduces the potential

for negative impacts from rodenticide bait on non-

target species, thereby minimizing the hazard to the

natural environment.

TABLE 1.

Approximate initial start-up costs for a roof rat and/or deer

mouse control program on an 80-acre orchard (Estimates do

not include labor costs for placing and filling bait stations.)

Expense 164-foot spacing * 98-foot spacing *

Quantity Cost Quantity Cost

Bait stations

(for 80 acres) 121 $571.12 † 361 $1,703.92 †

Bait

(for 80 acres) 30.25 lb $52.94 ‡ 90.25 lb $157.12 ‡

T O TA L $624.06 § $1,861.86 §

* We recommend 164-foot spacing between bait stations when only roof rats are present in the orchard. If both roof rats and deer mice are present in the orchard or only deer mice are present in the orchard, we recommend a 98-foot spacing.† Estimates were based on a cost of $4.72 per bait station.‡ Estimates were based on a cost of $1.75 per pound.§ Miscellaneous cost may vary depending on the attaching method. The cost of bungee cords was approximately $0.30 each, while the cost of wire was approximately $0.02 per bait station.

ANR Publication 8513 | Managing Roof Rats and Deer Mice in Nut and Fruit Orchards | December 2014 | 6

References

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ANR Publication 8513 | Managing Roof Rats and Deer Mice in Nut and Fruit Orchards | December 2014 | 7

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12/14, 3/18-WJC,SB/RW/CR

Measurement Conversion Table

U.S. Customary

Conversion factor

for U.S. Customary

to Metric

Conversion factor

for Metric to

U.S. Customary Metric

Length

foot (ft) 0.3048 3.28 meter (m)

Area

acre (ac) 0.4047 2.47 hectare (ha)

Mass

pound (lb) 0.454 2.205 kilogram (kg)

pound per acre (lb/ac) 1.12 0.89 kilogram per hectare (kg/ha)