Christine E. Harris, Eric Simonne, Peggy Codreanu, and Joseph Eakes

The last census for Alabama reported a population of 4.2 million people in the state. At the same time, the white-tail deer population was estimated at 1.7 million. This makes a 3:1 people-to-deer ratio in the state. As the large cities of Mobile, Montgomery, Birmingham, and Huntsville expand, so do suburban areas around them. Urbanization often infringes on deer natural habitat, thereby increasing the interface between humans, agriculture, and deer.

Peoples’ perceptions of deer vary. For hunters and wildlife lovers, deer are a source of enjoyment. For vegetable growers and nursery producers, they are but another pest. Deer commonly feed on a wide range of horticultural crops. For example, deer are fond of sweetpotato. When sweetpotato defoliation due to feeding damage exceeds 30%, root yields are markedly reduced. Deer also feed on residential and commercial landscapes. The aesthetic value of a landscape is reduced by 10% feeding damage. When damage approaches 50%, the landscape requires replanting.

Strategies available to control deer include physical barriers, scare tactics, and harvesting. Fences must be at least eight feet tall to be effective. In many circumstances, the presence of a fence reduces the aesthetic value of the garden or landscape. Also, it is not economical to fence large fields to protect commercial vegetable crops. On the other hand, scare tactics (soap bars, aluminum foil, predator urine, human hair) are easy and inexpensive to use, but they are effective only for a short time because deer become accustomed to them and lose their fear of these devices. Finally, harvesting has helped control deer numbers. Yet, hunting has reduced the number of bucks more than that of does, thereby failing to significantly reduce the reproduction rate of white-tail deer in Alabama. Moreover, hunting is objectionable in suburban areas.

Chemical repellency is another method to control deer feeding damage. Chemical repellency occurs in the wild, as deer are known to avoid certain fragrant plants, and artificial chemical repellency can be achieved by applying a compound that decreases a plant’s pallatability to deer through an unpleasant smell or taste.

Ideally, the desirable characteristics of a good repellent are efficacy (reduces deer feeding damage below economical or aesthetic thresholds), invisibility (the product should not be detectable after application), ease of use (ready to use sprays are easier to apply than concentrated products that must be diluted), lack of phytotoxicity (the repellent should not interfere with plant growth), and affordability (the cost of protection should be much below that of what is protected). The most commonly used chemical repellents are Thiram (a bitter-tasting fungicide), rotten eggs, salts of fatty acid, bitrex, or combinations of these ingredients. Garlic juice and capsaicin also are used. Several products currently marketed for deer control contain these repellents, either alone or in combination.

An AAES research project was initiated in 1998 at Auburn University to compare the efficacy, phytotoxicity, and ease of use of chemical repellents for the control of feeding damage caused by white-tailed deer to horticultural commodities. This study was conducted at the AAES Deer Research Facility located near Auburn in the summer of 1999 using two separate groups of six does and six bucks each. Each group was confined in a fenced, one-acre study pen. Water and a balanced high-protein pelleted ration were available as needed.

Hosta, vinca, and sweetpotato were used as test plants because of their importance in Alabama. Plants were grown off-site in trade-gallon pots. Groups of eight homogeneous plants were sprayed with selected repellents outside the pens, while a group that was not sprayed was used as a control. Selected repellents (product) were putrescent egg-based spray (Havahart), ammonium salts of fatty acids (Grant’s and Hinder), bitrex (Ro-Pel), and Thiram (XP-20). They were applied following the manufacturer’s recommendations once at the beginning of each test. After spraying, pots were taken inside the pens. A damage scale was established to rate the severity of feeding damage each day: 0 = no damage; 1 = 33% damage; 2 = 66% damage; and 3 = 100% damage or uprooted plant.

Study plants were staked to the ground to avoid movement or spillage by the deer.

Damage data were taken daily for each plant species.

Products were tested on each plant species separately. This was done to ensure that the efficacy of the product was under evaluation, rather than deer plant preference. For each test, products were ranked according to decreasing efficacy (1 = highest efficacy, 8 = lowest efficacy). Results of all tests were combined by adding the ranks of each product. This cumulative ranking was called overall rank sum index (ORSI). The treatment with the lowest ORSI would, therefore, correspond to the product with the highest efficacy, and the treatment with the highest ORSI would correspond to the product with the lowest efficacy.

Under the conditions of these tests, more than 90% of the hosta and 45% of the sweetpotato plants were eaten after five days of exposure to the deer. Despite the high deer pressure of these tests, significant differences were found among treatments. For example, except for the rotten egg treatment, all treatments had at least 29% feeding damage after only one day in the pen for hosta (Table 1). After five days, only the hosta treated with rotten eggs were not completely eaten. In that test and except for the rotten-egg treatment, 10% damage was reached after only one day of feeding exposure. Fifty percent damage was reached within two days for Ro-Pel and Hinder, and within three days for Grant’s and XP-20.

Table 1. Effect of Selected Products on Deer Feeding Damage to Hosta1
Treatment (product)	Percent damage					Days to damage		Product rank2
	Day 1	2	3	4	5	10%	50%
Control (none)	56	75	87	93	93	1	1	6
Rotten egg (Havahart)	1	2	2	2	2	—	—	1
Fatty acid salts (Grant's)	29	33	66	66	72	1	3	2.5
Bitrex (Ro-Pel)	31	52	82	88	90	1	2	5.5
Fatty acid salts (Hinder)	46	55	80	92	92	1	2	5.5
Thiram (XP-20)	37	46	92	93	93	31	3	2.5
1 Percent destruction based on daily observation.
2 Decimal ranks were used to clarifying tied scores.

Surprisingly, no feeding damage at all (including to the nonsprayed control) was observed to the vinca until 12 days after exposure. Moderate feeding damage to vinca was observed in 1998 under similar conditions, but with another variety. This illustrates deer feeding preferences not for different plant species, but for different varieties within one species. The reason why the variety used in 1999 was not palatable is still under investigation.

Overall treatment ranks are presented for all three crops and summarized in Table 2. All the products tested reduced feeding damage as compared to the nonsprayed control. Rotten eggs, followed by Thiram, had the highest efficacy in reducing deer feeding damage. Yet, only rotten eggs maintained feeding damage under 10% for all crops during these tests. Feeding damage to plants sprayed with Havahart and the nontreated control after five days were respectively 2% and 93% for hosta, and 8% and 45% for sweetpotato.

Table 2. Summary of Ranks and Overall Rank Sum Index (ORSI) for Selected Products
Treatment (product)	Treatment ranking			ORSI	Product rank
	Hosta	Vinca	Sweetpotato
Control (none)	6	3.5	6	15.5	6
Rotten egg (Havahart)	1	3.5	1	5.5	1
Fatty acid salts (Grant's)	2.5	3.5	4	10	3
Bitrex (Ro-Pel)	5.5	3.5	4	13	4.5
Fatty acid salts (Hinder)	5.5	3.5	4	13	4.5
Thiram (XP-20)	2.5	3.5	2	8	2

None of the products tested were phytotoxic during the course of the experiment, and none of the products were visible on the foliage after application. All products were ready-to-use sprays, which made them easy to apply.

Despite increased pressure on remaining plants in the latter part of the tests and the extremely high deer pressure, differences were found among repellents. These results currently suggest that putrescent egg-based products are among the most effective repellents for deer feeding controls on vegetable and ornamental crops. Although a perfect deer repellent product has yet to be found, chemical repellency appears to be a promising alternative to traditional deer control methods. More research is underway on optimum application frequency for these repellents, and on selecting new chemical repellents.